Ian West,

Romsey, Hampshire

and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University,

Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.

|Home and List of Webpages

|Kimmeridge - Kimmeridge Bay and Introduction |Kimmeridge Clay Fossils |Kimmeridge - Kimmeridge Bay to Gad Cliff |Kimmeridge - East - Hen Cliff, Yellow Ledge and Cuddle |Kimmeridge - Blackstone, Oil Shale at Clavell's Hard |Kimmeridge - Burning Beach, Burning Cliffs |Kimmeridge - Rope Lake Head to Freshwater Steps |Chapman's Pool, Houns-tout. |Kimmeridge - Bibliography - Start |Kimmeridge - Bibliography Continued |Osmington Mills - Black Head |Corallian Fossils |Poxwell Quarry (Purbeck Formation). |Portland and Portland Stone - General Introduction |Portesham Quarry, Portland & Purbeck strata| |

Click here for the full LIST OF WEBPAGES

INTRODUCTION - Ridgeway Railway Cutting and the Bincombe Tunnel

In the 1850s a railway line was constructed from Dorchester to Weymouth. It reached Weymouth in 1857 and was subsequently operated by the Great Western Railway. The line had to cut northward from Bincombe, near Upway, through Ridgeway Hill, and a cutting and tunnel was needed here. The Bincombe Railway Cutting was a major Victorian engineering works which provided excellent exposures of the Purbeck Formation, the Wealden, the Oxford Clay and the Chalk.

It is of interest that both the railway cutting and the road cutting cross the Ridgeway-Abbotsbury Fault system, a great fault that bounds the English Channel Basin (the Weymouth area is really part of the English Channel Basin). This fault has a throw of about half a kilometre and is similar to the fault system which runs east-west through the Lulworth Cove area. The fault systems have been active from about 140 million years ago to about 10 million years ago. At Stair Hole, Lulworth, you can see a thin bed of Upper Purbeck, Unio Beds, lime mud which has undergone liquefaction by a Late Kimmerian earthquake shock. At Friars Cliff, between Mudeford and Highcliff the major dewatering structures coincide in age with early Tertiary, Alpine, movement (note Creechbarrow), and probably result from earthquake shocks. Mostly, though, there is very little obvious sign of the former violent earthquake activity which took place in south Dorset in the two major phases (Late Kimmerian stretching and Alpine compression). There seems to be no significant records of earthquake activity in historic times on the Ridgeway-Abbotsbury or the Purbeck Fault Systems (except possibly for some possible consequences on the Isle of Portland). The railway has been running safely across the great fault since the mid 19th century. Only minor adjustments seem to take place in the fault systems of the region; they result in very small earthquakes, particularly near Chichester.

The Bincombe Railway Cutting was particularly important in providing a very good section through the Purbeck Formation. These strata were already known in Victorian times for the fossil insects. Thus fossil insect enthusiasts like the Reverend Osmond Fisher and J.O. Westwood rushed to the exposures and made excellent discoveries of Mesozoic insect life. Now the Weymouth Relief Road excavations have again attracted insect specialists; Dr. Robert Coram and Dr. Ed. Jarzembowski have made a thorough search of the Purbeck exposures at Bincombe and found superb fossil insects. See the recent communication by Robert Coram.

INTRODUCTION

Ridgeway Cutting - Travelling to Weymouth in Victorian Times

The railway line at Ridgeway was on the route from Wimborne to Weymouth. The train went from Wimborne to Hamworthy Junction (Poole), to Wareham, Wool, Moreton, Dorchester, and then to Upwey and Weymouth. The travel time in 1882 was one hour to one a quarter hours. The single fares were: First Class - 6 shillings anbd 10 pence, Second Class - 4 shillings and 10 pence, Third Class - 2 shillings and eight and a half pence (fairly expensive in Victorian times?).

Between Dorchester and Weymouth the line is worked jointly by the London and South-Western and the Great Western Companies. Maumbury amphitheatre is immediately on the left on leaving Dorchester, while away on the right rises Maiden Castle, of which the train en route commands on the most striking views. The inner range of coast hills is passed at Ridgeway by a tunnel, immediately on emerging wherefrom there is a charming view right over the whole of the valley of the Wey and of the hills to the sea lin. On the left rises the commanding eminence of Bincombe Barrows.

29 miles. Upwey. A delightful and rapidly growing village; in effect a suburb of Weymouth, and a great resort for pleasure parties in the summer months, for whose accomodation there is ample provision. From Upwey the line descends and skirting Backwater sweeps into Weymouth station

Accompanying advertisement, - extract -
"Great Western Railway
FAMILY CARRIAGES (with lavatories and other conveniences), containing compartments for servants, can be engaged on payment of not less than Four First Class and Four Second Class Fares."

(Extracts from Worth, R.N. 1882. Tourist's Guide to Dorsetshire: Coast, Rail and Road. 112 pp with map.)

WEYMOUTH RELIEF ROAD

Weymouth Relief Road Geology - General

To see aerial photographs of the Weymouth Relief Road in progress go to:

Aerial Images of the Scheme, Dorset for You.

OXFORD CLAY:

Introduction

The Oxford Clay was examined by Ian West and James Codd briefly in the southern part of the Weymouth Relief Road cutting. More detailed studies may be available elsewhere. Most of the clay seen was of the Upper Oxford Clay (Oxfordian) but some Lower Oxford Clay (Callovian) was seen at the far southern end. The area around this southern part of the Weymouth Relief Road has previously been mapped on the 2000 edition of the BGS Geological Survey map 341 and part of 342. The Stewartby and Weymouth Members are shown undivided. The Weymouth Relief Road may provide some further information but is unlikely to change the mapping appreciably.

For up-to-date stratigraphy of the Oxford Clay, a recent paper is useful, and particularly so for magnetostratigraphy. The Oxford Clay correspond to marine magnetic anomalies M30 through to M37. See: Ogg, Coe, Przybylski and Wright (2009).

The details of the Oxford Clay were not easy to resolve in the road cutting because the soft muddy clay had been much disturbed by excavation machinery. In general terms the upper part of the Oxford Clay was found to be largely of mudstone containing siderite nodules and the oyster - Gryphaea dilatata.

Further south some hard bituminous shale was encountered. This more organic-rich part of the Oxford Clay resembles the Kimmeridge Clay and is probably the Peterborough Member of the Oxford Clay (i.e. Lower Oxford Clay - Callovian). For more on the general characteristics of the Peterborough Member (not specifically in the Weymouth Relief Road) see Martill et al. (1994).

JURASSIC - CORALLIAN STRATA
Introduction

See more Corallian information see also:

| Osmington Mills - Introduction |Osmington - Osmington Mills to Ringstead |Osmington - Bencliff Grit |Osmington - Osmington Oolite | |Osmington - Black Head | Osmington - Corallian Fossils | Osmington Mills and Corallian - Bibliography |

The Corallian strata have been exposed in the cutting for the Weymouth Relief Road at Southdown Ridge, an east-west hill, directly south of the Littlemoor Housing Estate. The area is in the vicinity of Lorton House.

CORALLIAN - RAILWAY CUTTING EXPOSURES

Near Upwey Station

The Corallian limestones and clays have been studied and collected from in this area since Victorian times. Arkell (1936) summarised previous work in the Upwey-Broadwey area, near the railway station:

"[Corallian Beds of Dorset] VIII. BROADWAY TO ABBOTSBURY.

At Bowleaze Cove the Corallian outcrop leaves the coast and runs by Broadway to Abbotsbury, forming a narrow ridge along the north side of the Weymouth Anticline. It is lower and more rounded than the steep ridge formed by the Portland Beds, which runs· parallel to it as far as Portisham, the two ridges separated by a grass vale underlain by the Kimeridge Clay. At first the Nothe Grits give rise to a small subsidiary escarpment, but, as noted by Blake and Hudleston, they seem to thin away westwards. The only large sections until just before Abbotsbury is reached are in the railway-cuttings on the Abbotsbury branch line on either side of Upwey Station (Broadway) [this is just to the west of the route of the Weymouth Relief Road at the hill, south of Littlemoor Estate]. The cutting at and west of the station is now mainly overgrown, though bands of limestone crowded with Trigonia clavellata protrude here and there. East of the station and the road bridge, below the cemetery wall, there is still a clean section between twenty and thirty feet deep, exposing the whole of the Trigonia clavellata Beds with about 6 ft. of marly oolite and clay at the bottom belonging to the Osmington Oolite Series. It is not favourably situated for study, being almost vertical and close to the line. When first made, this cutting showed the following section, according to Woodward [1895, p. 92] and Strahan [1898, p. 35] :-

Flint gravel

PART OF THE SANDSFOOT CLAY AND THE TRlGONIA BEDS.
Clay and oolitic limestone - 20 feet, [6 metres]
Calcareous and shelly grit with a band full of Trigoniae - 12 feet [3.7 metres]
Oolitic limestone and clay full of ooliths (Osmington Oolite Series) - 6 feet [1.8 metres - seen]

Another railway-cutting, on the main line 100 yards [100 metres] south of Upwey Junction, formerly showed the junction of the Nothe Grit and Oxford Clay; and Damon collected many fossils from the Trigonia Beds in a quarry" south of Broadway House" [1884, p. 46]."

CORALLIAN - WEYMOUTH RELIEF ROAD:

Introduction

The Corallian strata in the Weymouth Relief Road cutting are discussed briefly below in upward sequence. The lowest beds are to the south and the exposures young northward, with the top of the Corallian at the (southern) edge of the Littlemoor Estate.

CORALLIAN continued:

Nothe Clay

In June 2009 the lowest part of the Corallian seen was the Nothe Clay. This is often the cause of major landslides in the Weymouth area. It is a dark grey clay when seen unweathered in the cliff sections. It is about 12 metres thick at Redcliff Point (Arkell,1949). Here in the Weymouth Relief Road cutting it is in weathered, gleyed condition and contains the pedogenic calcite, moonmilk.

Moonmilk of the Nothe Clay

The Nothe Clay has, so far, only been seen in weathered condition. A deeper cutting at a later stage might show in better condition. The soil is interesting because it contains small patches of moonmilk scattered through it. Moonmilk is not a rare substance in Dorset and would be expected in association with cave (e.g. Portland), but it is not so commonly found dispersed in a gleyed clay soil.

Moonmilk or Calcite Moonmilk is a common and familiar white substance found in caves and sometimes in soils. It is usually, although not always, lublinite, named after Lublin in Poland. This is a variety of calcite usually developed as a chain of rhombs. Needles are formed by the elongatation of the chain of rhombs on rhombs (i.e. it is elongated more or less in a direction of an A - axis). Its name, Moonmilk, comes from the German-speaking part of Switzerland, where in the 16th Century people called it Monmilch, Mon being an old spelling for Mond, the present German word for moon. The people then believed that this underground white material was formed by the rays of the Moon. In the 16th Century, it was generally thought that the nightime dew was produced by the Moon.

The Nothe Clay (Corallian Group) in which the moonmilk occurs is a Calcareous Gley Soil (see Burnham, 1980). Such soils originate in seasonally waterlogged conditions on a slope, and this is compatible with the environment on the south slope of Southdown Ridge, near Lorton House. No check has been made here, but the pH of such soils tends to be rather high (obviously it cannot be low).

CORALLIAN continued:

Bencliff Grit

The Bencliff Grit is an obvious reddish-brown sand that is well-exposed in the cutting of the Weymouth Relief Road. It is seen in the hill ridge, Southdown Ridge, to the south of the Littlemoor Estate and in in the Lorton area.

However, the effects of scraping by heavy machinery has resulted in crumbling, crushing and smearing. At present the details visible in the cliff section cannot be made out in the broken rock. Large doggers or carbonate-cemented sandstone concretions are present in place, sometimes fractured by the machinery. Compared to the coast section at Osmington Mills the striking feature is that the iron content, present probably both as ferroan calcite and pyrite, is oxidised to a very reddish colour. Thus the product in the excavations is conspicuously coloured sand. The Bencliff Grit is not very thick and thus it only occupies a relatively small outcrop.

It was a little disappointing not to see the almost spherical nodules that occur at the base in the cliff section at Osmington Mills. I did not find one spherical nodule even though the section cut through to the underlying Nothe Clay. Most of the nodules seem very flattened and some fracture quite easily in the plane of bedding. An interesting feature is that they contain fine plant debris in places. This is present in contained clay laminae of temporary low-energy origin. These plant-debris laminae ought to contain some fossil insect remains (as in the Purbecks), but I have not searched for them and leave this to the insect palaeontologists; I hope that they make some finds.

Because of the oil seeps on the coast, one might expect to notice some odour of oil in the Bencliff Grit. I did not find this, although I only examined a part of the exposure and possibly this might be present deeper. In contrast, the smell of oil is very strong in parts of the Purbeck Formation, including a bed with the basal sequence (noticed by Fisher) and in the region of the Cherty Freshwater Member. There may even have been a former oil or gas seep at Bincombe (because of down-dip Purbeck connections with the fault systems).

The queston arises as to why is there no obvious oil seep in the Bencliff Grit at Southdown Ridge, while there is at Osmington Mills? The problem is probably one of former traps. The oil at Osmington Mills is not now in a closed trap now but it is in an axial region and might have been trapped at some time in the past. It might at one time have held significant quantities of oil. Where the Bencliff Grit is seen in the Weymouth Relief Road, however, it is on the north limb of the Weymouth Anticline and not near an axis or crest. It remains to be seen as to whether any oil indications appear in the Weymouth Relief Cuttings in the future.

CORALLIAN continued:

Osmington Oolite

The Osmington Oolite Formation was well-exposed but much crushed by machinery. The limestones are easily seen but heavily fractured. The ooids of the oolite are very easily recognised with the aid of a hand-lens.

For the Weymouth Relief Road a bridge was being constructed in October 2009 at the hill directly south of the Littlemore Estate. This required, at least for a time, the presence of a vertical face, unusual in the Weymouth Relief Road excavations. This section showed Corallian strata, with in particular the upper part of the Osmington Oolite Formation. The Upper White Oolite seems to be present with the Nodular Rubble a short distance above. At the top of the cliff the Red Beds of the clavellata Formation are present.

CORALLIAN continued:

Clavellata Member

The reddish-brown sandy limestones of the Clavella Member, including the Red Beds were initially exposed when the Weymouth Relief Road cutting first extended south of the Littlemoor Estate. On the top of the hill, which is part of the Southdown Ridge, quite a broad exposure was seen, with much rubble broken out from these strata.

Here the large flat oyster Deltoideum delta was seen to be abundant in the limestone blocks. The corrugated oyster - Lopha is quite abundant in the clay just above the limestones on the hill top. This does not seem as elongate as the common Lopha gregaria (J. Sowerby) and might, perhaps be Lopha genuflecta Arkell, which occurs in the Ringstead Coral Bed.

JURASSIC - KIMMERIDGE CLAY:

Introduction

Associated webpages provide further information on the Kimmeridge Clay, photographs of this Formation in the Dorset Cliffs, details and images of Kimmeridge Clay fossils, and also a Kimmeridge Clay Geological Bibliography. Refer to some of the webpages listed below:

Kimmeridge - Kimmeridge Bay and Kimmeridge Clay Introduction
Kimmeridge Clay - Fossils
Kimmeridge - Kimmeridge Bay westward to Gad Cliff
Kimmeridge - East - Hen Cliff, Yellow Ledge and Cuddle
Kimmeridge - Blackstone, Oil Shale at Clavell's Hard
Kimmeridge - Burning Beach, Burning Cliffs
Kimmeridge - Rope Lake Head to Freshwater Steps
Kimmeridge - Egmont Bight, Houns-tout Cliff and Chapman's Pool
Ringstead and White Nothe

KIMMERIDGE CLAY continued:

Kimmeridge Clay - Stages and Zones

Over the years since Salfeld established the Kimmeridge ammonite zones in 1913 there have been a number of changes. Salfeld classified the whole sequence as Kimmeridgian Stage. The Kimmeridge Clay is now considered to be the equivalent of two stages, the Kimmeridgian and the Bolonian Taylor et al. (2001). The zonal scheme shown here is that of Cope (1978), as used by Taylor et al. (2001).

STAGE	CHRONOZONE	ZONAL AMMONITE
Bolonian (Upper part of broader "Kimmeridgian" according to some previous authors. At Kimmeridge, the Upper Kimmeridge Clay, in the cliffs east of Kimmeridge Bay, and also west in part of Brandy Bay, corresponds to this. Includes Yellow Ledge, Kimmeridge oil shale, Rope Lake Head SB, White Band and associated shales etc.)
	Fittoni	Virgatopavlovia fittoni
	Rotunda	Pavlovia rotunda
	Pallasioides	Pavlovia pallasioides
	Pectinatus	Pectinatites pectinatus
	Hudlestoni	Pectinatites hudlestoni
	Wheatleyensis	Pectinatites wheatleyensis
	Scitulus	Pectinatites scitulus
	Elegans	Pectinatites elegans
Kimmeridgian (Lower part of the broader "Kimmeridgian" as used by some previous authors. At Kimmeridge the Lower Kimmeridge Clay, within Kimmeridge Bay, corresponds to this. Includes the Flats , Washing Ledge, Maple Ledge Dolomite Beds and associated shales etc.)
	Autissiodorensis	Aulacostephanus autissiodorensis
	Eudoxus	Aulacostephanus eudoxus
	Mutabilis	Aulacostephanus mutabilis
	Cymodoce	Rasenia cymodoce
	Bayliei	Pictonia baylei

KIMMERIDGE CLAY continued:

New Exposures

The Kimmeridge Clay occupies a significant area of low ground in the Littlemoor area, near Weymouth and further north. The Weymouth Relief Road crosses this. A septarian nodule was found at Littlemoor, and it resembles those present in the Kimmeridge Clay of the nearby cliff sections. The exact horizon was not determined but this must be very low in the Kimmeridge sequence.

North of Littlemore there is quite a long stretch with some exposures of Kimmeridge Clay. It is a little disappointing in that the clay is not excavated very deeply and the side slopes are maintained at low angles (20 degrees). The uppermost part of the Kimmeridge Clay is quite weathered. Intergrown selenite (gypsum) crystals of up to about 2 or 3 centimetres length are common. These are the usual types found in somewhat calcareous clay formations where pyrite has been oxidised. The selenite crystals are scattered through the uppermost halves of the side banks. The lower part is in better condition but still somewhat weathered.

The well-known, Kimmeridge Clay expert, Dr. Ramues Gallois has found several marker horizons. These include the Blackstone, the main Kimmeridge oil shale. This is at the top of the Pectinatites wheatleyensis Zone of the Bolonian (old Portlandien of French stratigraphy and represented by the Upper Kimmeridge Clay in Britain). The White Band, a conspicuous coccolith laminite (limestone) and a dramatic feature of the cliffs east of Rope Lake Head, Kimmeridge is present. The White Stone Band is at the base of the Pectinatites pectinatus Zone. Ramues has also found the Middle White Stone Band in the section. I have heard that, in addition, he has discovered a plesiosaur rib bone in the exposures, and other fossils (the peculiar, disc-shaped brachiopod Discinisca etc.). In contrast, the casual observer is not likely to find much, since well-preserved fossils are much rarer ind than in the coastal sections. This is mainly the result of weathering and, unfortunately the excavations are rarely deep enough to extend below this.

The distribution in the area of the Kimmeridge Oil Shale or Blackstone is of interest. The most well-known outcrop is at Clavell's Hard and Rope Lake Head, east of Kimmeridge Bay. It is also present west of Kimmeridge Bay. These two are the best coastal exposures. It can be seen near Burning Cliff at Ringstead Bay, but the section is rather degraded now and only rather weathered oil shale is obvious. It is also present on the Isle of Portland, but not exposed at present. Inland it extends from Ringstead Bay to Upwey, the site of the Weymouth Relief Road, then on westward to Portesham and finally Abbotsbury. There are very few good sections inland.

It does not seem to have been used in the Upwey-Broadwey area. However if the old railway branch line (Great Western Railway) is followed from Junction Station (Upwey Station) westward to Portesham the oil shale is encountered. In Victorian times quite an energetic attempt was made largely by Mr. Manfield to work the oil shale (presumably this was Mr. William Hardy Manfield, a quarry owner who lived at Portesham House, where Admiral Hardy had been brought up). See: Mark Godden's Little Bit Of Cyberspace Mk.II, by Mark Godden, well-known quarry manager and geologist of the Isle of Portland.)

In about 1856 the Blackstone had been discovered in the Portesham Dairy (100 yards west of the station) at 14 feet depth and a few tons were extracted. (Strahan, 1898). A pit was then sunk on the east side of the building and intersected the bed at 12 feet depth, but in both cases the works were flooded out. It was in 1877 that the Abbotsbury branch line of the railway (from Junction Station, Upwey) revealed the outcrop, much as it has been revealed again by the Weymouth Relief Road excavations. Whereas nothing has been done about its discovery now north of Littlemoor, in Victorian times it was soon put to use. The "Kimmeridge Coal" was dug out and mixed with clay. As much as 2000 tons of this were set on fire and burnt to make railway ballast ( Strahan, 1898). Unfortunately, the Kimmeridge oil shale is very sulphurous and gives off very unpleasant fumes, before leaving carcinogenic ash (but has been used in the Kimmeridge area as domestic fuel as well as industrial fuel). The fumes from this large combustion must drifted rather poisonously over the countryside.

Various other shafts were sunk with the objective of mining the Blackstone. All were flooded until the Manfield Shaft of Mr. Manfield was made in 1883. This was dry until the "coal" was reached, when, unfortunately, a fissure or joint was encountered which yielded 11,000 gallons of water per hour. According to Strahan (1898) a very rich oil-shale was met with in the shaft at 46 feet, 7 inches deep, but the principal bed was at 137 feet, a hard rock bed (a Kimmeridge cementstone - probably dolomite) occurring 33 feet below it. The main bed conisted of 10 inches of oil shale, then (above) 1 foot of clay (probably bituminous, but not specified), and then 2 feet of oil shale. The shaft was sunk to a total depth of 189 feet with a boring to a further depth of 126 feet. It was situated 170 yards (metres) north of the railway, and an incline was driven to it in the Blackstone from the outcrop by the side of the railway line. One hundred tons of both parts of the principal bed (the Blackstone), were sent to Scotland for distillation, but, unfortunately Mr Manfield died and the trial seems to have been abandoned (Strahan, 1898). A benefit of Mr. Manfield's investigation is that he had a section levelled from Rodden to Portesham, and determined the thickness of the Kimmeridge Clay here to be between 1,100 feet (335 metres) and 1,300 feet (396 metres). This is less than at Kimmeridge (1650 feet or 502 metres) but rather greater than at Ringstead (this is what you would expect for the back basin, north of Weymouth). The thickness at Littlemoor and Upwey is not known exactly, but it is probably rather similar to the figure for Portesham.

In 1917-1918, a time of war, three boreholes were made by the Government at Corton and Coryates to prove the oil shale. It was found to be no better than at Kimmeridge and has the same problems of low oil content and high sulphur content. Like the Blackstone at Kimmeridge it was found to contain abundant Saccoma (the pelagic crinoid), and associated "Subplanites" (ammonites of the genus Pectinatites) Arkell (1947). Saccoma is present in the cutting for the Weymouth Relief Road, north of Littlemoor.

For further information on the oil shale, go to the webpage: Kimmeridge Blackstone. See also: Kimmeridge geological bibliography.

KIMMERIDGE CLAY continued:

Drainage Trench, just north of Littlemoor and north of the Broadwey - Preston Road

As part of the Weymouth Relief works, as seen in October, 2009, a drainage pond and associated drains was being established north of the Littlemoor Estate. This is north of the original Broadwey to Preston Road, and north of the new bridge which was being constructed to allow the Weymouth Relief Road to pass underneath. A small drain cutting about 150 metres or so to the north of the road revealed some good, fossiliferous, Lower Kimmeridge Clay (eudoxus Zone?). I accompanied the engineering geologist (of Amey), James Codd. This section which we examined had already been investigated by the Kimmeridge Clay specialist Dr. Ramues Gallois. James had already received a report from him, and thus the stratigraphy and general features were already known. Thus the illustrations which follow are of features which probably had been already been studied in more detail and with greater accuracy by Dr. Ramues Gallois.

An interesting curiosity of the Lower Kimmeridge Clay in the Weymouth Relief Road excavations is the presence of ammonite eggs, as shown above. Such eggs have been described by Etches, Clarke and Callomon (1929). Their paper is summarised as follows, and is published in the journal - Lethaia:

"Eight clusters of small spherical and subspherical objects, some isolated and some associated with shells of perisphinctid ammonites, have been recovered from the Lower and Upper Kimmeridge Clay (Upper Jurassic) of the Dorset coast, England. They have been interpreted as ammonite egg sacs and represent the freshest and best-preserved examples known so far. Their structures and the ecological framework in which they occur are discussed. The parents are thought to be members of the two eudemic genera Aulacostephanus and Pectinatites that dominate the biostratigraphy of the ammonites in the range of the Kimmeridge Clay in which they occur. Isolated nuclei of ammonitellae have also been recovered."

The examples shown here were found by James Codd, who had been shown them by Dr. Ramues Gallois. They seemed fairly common at this particular Lower Kimmeridge Clay location, once the appearance was known. They were seen as a sheets, as shown in the photograph above, and also as smaller patches.

JURASSIC - PORTLAND SAND

Portland Sand

The Portland Sand was not properly examined. It occurred at The Knoll, south of the Bincombe Valley. It proved to be in a rather soft and crumbly condition without obvious hard beds, at least at the surface. It was mostly a greenish-brown, sandy marl. The excavating machines and bulldozers dealt with this very easily. They ploughed through it, pushed it and transported it into the valley to the north of the outcrop where it was used, with soft Portland Stone, for embankments. No good exposures were seen by me, only soft rubble. This does not mean that the Portland Sand may not have been visible as some stage in a temporary manner. Arkell (1947) has discussed the exposures of cementstones at East Hill, Sutton Poyntz and elsewhere in the local region.

PORTLAND GROUP (Upper Jurassic)

Portland Stone

In the Weymouth Relief Road excavations the chalky nature of the Portland Stone has been a benefit in terms of excavation. No quarrying of Portland Stone is needed; it is easy broken up and moved by bulldozers and lifted by digger machines into dumper trunks. A consequence of this, though, is that clean sections of Portland Stone are not normally available in the road excavations. It is mostly seen as whitish rubble. The work sometimes reveals large Portland ammonites.

.

.

.

PURBECK GROUP (Lulworth and Durlston Formations) -

Introduction

The Ridgeway Cutting exposures of the Purbeck Formation confirm the impression given elsewhere:

The bivalves and gastropods of the Purbeck Formation are very similar to living forms, even though 140 million years has passed. The freshwater forms are difficult to distinguish from modern ones. The insects are broadly similar to modern forms. The trees were not strikingly different, being largely Cypress-type trees such as grow today in Cyprus. In many respects it resembles some forested modern Mediterranean environment. The climate was very seasonal and dry in summer. In the lagoon and lakes, there were fish, turtles and crocodiles. On land there were rat-like mammals, many small flies, and some birds. Dinosaurs were also present.

PURBECK GROUP:

Related webpages:

See particularly for a nearby section of the Purbeck strata:Poxwell Quarry (Purbeck Formation). See particularly the main set Purbeck web pages, including the type section at Durlston Bay:

PURBECK FORMATION, UPPER JURASSIC - LOWER CRET, DORSET
Purbeck - Formation, - Facies (Jurassic to Lower Cretaceous)
Purbeck - Evaporites (late Jurassic)
Purbeck - Durlston Bay, Swanage - Peveril Point, Upper Purbeck
Purbeck - Durlston Bay, Middle Purbeck Formation
Purbeck - Durlston Bay - Lower Purbeck Formation.
Purbeck - Durlston Bay - Central Zigzag Part & Erosion
Purbeck - Durlston Head - Lower Purbeck & Portland Stone
Purbeck - Formation - Geological Bibliography - General
Purbeck - Formation - Geological Bibliography - Topics, Alphabetic.
Purbeck - Formation - Fossil Forest Exposure, thrombolites, evaporites
Purbeck - Formation - Fossil Forest and Isle of Portland Fossil Trees
Purbeck - Ridgway Railway Cutting, Weymouth - Purbeck Formation
Purbeck - Portesham Quarry, north of Weymouth.
Purbeck - Poxwell Quarry, east of Weymouth.
Purbeck - Formation - Dinosaur Footprints of the Isle of Portland
Purbeck - Formation - Bibliography - Vertebrates
Purbeck - Formation - Analogues

PURBECK - FISHER'S LOG - RAILWAY CUTTING

Purbeck Strata of the Bincombe (Ridgway) Railway Cutting

In 1856 the Reverend Osmond Fisher, M.A., F.C.P.S., F.G.S., Fellow and Tutor of Jesus College, Cambridge, published an excellent account of the succession of Purbeck strata at Durlston Bay, Swanage and in the Ridgeway railway cutting. Most of the sequence in the railway cutting has not been seen since that time, except for limited exposures in adjacent old quarries. The new excavations for the Weymouth Relief Road have realed a substantial part of the sequence described by Fisher. The Purbeck sequences are shown in photographs on this webpage. Study of the new exposures has shown that Fisher's work was very accurate and reliable. The road cutting could be studied in a limited period of time (necessary because the section was only open for about a week or so), using Fisher's log, and noting additional sedimentary structures etc. His log is reproduced below.

It is of interest to summarise something of the history of this Victorian geologist. Osmond Fisher was of local origin. He was born on November 17, 1817 at Osmington, Dorset. He was interested in geology from a very young age. He graduated in mathematics at Jesus College, Cambridge in 1841, then took holy orders, and in 1845 was appointed Curate-in-charge of the newly rebuilt All Saints' Church, Dorchester, where he stayed for eight years. While at Dorchester he made very useful observations on the geology of Dorset, including his study of the Bincombe or Ridgeway Railway Cutting. He was particularly interested in finding fossil insects in the ancient Purbeck strata. He was a very early theorist on the subject of continental drift and was also the author of the first geophysics textbook (Fisher, O. 1881. Physics of the Earth's Crust). He anticipated many of the ideas that are now part of plate tectonics theory. The Rev. Osmond Fisher was the recipient of the Murchison Medal in 1893 and the Wollaston Medal in 1913. For 39 years he was rector of Harlton, near Cambridge, but he returned to Dorset as often as he could to study and write about its geology. He died in 1914 in his 97th year. He was still publishing when he was 96.
(For more information see: Wilding (1988))

With regard to the Purbeck strata of Upwey and Bincombe, I recognise the valuable contribution made by the late Rev. Osmond Fisher about 150 years ago. A recent restudy of the strata at Bincombe showed his work to be very accurate, and that he recorded much good detail, particularly of fossil content.

PURBECK GROUP

General Introduction - Basin Analysis - Isopach Map

The Purbeck Group (or Purbeck Formation) is a well-known lagoonal facies that overlaps the Jurassic Cretaceous boundary. It was deposited in a very extensive and very shallow lagoon or inland sea, as shown above. It represent the end of phase of marine Jurassic sedimentation. The salinity of the water varied greatly from hypersaline, mostly in the basal and lower parts, to freshwater and to near marine elsewhere in the sequence. The Purbeck Group is thin-bedded, very fossiliferous (with mostly non-marine fossils) and very varied lithologically and petrographically. It has mostly been zoned in the past by means of the ostracods, but now magnetostratigraphy is becoming an important tool for understanding the Purbeck Group.

PURBECK SEQUENCE - INTRODUCTION

The Ridgeway Cutting - Introduction to the Purbeck Sequence

A summary of the Purbeck Succession in the Ridgeway Railway Cutting is given below. This is an introduction to the detailed work of Fisher and others. It is based on Ibbetson in Weston (1852), Fisher (1856) and Damon (1884). The latter author had grouped Fisher's sequence into Forbes Purbeck subdivisions and given faunal content in old terminology. The fauna is mentioned with old names and probable modern equivalents (where possible).

PURBECK FORMATION (with main faunal elements)

UPPER PURBECK

Viviparus Clays Member - 1.77m. (5ft, 10 inches)

Greenish (glauconitc) clays and alum clays and ferruginous beds. Viviparus, ostracods

Upper Ostracod Clays and Shales Member -3.15m. (10ft, 4 inches)

Marls and blue clays, some beds glauconitic. Viviparus marl is related to Purbeck Marble. Fish teeth and scales, turtle, Unio, Viviparus, "Cyclas" (bivalve), lignitic wood. Many coprolites (presumably phosphorite) in Fisher Bed 25.

Unio Member - 3.81m. (12ft. 6 inches)

Brown sandy limestones, including The Unio Bed, Bed 30, which at Friar Waddon has reworked Portland debris. Unio (two species present, excellently preserved in The Unio Bed), fish teeth, Viviparus, and Anodonta purbeckensis Forbes.

MIDDLE PURBECK

Upper Broken Shell Limestone - 3.20m.

Shelly limestone and marlstone. Comminuted shells.

(total Upper Purbeck - 8.75m. or 28ft 8 inches)

Chief Beef Member - 1.37m. (4ft. 6 inches)

Compressed aragonitic bivalves, beef and limestone

Corbula Member - 3.81m. (12ft. 6 inches)

Limestone and shale with aragonitic bivalve beds, some pyritic shale ("alum shale") with small oysters (Praeexogyra distorta?). Fish teeth, turtle remains.

Intermarine Member - 1.82m.

Coarse, brown sandy limestone and sandstone, with a good bivalve fauna of near-marine type: Thracia, Modiola, Perna, Pecten

Freshwater above Cinder (i.e. Downs Vein - Lias Rag etc of Durlston) - 2.29m. (7ft 5 inches)
Limestones, marl, beef and aragonitic bivalves. At the base there are broken oyster shells washed out of the top of the Cinder Bed. (Fauna not known, but freshwater at Durlston Bay)

Cinder Bed - 1.57m (5ft 2 inches)

(NB. In the Fisher Log, also given here, the thickness for Bed 77, upper Cinder, is missed (listed as 0 inches), and hence the Cinder will be underestimated if only the basal part is counted)
With abundant Praeexogyra distorta and with Protocardia in the middle as at Durlston Bay. Bones of saurians and of turtles, (and with the freshwater gastropod Physa, according to Damon (1884), although this might be an error as it present as expected in the underlying CFM).

Cherty Freshwater Member - 1.52m. (5 ft)

Limestone and marl with chert. Good fauna, mostly freshwater and including: ostracods, "Cyclas", Physa, Viviparus, Planorbis, Valvata, Melanopsis and fish remains. The record of Pinna by Fisher (1856) is unexpected for a freshwater facies (is it really Pinna or is it a Unio?). Plant remains. Also with some Praeexogyra distorta in a dark carbonaceous bed.

Marly Freshwater Member - 3.50m. (11ft. 6 inches)

Limestone, marlstones, marl and thin beds of calcareous yellow clay. Plant debris, Physa, Hydrobia, "Cyclas" and ostracods.

(total Middle Purbeck -

LOWER PURBECK

Upper Insect Beds - uppermost Soft Cockle Member - 1.55m. (5ft 1 inch)

Cyclical, cream-coloured or yellowish marls and marlstones with clay partings. Carbonaceous specks. Ostracods - Cypris purbeckensis. Insects including numerous elytra and sometimes bodies of beetles.

Lowermost and Middle Soft Cockle Member (Gypsum Beds and Serpulid Beds above) - 4.98m. (16ft 4 inches)

The middle Soft Cockle Member with abundant small Serpula coacervata (as in the Serpulite in Germany) are well developed. The lowest part of the Soft Cockle Member, the part with gypsum at Durlston Bay and Worbarrow Bay, is condensed into "ribbony" (laminated) marls and shales with pseudomorphs after halite (about 1.21 metres or slightly more). There are no major evaporites, only traces. Fauna includes Protocardia purbeckensis, the euryhaline bivalve of hypersaline lakes, but also Leda and Modiola. These two are more "normal marine" but were perhaps living in a pseudomarine, restricted lake or inland sea (more like the large Aral Sea than the small pseudomarine Lake Qarun of the Faiyum Depression). However, note that in Sussex Praeexogyra distorta occurs in the Blues (Soft Cockle equivalent) and thus indicates that at least temporarily there was near-marine salinity (Howitt (1964).

Hard Cockle Member (the limestones, without the linsen shale underneath) - 2.64m. (8ft. 8 inches)

Shelly limestones and hard marlstones. With Protocardia purbeckensis (common) and with Rissoa (found in Fisher's Bed 131, top of Hard Cockle; but what actually is this gastropod?). A bed low in the Member, the Hard Rock or Fisher's Bed 134, has been quarried. Note that the underlying beds, represented in the Worbarrow - Lulworth area by linsen shale (heterolithic with sand linsens in shale), may be included within the Hard Cockle Member in some classifications.

Lower Insect Beds (the Linsen Shale of Lulworth Cove) - 2.21m. (7ft 3 inches)

Hard marly rock with streaky bedding. A chrysalis common, wings of libellula (darter dragonflies), dipterous (flies), hemipterous, and orthopterous (grasshoppers etc) insects. Elytra of beetles. Carbonaceous specs - plant debris. (carbonate-rich clay with sand linsens at Lulworth - "tidal" flat deposit - under the overhanging HCM limestones).

Cypris Freestones - 7.65m. (25ft. 1 inch)

Laminated and ripple-laminated, ostracod-rich limestones with some marls and thin calcareous clay laminae. Fish scales, ostracods (Cypris purbeckensis?).

Broken Beds Equivalent (Broken Beds - evaporite breccia or cargneule elsewhere) - 2.82 (9ft 3 inches)

The evaporite facies is very thin here at the margin of the basin (West (1975)) and thus the evaporite breccia is not properly developed. Corellation with the breccia is difficult and really a guess. The scheme given here actually includes at least the Soft Cap and probably the Great Dirt Bed (in other words the error is downwards!). Further information of the basal Purbeck strata of Upwey and their evaporites is given below. See image of chert replacing gypsum and log of evaporite distribution.

A particularly interesting record is the occurrence of Valvata helicoides (see illustration below) in two beds (150 at about the GDB and 155 in the real Hard Cap) and "Lymneus physoides" in 150 (see also illustration below). This species of Valvata occurs in places in the basal Purbecks, but the Lymnaea would not usually be expected in the hypersaline facies of the basal Purbecks. However, the Portesham Charophyte Chert (West (1961), Barker et al. (1975) ) has already revealed a surprising association of evaporites and freshwater fossils. Portesham is only about 6km. to the west of Ridgeway Cutting. The main evidence for freshwater condition is the presence of the freshwater ostracod Cypridea dunkeri papulata, also known from the Purbeck facies of Swindon. Charophytes remains are abundant, and are probably of freshwater origin (although some modern charophytes can tolerate brackish water). The gastropods of the Portesham charophyte chert have not been studied, but a species of Valvata is certainly present. There seems little doubt that Fisher's Bed 150, Cherty Marl with flint at the bottom is the equivalent of the Portesham Charophyte Chert. It is of the same general lithology although charophytes have not (as yet) been reported. More information on Lymnaea physoides is given in a section below. Although Fisher recorded the chert with freshwater gastropods as Broken Beds equivalent, it is more likely to be the equivalent of the Great Dirt Bed, discussed below.

Supposed "Hard and Soft Cap" (actually part Hard Cap down to Skull Cap or Transition Bed and including LDB) - 2.03m. (6ft. 8 inches)

The Lower Dirt Bed is present in this area and there are silificied tree remains above it. This was seen in the Ridgeway Cutting by Fisher but has not been clearly seen as yet in the adjacent Weymouth Relief Road excavations. There is little doubt that it is present here because the Lower Dirt and a silificified tree are known from Chalbury Camp, about 2km. southeast of the cutting. At neither Ridgeway or Chalbury is the Great Dirt Bed with trees present (but as mentioned above may be represented in the area by the Portesham Charophyte Chert and equivalents). In addition to silificied wood, fish remains (Histionotus breviceps) are present. Ostracods occur. An unusual feature is the record of "Paludina" or Viviparus in the Hard Cap (Fisher's Bed 162). If the Portesham Charophyte Chert was not known to exist, this record would have probably been regarded as an error. However, it is well-established that the "West Basin", the local basin north of Weymouth occasionally changed from the normal basal Purbeck hypersaline conditions to freshwater. More study of this area is needed.

Total Thickness of Purbecks

Adding all the unit above results in a total figure of 169ft 1 inch or totalled in metres - 51.71 metres. The figure given by Fisher was 190ft. 9inches or 58.13 metres. Some error is present in the literature, possibly with regard to the upper limit of the Purbeck Formation under the Wealden. Thus about 58 metres should be perhaps be taken for the present, but with caution as the actual figure may be a little less.

The Purbeck Formation is 120.7 metres thick at Durlston Bay, 88.3 metres thick at Worbarrow Tout, and variable but very thin in the Lulworth district. In general terms the Ridgeway Purbeck section is of about the thickness of the Purbecks at Stair Hole, but partly lacks the very shallow facies of the Caps with the Great Dirt Bed (only the lowest part having this character). The evaporite facies of the basal Purbecks at Ridgeway is very thin and almost at the northwestern feather edge. The Soft Cockle evaporites (less widespread than the basal evaporites) have almost disappeared at Ridgeway. Most of the other facies are fairly normal for the Purbecks and this marginal area does not show a great facies change (although reworking appears in the Unio Bed in this region - i.e. Friar Waddon).

Purbeck Formation - Ridgeway Cutting - Lymnaea physoides

An unusual discovery of Osmond Fisher when searching the Lower Purbeck Formation of the Ridgeway Cutting was a specimen or specimens of "Lymneus phyoides". This was a lake-snail resembling the modern Lymnaea, and would have lived in freshwater conditions. Most of the basal Purbeck strata of Dorset are of hypersaline origin, and the remains of gypsum crystals, usually replaced by calcite or silica, are very common (West, 1975).

The record has been discussed by (Arkell, 1940). He was unaware of the freshwater fauna in the basal Purbecks of this region. Thus he recorded the distribution wrongly as:
"?Upwey, presumed Cherty Freshwater Bed, Middle Purbeck."
(Fisher (1856) had already recorded it as from his bed 150, "Broken Bands", Cherty Marl with flint at the bottom, and there is no reason to doubt this. Arkell's synonomy list and some comments are as follows:

Lymnaea physoides (Forbes MS.) de Loriol. [see Arkell's fig. 63]

Limneus. sp. Forbes 1851, p. 80, (basal Lower Purbeck Beds).
Lymneus physoides (Forebes MS.) Fisher 1856, p. 581 (Lower Purbeck, Broken Beds, Ridgeway) (nomen nudem).
Lymnaea physoides (Forbes MS.) de Loriol 1865, p. 127, pl.ii, fig. 14 (holotype figured from Dorset).
Limneus physoides Sandberger 1870, p. 43, pl. i, fig. 31 (copy of de Loriol's figure).
? Lymnaeus physoides Maillard 1884, p. 35, pl. i, fig 12.
Limnaea sp. Woodward 1895 (pars) (Broken Beds)

Remarks. - Forbe's type has disappeared and I have not discovered any authentic specimen. The species has never been described and rests solely on "Copy of a drawing by Mr. Bone sent to Mr. Bristow, from the Purbeck of England, of the type in the Museum of Practical Geology in London", figured as plate ii, fig. 14, of de Loriol's monograph and reproduced below (Fig 63, p. 119 [of Arkell(1940)]).
The only specimen known to me which might belong to L. physoides is that represented in Fig. 12, but it is broken at the anterior and not generically determinable. The spire is taller and the whorls are less inflated than in de Loriol's figure.
Comparisons. If fresh specimens are found they should be compared to L. hopii Robinson (1915, p. 649) from the Morrison Formation of Arizona.

PURBECK SEQUENCE

Fisher's Log of the Purbeck Strata of the Ridgeway Cutting - in full

PURBECK

Basal Purbeck Limestones and Evaporites - General

Derek Barton, the Principal Geotechnical Engineer, and James Codd, Engineering Geologist, both of Amey Consulting, and working on the Weymouth Relief Road, very kindly arranged an excavation of the strata around the junction of the Portland Freestone and the basal Purbeck Formation. The uppermost Portland Stone here is lime mudstone (micrite), sometimes referred to as of chalky facies. It is very fine-grained, mostly of calcite crystals of less than 1 micron across but not of coccoliths, as is the Chalk ( Townson (1975). This lime mudstone facies occurs mostly in the West Basin of Portland deposition in Dorset. It extends from Ringstead Bay, through Poxwell, Chalbury, Upwey, Waddon Farm to Portesham.

The Purbeck sequence is stratigraphically above the Portland Stone. Because of the steep dip to the north the Purbeck succession is seen in the northern part of the trench, which is examined here, with the sequence younging northward.

Chert or silica in the basal Purbeck strata is usually either a replacement of tree remains (easily recognised because of the fibrous appearance), after stromatolitic limestone (of lumpy appearance) or a replacement of evaporites (needs high-power hand-lens or, better, microscopic study). Evaporite replacement chert in the Purbecks is either after anhydrite or after minute lenticular crystals of gypsum. Anhydrite chert is only present towards the basin centres and is found at Durlston Head and Connor's Cove (Fisherman's Ledge) near Swanage and at Brightling and Mountfield gypsum mines in Sussex. The Purbeck silica replacements of gypsum were first found at Upwey in an old quarry in the garden of a house to the west of the railway bridge (over the Dorchester Road). Details are given in West (1964; 1975). Similar chert, as shown on the left hand side of the image above, was found in the excavated material from the basal Purbeck trench of the Weymouth Relief Road excavations. The exact stratigraphical location will probably be fixed when the samples collected have been examined microscopically. The evaporite distribution in the basal Purbecks at Upwey has already been given in West (1975). A version of a distribution diagram is reproduced below. Please examine the Upwey log within this. Note that the Purbeck silificied gypsum closely resembles modern, minute lenticular crystals of gypsum at the Dead Sea and other salt lakes and lagoons.

PURBECK GROUP - BASAL PURBECK

Purple-grey Lacustrine Marl in the Hard Cap (Bed PB 9)

A feature of interest in the Hard Cap sequence of the basal Purbeck strata is bed PB 9, the purple-grey, lacustrine marl. This is of similar facies to Bed 81, the purple marl of the Cherty Freshwater Member. Both contain gastropods, including Valvata, although PB 9 does not seem to have an abundant freshwater fauna. Both have almost certainly received a purple colour from gleying, that is early, partial oxidation when the sediment was at surface and periodically exposed. It is thus a type of palaeosol, but not to be confused, of course, with the well-developed rendzina soil with trees that occurs as the Lower Dirt Bed, lower down.

BASAL PURBECK GROUP (LULWORTH FORMATION IN PART) continued

Basal Purbeck Limestones - Upwey Great Quarry and the Bincombe Arch Trench of the Weymouth Relief Road

A trench was excavated at the site of Bincombe Arch, Weymouth Relief Road, so as to expose the Basal Purbeck Formation and the junction with the Portland Stone. These strata have been previously logged and sampled at slightly different nearby localities and with different objectives by Fisher (1856) and West (1964; 1975). The basal Purbeck Formation was logged and resampled in May 2009 by Ian West with the help of Derek Barton, James Codd and Richard Edmonds. The log is as follows:

(record in preparation - to be continued)

Bed numbers in the form PB1 or PT1 given here are temporary and just for immediate practical use. Correlation with Fisher's (1856) bed numbers will be given, where possible.

PURBECK GROUP continued

Middle Purbeck (Lulworth Formation, Upper Part) - Cherty Freshwater Member

- Black Band (Fisher 83) and Plant Bed (Fisher 85)

The Cherty Freshwater Member was seen in the pit of the 10th May and the bed identifications in relation to Fisher's Section seem fairly reliable. However, in the initial excavation I did not see Bed 81, a shell bed described by Fisher as "Purplish dirt full of shells" in the first excavations. At the top of Fisher's page 579 the highest bed shown is Bed 82 - "Chert, containing the same shells, better preserved, 1 foot 4 inches (41cm.)". There seems to be something wrong just here. No evidence was seen in the Weymouth Relief Road exposure of chert this thick, and, indeed, this would be quite abnormal for the Purbeck Formation. A thick limestone with chert would be expected, not a bed of chert.

Now, unfortunately the Cherty Freshwater Member was at the top of the exposure of the early excavation. There was no clear exposure of the relationship of the Cherty Freshwater Member to the Cinder Bed at that date. The Cinder Bed was seen to be poorly exposed in the bank above, and out of, the main excation. However, later, on the 4th August a new temporary exposure of part of what had been a sloping bank revealed the Cinder Bed, 77, much more clearly. This is shown in a photograph above. The Cinder was seen to be a short distance above Bed 79, Beef, and this in turn over the distinctive purplish clay ("Purplish dirt") referred to by Fisher. The purplish clay contains with abundant shells, particularly Planorbis and Viviparus. The details of this are shown in a photograph.

Thus we now know that Fisher's Bed 81, the "Purplish Dirt" is present in the Weymouth Relief Road excavations, but is above the original pit at the new Bincombe tunnel. This means that the Cherty Freshwater Member between the Black Bed (carbonaceous bed at the base of Fisher's Bed 83) and the "Purplish Dirt" is not clearly explained in Fisher and that the Flint Bed and/or adjacent strata of the Cherty Freshwater Member may be a little thicker than expected. A new clean excavation has been dug for a new cycle track near the Bincombe new tunnel. This was initially in very broken and disrupted condition but should clear with natural weathering processes. Thus the details should become clearer.

In the excavations for the supports of a bridge for the Weymouth Relief Road at Bincombe, a good but temporary exposure was made of the Cherty Freshwater Limestone Member and underlying strata. The sequence is somewhat condensed compared to the type-section of the Purbeck Formation at Durlston Bay, Swanage. In total thickness the Purbeck Formation of the Bincombe (Ridgeway) railway cutting is 60m. compared to about 120m. at Durlston Bay. Thus the Cherty Freshwater Limestone and Marly Freshwater Limestone Members are rather thin (probably even thinner proportionally).

The Flint Bed (Fisher's Beds 81-83) of the Cherty Freshwater Member is in fairly typical limestone form, with chert nodules containing good cross-sections of charophyte stems. The chert nodules are concentrated at the base of the bed. Just underneath this chert is a black carbonaceous bed, referred to here as the "Black Band" (part of Bed 83). The carbonaceous matter in this is very powdery, but it has not been studied in any detail or under the microscope. About 8cm. of oyster-Neomiodon marl (referred to as "sand" by Fisher) occurs under the Black Band. Then comes the "Plant Bed" (Fisher Bed 85) an argillaceous laminated limestone, about 25cm thick and it is conspicuous for containing an horizon of branching lignitic plant remains. Small quantities of conifer foliage also occur, and also some unidentified plant remains. This plant bed and its organic content is shown in several photographs above. It was excavated to some extent by Richard Edmonds on the 10th May 2009.

Fisher (1856) described this bed as:
"Laminated light-coloured clays and sands [not really sand], somewhat indurated (shaly). 10 inches [25cm.]."

In the text (p.561) Osmond Fisher describes (descending the sequence) the Plant Bed in a little more detail:

"ext we come across a bed of shaly stone, ten inches thick [25cm], chiefly composed of Cyprides [ostracods - confirmed] but full of carbonized impressions of a branching aquatic [his interpretation] plant, which must have lived on the spot. I have placed a fine specimen in the Museum of Practical Geology in Jermyn Street [London]. My impression is that this is the equivalent of the Zostera-band [not really sea-grass - Zostera] mentioned by Professor Forbes in his paper [presumably Forbes (1950) or Forbes (1951) - On the succession of strata and distribution of organic remains in the Dorsetshire Purbecks], as separating the middle and lower Purbecks in Mupe Bay. [probably bed 57 of my own log of Bacon Hole - soft very argillaceous laminated marl with plant debris, 1.5m. thick and overlying bed 56, a chocolate coloured shale of 0.46m. thick.]"

Obviously a detailed comparison of the Bincombe Plant Bed with the probable equivalent at Bacon Hole, Mupe Bay would be useful. The Bincombe Plant Bed needs to be seen in perspective, though. It is not an unusual facies in the Middle Purbecks and is an indication of the rather wetter climatic conditions that came in above the Soft Cockle evaporitic beds. This wetter facies is well-known in Sussex. The equivalent of the Cherty Freshwater Member and the Marly Freshwater Member are the Plant and Bone Beds of Howitt (1964). Note for example the occurrence of the following bed in the lower part of the Plant and Bone Beds in the Gypsum Mines Goldspur Drift:

"Silt with abundant carbonised tree remains, including conifer and fern. Jet and pyrite at the base." (Howitt (1964) fig. 3, p. 85).

Beds like the Bincombe plant bed are not rare in the Middle Purbecks and may be very common in Sussex. However, it was wise to sample the bed at Bincombe and take material for research use by palaeobotanists. The material may shed light on those Purbeck environments where the rainfall was slightly over 400mm. per annum (in contrast to the Soft Cockle rainfall), although there is no particular reason to suggest that the marked seasonality had decreased. Note also that a greater change to wetter conditions with influx of kaolinite in addition to sand occurs above the Cinder Bed (Red Rag area).

PURBECK GROUP (MIDDLE PURBECKS)

Cherty Freshwater Member in the Cycle Track Exposure

PURBECK GROUP (LULWORTH FORMATION) continued

Cherty Freshwater - Cinder Bed Junction

PURBECK GROUP:

Cinder Member (Cinder Bed)

The Cinder Bed was well-exposed in a temporary section at the northeast corner of the new tunnel for Bincombe Lane, and down the hill from the old hairpin bend on the Dorchester Road. The shells of Praeexogyra distorta are quite well preserved here, and show only a limited amount of additional distortion and compaction. The thickness of the Cinder Bed was not accurately determined in the temporary exposure in the time available, although it is obviously very thin compared to the section at Durlston Bay. Fisher (1856) did not give the thickness.

The Cinder Bed is well-exposed in the Cycle Track section. It is very easy to find, and probably the most obvious bed in the section. It is, of course, in the middle of the Purbeck sequence. It shows the normal features and contains abundant remains of the small, dark blue-grey, lagoonal oyster Praeexogrya distorta. This bivalve is matrix supported in micrite or microsparite. A thickness determined in the east bank seems to be about 1m or a little less but more accurate measurement is needed. It is of about this thickness (0.86m) in the Poxwell Road Cutting described by Sylvester-Bradley (1949).

PURBECK GROUP - LULWORTH FORMATION

Palaeontology: Fossil Insects

Fossil Insects in the Purbeck Formation at the Ridgeway Cuttings (with Dr. Robert Coram and Professor Ed. Jarzembowski).

Westwood (1854) commented on early finds of Purbeck fossil insects:

"If we take into consideration the small, and even minute size of the great majority of the insects, and indeed of the whole of the Coleoptera, which have been passed under review, the idea, that we have before us the wreck of an Insect Fauna of a temperate region, is at once raised; for although it would be rash to assert that a mass of remains of the existing tropical insects might not be accumulated in which a large quantity of minute beetles and flies would not be present, yet I cannot conceive any process, either arising from currents of water, or chemical dissolution of insect matter, which would carry off or destroy the many gigantic forms of insect life always occurring in the tropics."

For more on fossil insects and, in particular, new discoveries in the exacations for the Weymouth Relief Road see the excellent report:

Coram, R. 2009. Purbeck Fossil Insects; Weymouth Relief Road, Bincombe. 10 pp. pdf file, May 2009 by Dr. Robert Coram. Extract: "In May 2009 excavations for the Weymouth Relief Road at Bincombe exposed a Purbeck section displaying both of Fisher’s insect beds. Along with Ed and Biddy Jarzembowski, I had the opportunity to visit the site and collect insect specimens. The upper of Fisher’s beds yielded a small amount of material; his Lower Insect Bed, near the base of the Purbeck, proved much more productive. A selection of specimens from this horizon are shown on the next few pages, followed by comments on the likely palaeoenvironment." [Remains of Trichoptera (Caddis Flies), Neuroptera (Lacewings), Coleoptera (Beetles), Orthoptera (Grasshopers and Crickets), Odonata (Dragonflies and Damselflies), Hemiptera (Bugs), Diptera (True Flies), Blattodia (Cockroaches) and Mecoptera (Scorpion Flies) have recorded by Robert Coram and many are illustrated in photographs.]

CRETACEOUS - WEALDEN GROUP

Notes to be added

CRETACEOUS - CHALK

The Chalk of the Weymouth Relief Road Works

The diagram above is provided as a simple guide to the relationships between the terminology for the Chalk used by Mortimore and the British Geological Survey (BGS) and the traditional terminology of Chalk "zones" used by Rowe, Arkell, House etc., and well-known to older geologists. It is not a problem to have two schemes in use, provided you can translate one to the other (and also know the stage names). If you use an older geological survey memoir then the zonal names will be needed. If you use the new editions of the BGS geological maps then you need to know the new names. Should you forget a name or need a reminder just use the chart above.

The locations of the Chalk cutting of the Weymouth Relief Road is shown above. Positions of faults etc are shown as accurately as possible from older work but have not been accurately surveyed. The general structure is of a faulted moncline with two faults of different dates. The Chalk dips very steeply in the north limb of the monocline (near the tunnel). Further north, past the foresyncline axis, it roughly levels out. The Chalk only extends up as far at the upper part of the Newhaven Chalk Formation or Offaster pilula Zone. Notice the area of Tertiary deposits piped into the Chalk; this just impinges on the Weymouth Relief Road cutting, mostly on the west side.

The two photograph above of the southernmost part of the Chalk cutting of the Weymouth Relief Road (at an early phase of excavation) show an unusually thick flint bed. This is dipping north at about 80 degrees, and is almost as steep as the dip of 85 degrees recorded in the entrance to Bincombe Tunnel ( Arkell, 1947). The tunnel is almost due west of this location. The Chalk shown is probably about the lowest seen in the Weymouth Relief Road cutting and may be in the Lewes Nodular Chalk (i.e. Holaster planus Zone to Micraster cortestudinarium Zone)

The photographs above shows Chalk with flints that is steeply dipping towards the north, probably at about 60 degrees. This area is in the southern part of the Chalk cutting and roughly east of the Bincombe Railway Tunnel mouth (It is a little further north than the photograph above, which shows flint dipping at almost 80 degrees).The Chalk here is sheared with slickensides. The shear planes are in various directions and are not all north-south. The flint is crushed into angular fragments in association with shear planes. These features resemble in some respects those of the crushed Micraster Chalk in the middle limb on the monocline at Durdle Cove, Lulworth area.

An interesting aspect at the Ridgeway Chalk cutting is that are also open joints, which seem to be extensional fractures. These might suggest that Sigma 1, the maximum principal compressive stress, was from south and upward at about 45 degrees (i.e. in general from the nearby Ridgeway Fault). However, it is not known whether the opening (contrasing with the earlier intense compaction and crushing) is a result of stress release during excavation.

In terms of lithology this chalk does not appear obviously nodular, but has the unusual feature of continuous flint beds, some of which, as shown in a higher photograph, are quite thick. The precise horizon is not yet known but it may be somewhere in the Cortestudinarium - Coranguinum part (Lewes Nodular Chalk Formation to Seaford Chalk Formation in BGS nomenclature).

This photograph above shows the Chalk in the western bank at chainage location 335, and near the southern end of the Chalk cutting. You can see that the ground surface is relatively low in relation to the future road surface here. This is quite close to the Ridgeway Fault, which is off to the left (south) of the photograph. The conditions were poor at the time of photography, with rain falling and much rubble over the Chalk. It was difficult to determine with certainty the dip of the flint beds. There is no doubt that they are very steep, but the appearance of overturning may be a false impression and a result of an oblique view of a sloping bank. Note that younging is northward, towards the right. A cleaner exposure is really needed.

The next photograph, which is shown above, is a closer view of the same location and helps explain the situation. It suggests that the Chalk is here dips very steeply towards the north. The dip seems to be about 85 degrees. I think that this is fairly reliable. It also agrees with the figure given for the dip at the entrance to the Bincombe Tunnel (a short distance further west). It is probably best to take this as the angle of dip at chainage 335.

We move northward a short distance (perhaps about 30 or 40 metres) along the west bank of the cutting. It is difficult to see the dip. Here, though, is Dr. Mark Woods, a Cretaceous specialist, from the British Geological Survey, hammering out a large type of "Inoceramus", now under the subgenus Platyceramus, and in another photograph finding some examples of the echinoids Micraster praecursor and Echinocorys. Apparently the Platyceramus tend to occur near the top of the Lewes Nodular Chalk (Micraster cortestudinarium to Micraster coranguinum Zones). The echinoid specimens are compatible with this interpretation.

Mark is on the west bank of the cutting, just above the road level. The cutting is seen here at a fairly advanced stage, but before the road has been constructed. The photograph is towards the southwest. The Chalk looks very grey because it was raining and the sky was overcast. Because of the rain the machines were not operating on that day and access to the Chalk cutting was possible. When work is in progress there is no access here, except when the dumper truck drivers have short tea breaks.

Notice that the Chalk is not well-exposed as in a cliff because rubble and dust covers much of the section. It is quite difficult to even see the dip direction (probably at a low angle here). The section might improve with rain and weathering but, unfortunately, it will covered with earth and grass in due course. In general, the Weymouth Relief Road is providing interesting temporary sections but very few permanent exposures.

The section above is diagrammatic and not exactly to scale. It shows the general structure of the folded Chalk north of the Ridgeway Fault in the general direction of the old Dorchester Road, the Ridgeway Railway Cutting and Bincombe Tunnel and the Weymouth Relief Road. It is based quite largely on the work of Arkell and Osborne White in Arkell (1947), but has been modified in the area of the Ridgeway Fault. A dip as steep as 85 degrees, towards the north, has been recorded by Arkell (1947) at the mouth of the Bincombe Tunnel, in the Ridgeway Railway Cutting. This is the steepest dip in the Chalk north of Weymouth. Neither here nor in the Weymouth Relief Road excavations has any overturning been seen. Thus the structure is different from that at Durdle Cove, Lulworth. The diagram above takes into account the steep dips north of the fault and the lack of overturning. As aspect that is uncertain, though, is whether the folding is generally in a smooth curve or whether there is an abrupt change in angle. Unfortunately, so far the largely rubble-covered state of the exposures in the Weymouth Relief Road cutting have not given a clear answer to this.

Where the dip is quite steep in the Weymouth Relief Road there is evidence of shearing and also some extensional joints. The tectonism does not seem as severe as at Durdle Cove. Some of the flint is fractured and some slickensides are present. Fossils are not very obvious and are rather difficult to see because of both the partial crushing of the chalk and the amount of chalk dust and debris on the surfaces. Dr. Mark Wood from the British Geological Survey and Dr. Rory Mortimore are studying the Chalk here in detail, so more will be know later.

TERTIARY AND QUATERNARY:

Solution Pipes in the Chalk

The Bincombe Tertiary Outlier

A short distance to the north of Bincombe, up on the Chalk down (Ridgeway) immediately above the tunnel is a Tertiary outcrop. This is shown on the British Geological Survey Map, Sheet No. 328, Dorchester.

The Bincombe Tertiary outlier has been much dug in the past for railway ballast (presumably for the adjacent railway) and for road metal Strahan (1898). It consists of irregular, interbedded gravel, clay and sand, of the usual "Bagshot" type. The gravel is parly a shingle of well-rolled flints with numerous small pebbles of white vein quartz and other rocks. It is partly, however, of subangular pebbles. The outlier extends to within 200 metres of the Ridgeway Fault, which inclines the Lower Chalk to an angle of 85 degrees. A large gravel pit was opened on Ridgeway, above Bincombe in 1855. This showed vertical Tertiary strata Fisher (1896). Some comments of Fisher are reproduced below:

"The excavation was 24 feet [7.3m.] deep, and the vertical strata occurred in the following sequence from north to south [presumably descending]: sand coarse pipe clay [on lithology presumably Poole Formation], round gravel of flint pebbles, black [unweathered] inside; sand and clay; block of cherty grey flint with numerous casts of fossils; subangular flint gravel, not ochreous, used for road metal."

The vertical orientation of the Tertiary strata adjacent to the fault, might be expected here as a result of the monoclinal folding like that at Lulworth, Alum Bay and Whitecliff Bay, Isle of Wight, and this theory has been applied by the earlier authors (Fisher and Strahan). However, in addition there has also been very extensive piping as mentioned by Strahan (1898). This is reminiscent of the Red Hole piping of Tertiary strata into the overturned Chalk at St. Oswald's Bay between Dungy Head and Durdle Door, Lulworth. Arkell (1947, p. 232) did not consider the vertical orientation at Bincombe to be a result of tectonism because "the Chalk beneath is almost certainly horizontal". He quoted Strahan (1898) regarding the Hardy Monument Tertiary exposures, further west: "some pits 100 yds. south-west of the Hardy Monument show stratified pebble gravels dipping at 15 degrees, 45 degrees and even 65 degrees, while in another pit, 600 yards southwest of the monument, the gravel is partly vertical and partly horizontal. These dips are none of them due to earth-movements, but merely to the dissolution of the Chalk and the caving in of the gravels." Arkell then proceeded to make a very sensible comparison with the pipes of Hambury Tout (Red Hole etc). He argued that the pipes there were formed as the fold began to rise, and this seems very likely.

TERTIARY

Pipes of Tertiary Gravels with Jasper

The Tertiary gravels and sands adjacent to the Weymouth Relief Road have been studied since Victorian times. They were a major source of ballast for the Dorchester-Weymouth railway line. Pipes containing these sands and gravels have been re-exposed in the cuttings of the Weymouth Relief Road. They contain many interesting features and further description will be added.

Of interest is the occurrence of pebbles of the red form of chalcedonic silica - jasper. These pebbles are not common but it is a curiosity that they are present. (Those which could be found after spending some time searching the cutting have already been collected - and there is no public access to the cutting). Although it is somewhat surprising to find jasper in the Dorset Tertiary, it has already recorded in these strata elsewhere and it is present as a rare constituent in the Chesil Beach (Damon, 1884). A strange aspect is that jasper is more common in the Tertiary gravels of the road cutting than is Budleigh Salterton type of quartzite (so abundant on the Chesil Beach).

TERTIARY:

Pipes with Jasper continued - Source of Jasper Pebbles

Sources of the jasper pebbles must be considered, but it is not a mineral that is normally found in the sedimentary strata of post-Permian age. It is more likely to have come from the older "hard rock" Palaeozoic strata of southwest England or the English Channel. Jasper occurs in Cornwall at several localities and a short list, unlikely to be comprehensive, is given below. This comes from the website of the Mindat Directory, a source of mineral data.
1. Greystones Quarry, Lezant, Callington District, Cornwall.
2. Wheal Phoenix, Linkenhorne, Caradon and Phoenix Area, S.E. Bodmin Moor, Liskeard District, Cornwall.
3. Wheal Unity, St. Day United Mines (Poldice Mines), Gwennap Area, Camborne - Redruth - St. Day District, Cornwall.
4. Gunheath China Clay Pit, Stenalees, Hensbarrow Down, Luxulyan Area, St. Austell District, Cornwall.
5. Botallick Mine, Botallick - Pendeen Area, St. Just, Cornwall.
6. Ding Dong Mine, Boskednan, Gulval, St. Ives District, Cornwall.
The Cornish localities seem too far distant to have provided a common occurrence of jasper pebbles north of Weymouth.

A more promising source area would be the eastern margin of the Palaeozoic peninsula of southwest England, if sufficient jasper is or was present there. The eastern marginal region of Dartmoor needs consideration. There has been a possible occurrence of jasper at Trusham, north of Chudleigh, Devon. This region contains Lower Carboniferous rocks with dolerite and some spilitic lavas. Although jasper has more than one type of origin, some jasper occurs with spilites, particularly between the pillows of pillow lavas. The reference to jasper at Trusham is an historic one. It is reported in a webpage: History of Chudleigh, Devon. At Whitway Mansion at Chudleigh some jasper table are or were present: "In some of the rooms there are valuable paintings.... also beautiful tables made from the various marbles of the district, others are of rich jasper cut from a block weighing several tons found in a field, at Trusham, called the Great Southdowns.... There are two similar tables at Ugbrooke House, each containing about fifty specimens."

Confirming the occurrence of jasper southeast of Dartmoor, and also mentioning Trusham (Bramble Brook) is the short paper of (Lowe, 1910). He reported on some boulders of "pseudo-jasper" found near Newton Abbot. His paper is given in the references below. This particular region southeast of Dartmoor is a feasible source area for the jasper of the Tertiary of Bincombe Down and other nearby localities. It is not implied that the jasper Lowedescribed is necessarily the source material. In any case at the time of erosion and transport of jasper to the Tertiary sediments the ground surface would have been at a higher geological level relative to the present surface. Dartmoor and its surrounds would not have been eroded down to the present extent. Thus the jasper could have come from higher deposits which no longer exist. Lowe's (1910) paper simply shows that the jasper could have come from the Culm deposits southeast of Dartmoor (or even above the granite). Being at the western side of Lyme Bay, this is also a feasible source area for Chesil Beach jasper (although take note that Carr in the 1970s actually deposited jasper-bearing pebbles on the Chesil Beach, as part of an experiment. However, jasper was recorded on the Chesil Beach by Damon long before).

(Footnote: As a matter of interest, it should be noted that there are occurrences of of gold at several places in south Devon, particularly at Torquay (Hopes Nose) and Loddiswell. It also occurs in panned samples in the South Hams district between Plymouth and Brixham. Gold exploration has been taking place in the Crediton Trough (details of these occurrences are not given here but are well-known and can be found easily on the internet). If the circumstance had been favourable, it might possibly occur in minute quantities reworked into Tertiary gravels. Your chances of finding it are remote, but if Devon jasper can come to Dorset, a trace amount of Devon gold might also do so!)

QUATERNARY FEATURES:

Piping and Dolines in Purbeck Strata at Bincombe

(The upper photograph is of solution pipe in the Purbeck Formation at Bincombe; the lower photograph, for comparison, is of a much larger but rather similar solution pipe in the Chalk at Man O'War Head, St Oswald's Bay, near Lulworth Cove, Dorset)

Of particular interest is the piping of brown, iron-stained but uncemented sands with some gravel into the Purbeck strata in the Weymouth Relief Road excavations at Bincombe south of the Ridgeway-Abbotsbury fault system. This piped material, occurring in small dolines into Purbeck limestone is almost certainly of Tertiary derivation.

There is no doubt that the dolines or pipes in the Purbeck strata at Bincombe resemble to some extent and on a smaller scale the pipes of the Red Hole cliffs. The Bincombe pipes contain some Chalk, but much less so than at Red Hole. The reddish-brown colour is an obvious similarity. With regard to the constituent material the pipes into the Bincombe Purbeck do not seem to contain rounded pebbles like those in the higher part of the main Bincombe Tertiary outlier. Instead they mostly contain ochreous sand and clay with small subangular pebbles. Thus the similarity is more with the stratigraphically lower part of the Tertiary succession at Bincombe.

The interesting matter is the southward overstep of the Tertiary onto the Chalk and then older strata, specifically the Purbeck Formation. Reid (1896) inferred that the "Bagshot" gravels with rounded Purbeck pebbles overstepped southward not only the Lower Eocene (and Palaeocene), but also the Chalk and came into contact with older rocks.

Here in the Purbeck exposure at Bincombe we probably have early Tertiary sands, clay and subangular gravel piped down in Tertiary times from a Tertiary erosion surface (probably during folding, as at Red Hole). The piping into as low a stratigraphic level as the Purbecks could be a probable consequence of the southward overstep of the Tertiary clastics. Thus the Bincombe Purbeck exposure might provide some insight into the Tertiary overstep, and be compatible with a relatively early date of the southern England "Alpine" folding (perhaps as shown by Creechbarrow, really more Pyrenean than Alpine).

The angle between the Purbeck limestone and the overstepping Tertiary (if this happened) need further discussion. Notice that in a photograph above there is dip of the sediments within the pipe or doline of about 20 degrees in a southerly direction. Now it would, of course, be expected that the present northerly dip of the Purbeck Formation is Tertiary in date (this is because the earlier and extensional, Late Kimmerian faulting has been downthrowing south whereas the compressive Alpine faulting has been downthrowing north). The problem is the present dip of the sediments within the pipe is broadly southward at about 20 degrees. If the present dip of the Purbecks (about 45 degrees) is theoretically returned to near-horizontal then an original dip of about 65 degrees is imposed on the intra-doline sediments. If this was considered a depositional dip it is not feasible because it is too steep.

Thus the apparently depositional dip within the doline, shown above, implies that the dip of the Purbeck Formation was already close to the present figure (perhaps 10 or at maximum 15 degrees difference could be allowed). If it was not then the original depositional dip would have been steeper. Thus any overstep that took place presumably happened when the dip was already within 10 or 15 degrees of the present figure.

The problem of the dolines into the Purbeck strata is clearly not solved but an origin at the time of or after the Bartonian Creechbarrow overstep Arkell (1947) is not out of the question.

STRUCTURAL GEOLOGY:

Weymouth Anticline - General and Seismic Section

Shown here is a north-south seismic section across the Weymouth Anticline, modified and with some notes added. See the original seismic section by going to the website:

UK Onshore Geophysical Library.

By agreement with the DECC and HMSO, the Library operates as a registered charity, funded by revenues raised from data sales and donations, with the long term objective of bringing all available UK onshore seismic data into secure archival storage, whilst providing efficient access to all interested parties:

* Oil and Gas Exploration
* Gas Storage, Mining, Coal Bed Methane Extraction
* Water Resources and Environmental Issues
* Planning and Engineering
* Academic Studies and Teaching Resources

RIDGEWAY RAILWAY CUTTING - STRUCTURE:

The Ridgeway and Abbotsbury Fault Systems

The Ridgeway Railway Cutting is important in revealing the structure of the northern margin of the English Channel Inversion. An "Inversion" is a term used particularly in petroleum geology for a basinal downwarping area that thus acquires a thick sedimentary sequence and then is later uplifted. A thick sedimentary sequence is necessary for the deposition of oil source rocks and then their burial to such temperatures (about 100 degrees Celsius) that oil is generated. The English Channel Inversion is the location for the generation of oil in the Wytch Farm Oilfield, one of the largest onshore oil fields in north west Europe and with possible reserves of as much as about 500 million barrels.

The Ridgeway Cutting and the Weymouth Relief Road show the nature of the margin of the oil-generating inversion. Perhaps not suprisingly, certain of the Purbeck limestones in the Weymouth Relief Road excavations smell very strongly of oil. Not too far away, at Osmington Mills oil is seeping into the sea. The search for oil in southern England started in the local area and a borehole for oil was put down at Poxwell in 1936.

In this petroleum geology context it is necessary to understand the nature of the faulting at Ridgeway and Bincombe. I have reproduced the diagram of the Ridgeway Cutting again, just above, for comparison with Arkell's interpretion of the structures at the remarkable Bincombe inlier [an inlier is an outcrop of older strata lying within younger strata]. Compare the two diagrams and note the locations of the Abbotsbury Fault and the Ridgeway Fault.

Now the Abbotsbury Fault is the old fault. It is of intra-Cretaceous age and it originated about 120 million years ago, just before the deposition of the Upper Greensand (Albian). It is sometimes referred to as Late Kimmerian [the name is after the Crimea where structures of similar age occur - this has nothing to do with the name "Kimmeridge"]. The Abbotsbury Fault clearly shows downthrowing to the south [because younger strata are alongside and to the south of older strata - Oxford Clay]. This old fault originated when Dorset first started to leave the North American continental shelf about 1000 km off to the southeast of Nova Scota. The Weymouth region was originally a small part of a greater North American continent known as Laurentia. The North Atlantic Ocean had not formed when the strata of the Ridgeway Cutting were deposited [that is why it is sensible to compare the Purbeck lagoonal formation with the Morrison lagoonal formation of the USA].

The Abbotsbury Fault originated as a result of extension (stretching) of the Laurentian continental shelf and is one of the first signs of the formation of the North Atlantic Ocean. The fault is developed above and as a continuation of an old thrust fault plane of Carboniferous-Permian age (round about 300 million years ago). The old structure was reactivated by the extensional pull [the orientation of the old structure is, of course, the reason why the Abbotsbury Fault is shown with a hade towards the south]. You will notice that on the old diagrams the fault plane is not shown curved to the south; this is because the geologists of the past did not understand the structure and, in addition, were unfamiliar with listric [curved] faults. The Abbotsbury Fault plane probably should be curved, but this is not important as long as it is shown hading south [Captain Ibbetson showed it vertical].

The Ridgeway Fault is much later. It is obvious that it is relatively young because it displaces the Chalk and the Upper Greensand. Thus, it has to be younger than about 65 million years. The exact date is a very interesting topic but it clearly relates to the post-Cretaceous tectonics and these are known as "Alpine" and are probably of Eocene, Oligocene and Miocene in age [although in the past often considered as just Miocene]. In other words the Ridgeway Fault is related to the Alpine Orogeny, when the Alps were formed and when major compression took place as a result of the northward impact of the African Plate on the European Plate. Of course, this is all a simplification and the details are more complicated.

In summary there are two faults:

The Abbotsbury Fault - intra-Cretaceous, Late Kimmerian - before the Chalk, and extensional,
The Ridgeway Fault - Tertiary, Alpine - after the deposition of the Chalk and compressional.

Note that the Abbotsbury Fault has a very large throw, about half a kilometre. This is one of the largest faults, in terms of displacement, in southern England. Numerous substantial earthquakes occurred during its phase of movement [liquefaction in the Unio Bed of the Purbeck Formation at Lulworth Cove are probably a minor effect of a local earthquake associated with the similar Late Kimmerian faulting at Lulworth].

The Ridgeway Fault has a smaller throw and its displacement is in the opposite direction to the Abbotsbury Fault and thus it has reduced the original throw. In spite of the lesser displacement the compressional movement was quite intense. It has crushed flints within the Chalk and caused the development of small thrust planes. Associated with the faulting was the development of monoclinal folding [perhaps just before the fault fractures]. Compressional earthquakes might have had quite a severe effect in the region [We do not see much effect of earthquakes other than liquifaction in the Eocene Boscombe Sands at Friars Cliff, Mudeford].

The Ridgeway-Bincombe Structures in Comparision with the Lulworth Cove Structures

The structure at the Ridgeway Cutting and Bincombe are useful in providing a simple introduction to the structures at Lulworth Cove and Durdle Door area. The Ridgeway-Bincombe structure have the Intra-Cretaceous and Tertiary faults physically separated and so they are easy to understand. In addition erosion has cut to a lower level. This is because there is a general, regional plunge towards the east. The Ridgeway and Bincombe area, although on a separate en-echelon structure provide insight into the deeper parts of the Inversion Margin structures. Examine the diagram above and try to separate Abbotsbury Fault type structure from Ridgway Fault type structure. You will find that to some extent the structures are superimposed. This is the main reason why at first sight the Lulworth structures seem rather more difficult to read.

ACKNOWLEDGEMENTS

Many people have helped in studying new road cutting exposures and we very much appreciate their help and cooperation. We are grateful to Dr. Robert Coram for information and photographs of fossil insects found by him in the Purbeck Formation of the Weymouth Relief Road excavations. We also very much appreciate the help of Professor Ed. Jarzembowski with regard to insect discoveries here. We thank Amey, the Civil Engineering company involved in designing the road cutting excavations. We are very grateful to Skanska, the civil engineering company excavating the Weymouth Relief Road. We much appreciate cooperation of the Clients, Dorset County Council and particularly thank Richard Edmonds who participated enthusiastically in the study of the new excavations. I am very much obliged to Doreen Smith for an old photograph of the Ridgeway Cutting and general discussion on the topics of this webpage.

ARCHAEOLOGY:

Saxon Burial Site on the Chalk

Fifty-one dismembered skeletons (heads separated and thrown to the south) have been found in a small chalk pit adjacent to the route of the Weymouth Relief Road. They are apparently the remains of young men executed between AD 890 and AD 1030.

For further information see:

Dismembered Skeletons' Saxon Link.

Geologically, the site is probably in low-dipping Newhaven Chalk. The nearest geologically recorded locality has been described by Osborne-White in 1936 and reproduced in summary by (Arkell, 1947):
Pit 13: Little disturbed, Newhaven Chalk Formation (lower part), White Chalk SubGroup. In old terminology this is from the Marsupites testudinarius Zone. (More details are available about these pits but not given here)
Pit 12, a little further south, on the east side of the main Dorchester Road, is also quite close: Newhaven Chalk Formation; Offaster pilula zone.

REFERENCES AND BIBLIOGRAPHY

See also:

Osmington Mills and Corallian - Bibliography
Kimmeridge Clay - Bibliography
Purbeck Formation - Bibliography |

.

.
Arkell , W.J. 1936. The Corallian Beds of Dorset. Part 1. The Coast. Proceedings of the Dorset Natural History and Archaeological Society, vol. 57 for 1935 (published 1936), 59-93. [Contents: Introductory and Historical, The Lithology and Bionomics of the Corallian Sea Bed, Ringstead Bay to Osmington Mills, Osmington Mills to Black Head and Shortlake, Ham Cliff, Redcliff and Jordan Hill, Weymouth: Nothe Point to Sandsfoot Castle, The East Fleet near Wyke, Broadway to Abbotsbury, The Abbotsbury Iron Ore, The Ammonite Succession, List of Works to which Reference is Made. .. The Corallian Beds of the Dorset coast attain a thickness of just over 200 feet. Deposited as they were during a predominantly shallow-water episode between the epochs of the Oxford and Kimmeridge Clays, they present exceptional variety and interest in both their palaeontology and their lithology....(This publication is the source of the standard maps and cliff sections and descriptions of the Corallian strata of Osmington Mills and elsewhere. Most have been reproduced in Arkell, 1947 - Memoir. There are photographs and details, however, which have not been reproduced).]

Arkell , W.J. 1941. The gastropods of the Purbeck Beds. Quarterly Journal of the Geological Society, London , vol. 97, part 1, 79-128. [Example extract - the start - follows:]
Isolated occurrences of genera such as Viviparus and Valvata are known in Bathonian, Liassic and more doubtfully even earlier rocks, but the earliest assemblage of a dozen genera of unequivocally freshwater molluscs is found in the wonderful fauna of the Purbeck Beds. Attention was first called to it by Thomas Webster (1816, pp. 191-2) (see below, p. 81). His remark, "It is rather surprising that this very ancient freshwater formation should not have excited more attention," might almost be said to be still true; for since J. de C. Sowerby figured a couple of species of Viviparus from the Purbeck Beds in 1826 and six species of lamellibranchs in that year and in 1836 (in Fitton's memoir), no further Purbeckian mollusca have been adequately figured in this country or described in the English language.
Edward Forbes was preparing a monograph on the invertebrate fauna of the Purbeck Beds when his work was cut short in 1854 by his death at the age of 39. All that appeared was a preliminary account, in which the genera Viviparus, Valvata. Lymnaea, Planorbis, Hydrobia, Physa, Melania, Cyclas, and Unio were recorded, and also many marine genera, but no species were mentioned. A number of MS. names were introduced by him on labels and in the Survey catalogues, and some of them have been used by other authors, but if any manuscript or type specimens existed they have disappeared.
In 1856 Osmund Fisher published a detailed account of the stratigraphy of the Dorset Purbeck Beds, recording all the genera mentioned by Forbes and assigning them to their precise horizons. H. W. Bristow also noted many of them in his vertical sections of the Geological Survey, sheet 22 (1857). A useful reprint of the Durlston section with the beds numbered was published by Damon (1884, pp. 201-209).... [continues with systematic descriptions from page 83 onwards].

Arkell , W.J. 1947. The Geology of the Country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey Great Britain. 386pp. With Wright, C.W.and Melville, R.V. 2nd edition - 1952 with Addenda and Corrigenda.
.
Barker , D., Brown, C.E., Bugg, S.C. and Costin, J. 1975. Ostracods, land plants and charales of the basal Purbeck Beds of Portesham Quarry. Palaeontology, 18, 419-436.
.
Borsato, A., Frisia, S., Jones, B. and Van Der Borg, K. 2000. Calcite Moonmilk: Crystal Morphology and Environment of Formation in Caves in the Italian Alps. By Andrea Borsato, Silvia Frisia, Brian Jones and Klaas Van Der Borg. Journal of Sedimentary Research, Vol. 70, No. 5, pp. 1171-1182.
Abstract:
Calcite moonmilk, which is a cave deposit formed of calcite crystals and water, is found in many caves in the Italian Alps. These modern and ancient deposits are formed of fiber calcite crystals, 50-500 nm wide and 1 to > 10 µm long, and polycrystalline chains that have few crystal defects. Radiocarbon dating indicates that most moonmilk deposits in these caves are fossil and that for most precipitation ceased at about 6400 cal years BP, at the end of the mid-Holocene Hypsithermal. In the caves of the Italian Alps, the optimal conditions for formation of calcite moonmilk are: (1) a temperature range of 3.5 - 5.5 degrees C, (2) low discharge volumes of seepage waters that are slightly supersaturated (SICAL = 0.0 to about 0.2), and (3) relative humidity that is at or close to 100 percent. Microbial activity apparently did not play an active role in the formation of the calcite moonmilk. Conditions for moonmilk formation are typically found in caves that are located beneath land surfaces, which are soil covered and support a conifer forest. Precipitation of the fiber calcite crystals apparently involved very slow flow of slightly supersaturated fluids. The fact that moonmilk appears to form under a narrow range of environmental conditions means that this cave deposit has potential as a paleoclimatic indicator in high alpine karst areas.
.
Burnham, C.P. 1980. The Soils of England and Wales. Field Studies, vol. 5, pp. 349-363. Available online as a pdf file. With an Appendix and a 1:2,000,000 map by B.W. Avery, D.C. Findlay and D. Mackney, Soil Survey of England and Wales.
Abstract: Soil variation is great, but not random. Most of it results from relatively few processes, and arises from five factor groups: parent material, climate, relief, biotic factors and time. Examples of their effects are given from South Shropshire, and their wider regional implications are also discussed. The description and classification in standard terms requires a specialised vocabulary, which is explained in the Appendix, facilitating the use of the annexed soil map of England and Wales (1:2,000,000).
.
Coram, R. 2009. Purbeck Fossil Insects; Weymouth Relief Road, Bincombe. 10 pp. pdf file, May 2009 by Dr. Robert Coram. Extract: "In May 2009 excavations for the Weymouth Relief Road at Bincombe exposed a Purbeck section displaying both of Fisher’s insect beds. Along with Ed and Biddy Jarzembowski, I had the opportunity to visit the site and collect insect specimens. The upper of Fisher’s beds yielded a small amount of material; his Lower Insect Bed, near the base of the Purbeck, proved much more productive. A selection of specimens from this horizon are shown on the next few pages, followed by comments on the likely palaeoenvironment." [Remains of Trichoptera (Caddis Flies), Neuroptera (Lacewings), Coleoptera (Beetles), Orthoptera (Grasshopers and Crickets), Odonata (Dragonflies and Damselflies), Hemiptera (Bugs), Diptera (True Flies), Blattodia (Cockroaches) and Mecoptera (Scorpion Flies) have recorded by Robert Coram and many are illustrated in photographs.]

.
Damon, R. 1884. Geology of Weymouth, Portland, and Coast of Dorsetshire, from Swanage to Bridport-on-the-Sea: with Natural History and Archaeological Notes. New and Enlarged Edition (2nd Ed.), Weymouth, R.F. Damon, London, Edward Stanford. 250p. With a colour geological map of part of the Dorset coast, and including a log of the Purbeck strata of Durlston Bay, Swanage, by H. W. Bristow and Prof. E. Forbes (although note that it contains a small error).
.
Fisher , O. 1856. On the Purbeck strata of Dorsetshire. Transactions of the Cambridge Philosophic Society, 9, 555-581. [Classic early log of the Durlston section with useful faunal information. See Wilding, 1988 for biography of Osmond Fisher]

Fisher, O. 1896. Vertical Tertiaries at Bincombe, Dorset. Geological Magazine, vol. 33, 246-247. By the Reverend Osmond Fisher.
.
Howitt , F. 1964. Stratigraphy and structure of the Purbeck inliers of Sussex (England). Quarterly Journal of the Geological Society, London, 120, 77-113. By Frank Howitt. Published 18th February 1964. With maps and a discussion.
Abstract: The Purbeck Beds exposed in Sussex are redescribed together with new sections exposed in gypsum mine workings and boreholes. Comparisons are made with Purbeck Beds elsewhere in southern England in boreholes and at outcrop. A new map of the the Purbeck inliers is presented and the structure is discussed. Conditions of sedimentation are illustrated by isopachyte maps.
Contents:
1. Introduction
2. Historical Review
3. Stratigraphy
(a) General
(b) Sussex type area
(c) Comparison of the Sussex Purbeck type-sections with the remainder of the Sussex inliers
4. Structure
5. Conditions of sedimentation
6. Appendix: Purbeck Beds in boreholes and outcrops of southern England.
7. References
Plates 3-4, Geological Map and Structural Map - Central Weald.

.
Lowe , H.J. 1910. On some boulders of pseudo-jasper found near Newton Abbot. Reports and Transactions of the Devonshire Association for the Advancement of Science, Literature and Art, July, 1910.

Introduction, - The rock masses which form the subject of these remarks are met with in various places, occasionally as isolated blocks on the wayside, but are most in evidence at Whiteway Barton, Hestow and Well farms, to the north-east of Newton Abbot, where the walls and buildings are mainly constructed with the material, and where large boulders lie about as unmanageable obstructions. In a grass field north of Hestow large blocks, partly covered by vegetation, are scattered as though brought there by human agency for some unaccomplished purpose. Across the valley of a small stream from Hestow is the farmstead called Well, where the same rock material is met with in the walls, in obstructive boulder masses, and protruding through the ground surface. Similar rock is seen in situ, and the most suggestive position, in a field on the east of Ramshorn Down, north-west of Newton. There the masses lie together, crowning an eminence which forms a feature in the landscape.
General Description, - The blocks are generally of an irregular spheroidal form, the material is of intense hardness, though somewhat brittle. The rounded shape they assume, and their generally large size, have served to preserve them from being broken up or otherwise used. The larger blocks, which have defied both the farmer and builder, range from three to six and even nine feet in diameter, and no hammer can do more with them than splinter off the angular projections, and thus render them still more unmanageable. The dominant colour is red, which varies from a reddish yellow to the rich deep red of jasper. The stone is of very fine texture generally, and much veined by pure quartz; the contrast in colour giving it a striking appearance. Frequently cavities are met with in the mass, which are always lined with quartz crystals; the facets which terminate these inward-pointing crystals reflect the light and often sparkle like a nest of polished gems. Sometimes the fracture faces of the rock show defined patches of varying shades, suggesting a breccia composition of broken pieces of the same material with slight differences of hue, consolidated again by a cement similar in composition to the original material. A closer examination discovers, occasionally in fissures, instances of another variety of the same strong matter arranged in fine parallel layers of slightly differing colour, suggesting on a small scale the characters of the onyx and its relation thereto.
Geological Relationship. - The rock masses so far described occur amidst those widely extending shales, grits, and cherts that have been given the name of the Culm series, and which form in Devonshire the representatives of that great geological age termed the Carboniferous. At Hestow, Well, and Whiteway Barton junctions occur between the Culm and other rocks. At Well and Whiteway Baxton, they are inliers of the older Devonian series, while at Hestow the red sandy beds of the Permian are met with, where these altered rocks in question are exposed and protrude through the thinned-out edge of the overlying newer rock formation. Near Ramshorn Down the rock masses under review occur in the midst of the Culm beds, which around are but little altered and so more clearly indicate that these peculiar blocks belong to the Culm series.
Microscopic Characters. - When examined in thin sections by aid of the microscope the rock is found to be constituted almost entirely of silica or quartz, which, however, is presented in a variety of forms. The ground mass or main component of the rock is composed of grains of detrital quartz of slightly varying sizes, but for the most part very small and stained with iron oxide. In some sections a number of minute circular spaces are noticeable. These are occupied by radiating crystals of silica in a similar manner to those met with in the unaltered Radiolarian chert beds of the Culm, and thus declaring the source of some of the material. Through the main substance of the rock veins of clear quartz of varying widths are seen traversing it in all directions, the crystals of which are noticeably fresh, lie in arranged order, and are often larger than those of the ground mass. These veins were evidently formed within the rock subsequently to its deposition and original consolidation. Nests of secondary quartz are also seen, with other irregularly shaped concretions of the same clear mineral. More rarely another form of silica is met with, in which the initial or border stage of crystal development is only just reached. This crypto-crystalline or chalcedonic condition is usually presented in radiate or fan-like forms of needle-shaped crystals, probably mixed with non-crystalline or colloid silica, out of which the chalcedonic form has segregated. Slight differences in the purity of the colloid are sufficient to mark by staining the parallelism of layers in the immature crystallization, and so give the peculiarity of marking which distinguishes the onyx in silica minerals. The purely uncrystallized colloid material can also be detected in the section by its isotropic character.
The Problem and Solution. - It will be noticed that the composition of the rock is very simple, and mineralogically of very limited interest. Silica in some variety of condition constitutes the entire mass, which is given varying shades of colour by iron staining. But if the blocks in themselves and composition are of subordinate interest, their distribution and origin have long exercised the curiosity of local geologists without anything so far having been recorded to account for their erratic mode of occurrence, or respecting their other peculiarities. The problem has occupied attention for some time, and been considered from several points of view, but only one hypothesis seems to comprise all the facts and satisfy their conditions.
The key to the problem is presented in the heaped masses forming a rugged crest to a projecting shoulder of Ramshorn Down. This is surrounded by Culm material, some of which is altered, but most is in the general normal condition. The inference is that the masses in question are also composed of the same shale, grit, and chert rock matter, but very much altered by agencies that acted quite locally. The metamorphozing agent must have been that of water charged with silica in solution. Prior to the operation of the influences which brought about the peculiar local changes we are considering, the Culm series of rook deposits had been subjected to extraordinary disturbances by both lateral and vertical forces, the former of which taking place first had folded and faulted the beds to a remarkable degree. Along faults and fissures thus made the underground waters would circulate more freely, ultimately finding their way to the surface of the land, even though the same land might be itself covered by water. The next great episode in the geological history of the region was the slow but enormous upheaval of central Devon by an intrusion of fluid rock material that ultimately formed the granite mass of Dartmoor. With the welling up of this molten granite into the Culm deposits much internal heat would be brought nearer the surface. The rock that came in contact with the magma, and even within a wide border of proximity, was more or less altered by the heat of the enormous mass; and the subterranean waters for even a further distance would be affected in respect to temperature and solvent properties. Now heated water is a more ready solvent and greater absorbent of mineral matter than cold, which helps to account for the unusual amount of secondary quartz found in the rocks under notice. Inferentially we are led to conclude that these blocks, where found, are indications of the localities through which the heated water passed or issued. Much of the material presents the character of a breccia composed of fragmentary portions of the shale, grit, and chert which form the main materials of the Culm strata. These previously crumpled and shattered rocks would offer little resistance to water under any pressure, and fragments would collect wherever fissures or spaces were formed in the water's course. These collections of debris would be ultimately cemented together into masses by the silica-charged water, whenever the flow was sufficiently slow, and the other conditions prevailed to allow precipitation and crystallization taking place. Some portions of these rocks suggest a condition that might have arisen towards the end of the period during which they were forming. So remarkably and excessively veined are they that it would appear they must have been for a long time soaking or stewing in the solvent water, so that parts of the original rock were gradually dissolved, to be replaced by crystals of pure silica in veins and threads of extraordinary intricacy. In places the dissolved material was carried away by the moving water, thus forming fissures and hollows which, however, are always lined with rock crystals formed as the charged water became slower in movement and more saturated with the solvent material. An occasional occurrence of the chalcedonic form of silica in these masses points to conditions of supernormal temperature and pressure in the water solvent. Water above 200 degrees C. more readily dissolves silica and also combines with it in certain proportions forming a colloid which may be a liquid at the higher temperature, but is solid at ordinary temperatures (Van Hise). The opal is such a form of silica, while agate, onyx, and chalcedony are intermediate conditions between the purely colloid and the completely crystallized quartz. The crypto-crystalline and colloid forms of silica met with in these rock masses support the hypothesis that heated water under some pressure played an important part in endowing them with their peculiarities. Van Hise states that silicification is an earth-crust process which takes place at some depth, and but little near the surface under ordinary conditions, so that we must suppose favourable conditions of temperature and pressure prevailed during most part of the time these masses were attaining their peculiar character.
To restate our conclusions in general terms: Subsequent to the earth-crust movements which folded and faulted the Culm strata, and probably in connection with the up-welling of the granite magma therein, much heated water was circulating through portions of the upturned and fractured material that bordered the newly disturbed and rising area. Heat and pressure augmented the solvent property of the water, which became charged with silica, that it also combined with chemically to a greater or less degree, thus giving the water a cementing property where conditions of deposition and solidification prevailed in its course. The fractured rock and debris along courses of such subterranean flow would in time by silicification be consolidated into masses which, by reason of their constitution and the character of the cementing material, would be immune to all the ordinary forces of rock decay. Probably the jasperizing processes took place at some distance below the surface, and possibly beneath a depth of overspreading sea. But during the enormous lapse of time since the cessation of the rock-changing operations described, newer rock-formations have been deposited over the localities indicated, and these have again been removed by the ever-acting denuding forces of nature. The softer rock matter overlying and surrounding the silicified areas has been carried away, leaving exposed, as we see them now, the resistant masses, standing bare, defying time and the elements of decay as hardly any other rock material can do. As similar or identical conditions would prevail in other localities ordering the granite area during the period indicated, probably rock material of similar constitution and having a like history can be found in other places round Dartmoor. Of instances brought to notice, that of Bramble Brook, Trusham, is noteworthy as being the locality from which that fine block of jasper was obtained that now stands in the entrance hall of the Albert Memorial Museum, Exeter. The pale striped, so-called "ribbon jasper", of Ivybridge may be mentioned as another instance of rock altered under like conditions and by the same process ; while that peculiar silicification and deposition of chalce- dony, called Beekite, found in and on some of the conglomeratic material of the Permian of Livermead cliffs, near Torquay, may be attributed to the modified operation of similar agencies.
.
Martill , D.M., Taylor, M.A., Duff, K.L. with contributions by J.B. Riding and P.R. Bown. 1994. The trophic structure of the biota of the Peterborough Member, Oxford Clay Formation (Jurassic), UK. Journal of the Geological Society, February 1994; vol. 151; no. 1; pp. 173-194.

The Peterborough Member of the Oxford Clay Formation is organic-rich and contains an abundance of well-preserved vertebrate and invertebrate fossils. A high nutrient input supported a diverse biota. Phytoplankton was exceptionally abundant in the surface water, and formed the basis for an intricate food web in both surface and bottom waters. Top predators include some of the largest known Mesozoic marine reptiles. A giant teleost fish was analogous to modern filter feeding whales and sharks. Benthic faunas depended on organic matter sinking from surface waters, and two parallel food webs may have existed. Trophic partitioning allowed the higher level predators to become diverse, especially the plesiosaurs. Productivity was high in the surface waters, probably high in the mid-water column, and high on the sea floor at times, although benthic diversity may have been reduced due to substrate consistency and/or dysoxia. Bacterial activity within the sediment was also intense. The contribution of organic material to the sea floor was high, but heterotrophic reworking probably reduced the abundance considerably during early diagenesis. A decrease in organic carbon content in the Upper Callovian and Lower Oxfordian parts of the Oxford Clay Formation parallels a decreasing abundance of vertebrate fossils.
.
Ogg , J.G., Coe, A.L., Przybylski, P.A. and Wright, J.K. 2009. Oxfordian magnetostratigraphy of Britain and its correlation to Tethyan regions and Pacific marine magnetic anomalies. Earth and Planetary Science Letters.
Abstract:
A suite of 11 sections through the Oxfordian (Upper Jurassic) strata in the Dorset and Yorkshire regions of England and the Isle of Skye in Scotland yielded magnetic polarity patterns directly calibrated to the ammonite biostratigraphy of the Boreal and the Subboreal faunal provinces. The sections include the leading candidate for the global stratotype (GSSP) for the Callovian–Oxfordian stage boundary. The mean Oxfordian paleomagnetic pole derived from the Dorset and Yorkshire sections is 71.3 degrees N, 172.6 degrees E. The integrated magneto-biostratigraphic scale is consistent with results from the Sub-Mediterranean faunal province and extends the polarity pattern to the base of the Oxfordian. After adjusting for the estimated durations of ammonite subzones from cycle stratigraphy, the magnetostratigraphy confirms models for marine magnetic anomalies M30 through to M37, including some of the short-duration features recorded by deep-tow magnetic surveys in the western Pacific. The Callovian–Oxfordian boundary (base of Quenstedtoceras mariae Zone) occurs in a normal-polarity zone that is correlated to the youngest part of polarity chron M37n of this extension to the M-sequence.

.

Pugh , M.E. 1966. The Petrography of the Lower Purbeck Limestones of Dorset. Unpublished M.Sc. Thesis, Chelsea College, London.

Pugh, M.E. 1968. Algae from the Lower Purbeck limestones of Dorset. Proceedings of the Geologists' Association, London, 79, 513-523.

.
Reid , C. 1896. The Eocene deposits of Dorset. Quarterly Journal of the Geological Society of London, vol. 52, p. 90.

.

Strahan , A. 1898. The Geology of the Isle of Purbeck and Weymouth. Memoirs of the Geological Survey. Her Majesty's Stationery Office, London. 278 pages with a map.

.
Townson , W.G. 1971. Facies Analysis of the Portland Beds. Unpublished. D.Phil. thesis, Oxford University. 284 pp. By William Geoffrey Townson (Geoff. Townson). Supervised by Dr. A. Hallam.

Townson, W.G. 1974. Geology of Coombe Valley; remapped after investigation of a gas-main trench dug in October 1969. [near Chalbury and Bincombe Hill, north of Weymouth, Dorset.] Proceedings of Dorset Natural History and Archaeological Society, for 1973, published in July, 1974, vol. 95, pp. 7-8. Short paper.
[Extract: Introduction]
The Jurassic and Lower Cretaceous rocks in Dorset were laid down conformably but were folded and faulted before further deposition during the Upper Cretaceous period. In the Tertiary the strata were folded and faulted again and some of the pre-Albian high-angle faults were re-orientated to have the appearance of thrusts. Detailed information of this is given in Arkell (1947) together with an admirable map which illustrates these features on the scale of three inches to one mile (Arkell, 1947, Pl. XIX).
The geology of the area around Bincombe, Greenhill Barton and Coombe Valley is thought to differ from that depicted by Arkell. This is emphasised by Professor House (1969) in a note in these Proceedings, and implied by Miss Samuel (1969)[Samuel, E.M., 1970, Formations exposed in the trench for the natural gas main in the area southeast of Bincombe, Proceedings of Dorset Natural History and Archaeological Society, vol. 91, pp. 39-41.] in an article on the formations exposed in a trench across the area. During a study of the Portland Beds in Dorset I have examined all the outcrops and as many exposures as are available in the area shown on Arkells map. [This paper reports that the formerly termed "Black Sandstones", Parallel Bands and the West Weare "Sands" of the Isle of Purbeck and Portland are in fact dolomite, not sandstone. For more details see Townson (1975).]

Townson, W.G. 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society, London, vol. 131, pp. 619-638, 5 figures. Key paper for the Portland strata, with good diagrams. Available online in the Lyell Collection of the Geological Society of London. By William Geoffrey Townson (now a retired petroleum geologist, Dorset geologist and geological artist).
The "Portland Beds" of Dorset (Portlandian of English usage) are described in terms of a Group comprising two Formations and seven Members. Facies and thickness variations indicate the presence of a swell separating an East from a West Basin. The swell may be due to the movement of Triassic salt [a thick sequence of Triassic salt has since been proven under the Isle of Portland]. The environmental history of the Portland Group is described in terms of three cycles consisting of a major regressive and minor transgressive phases superimposed on an overall regression. The lower cycle consists of siliciclastics and dolomite deposited in a relatively deep marine environment. The dolomite formed by in situ replacement of lime mud. The middle cycle consists of cherty fine-grained limestones deposited on the outer part of a carbonate shelf. The abundance of replaced sponge spicules adequately accounts for the amount of chert. The upper cycle consists of cherty limestones passing up into shallow water grainstones. Ooid shoals developed over the swell. These marine limestones are overlain by stromatolites and evaporites which formed on the basin margin.

Townson, W.G. 1975. Information for a Geological Field Excursion to Dorset. Field Guide, A4 size. 2nd Edition (Revised). Shell U.K. Exploration and Production Limited. 83 pp.

Townson, W.G. 1976. Discussion of Portlandian faunas. Journal of the Geological Society, London, 132, 335-336.

.
Underhill , J.R. 2002. Evidence for structural controls on the deposition of the late Jurassic- early Cretaceous Purbeck Limestone Group, Dorset, southern England. Pp. 21-40 in: Milner, A.R. and Batten, D.J. (Editors) 2002. Life and environments in Purbeck times. Special Papers in Palaeontology, No. 68, Palaeontological Association, London, 268pp. Abstract: Integration of field studies with subsurface data demonstrate that deposition of the late Jurassic - early Cretaceous Purbeck Limestone Group occurred under semi-arid conditions during an important phase of syntectonic extensional activity in the Wessex Basin. Use of structural restorations and neotectonic analogue studies support the introduction of a new model to account for the regional and local along-strike variability in deposition of the Purbeck Limestone Group. It is proposed that the segmented nature of the episodically-active, basin-bounding Purbeck Fault explains observed thickness and sedimentological variations, including lateral facies changes and clast reworking. In addition to affording a tectonic framework in which to understand the rich palaeontological assemblages, the new depositional model also provides a mechanism for explaining the rapid death and unusual preservation of conifer forests through rapid submergence following co-seismic activity on the normal fault system. [By John R. Underhill, Department of Geology and Geophysics, University of Edinburgh.]

Underhill , J.R. and Stoneley, R. 1998. Introduction to the development, evolution and petroleum geology of the Wessex Basin. In: Underhill, J.R. (Ed.) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society, London, Special Publications, 133, 1-18.
.
West , I.M. 1961. Lower Purbeck Beds of Swindon facies in Dorset. Nature, London, 190, 526. Extract: ' .. a marl has been discovered with a fauna resembling that of the freshwater facies of the 'Swindon Series'. In the well-known Portisham [or Portesham] quarry with the 'fossil elephant' [stromatolitic limestone around a tree trunk], the tree-foot "impure marls with seams of chert" listed by Woodward as occurring 9 feet, 6 inches above the Portland Stone have been found to contain the Swindon ostracod Ulwellia papulata Anderson, and well-preserved charophytes in abundance. Gastropods are also abundant and perhaps have Swindon affinities... continues. [The fauna and flora discovered in the Portesham Charophyte Chert was subsequently described by Barker, D., Brown, C.E., Bugg, S.C. and Costin, J. 1975. Ostracods, land plants and charales of the basal Purbeck Beds of Portesham Quarry. Palaeontology, 18, 419-436.]

West, I.M. 1964a. Evaporite diagenesis in the Lower Purbeck Beds of Dorset. Proceedings of Yorkshire Geological Society, 34, 315-330. [Petrographic evidence of vanished evaporites in the Caps and Broken Beds - pseudomorphs, celestite, lutecite etc. Diagenetic history established.] Abstract: Five stages have been determined in the diagenetic history of the calcium sulphate beds of Lower Purbeck age in Dorset, particularly by reference to abundant pseudomorphs and other relict textures and structures preserved in late-formed gypsum and in secondary silica and calcite. The weight of the overburden is thought to have controlled the changes. Occurrences are described of minerals, including celestite, calciostrontianite, lutecite and quartz remaining after the removal in solution of gypsum. A description is also given of secondary limestones which occur particularly in the Caps and Broken Beds. They are shown to be almost entirely replacements of original sulphate deposits. [End of abstract].

West, I.M. 1975. Evaporites and associated sediments of the basal Purbeck Formation (Upper Jurassic) of Dorset. Proceedings of the Geologists' Association, London, 86, 205-225. Abstract: Four facies of limestones, each with particular contents of calcitised evaporites and of skeletal debris were recognised. They are compared with sediments of modern evaporite-depositing environments. The lowermost limestones, stromatolitic and pelletoid with foraminifera, probably originated in intertidal to shallow subtidal, moderately hypersaline, water. Overlying pelletoid limestones with algal-mats and some gypsum are products of high-intertidal flats. The main evaporite beds were originally gypsum, probably formed in supratidal to intertidal, very hypersaline, palaeoenvironments. The gypsum was converted to anhydrite and later brecciated in part, forming the Broken Beds. Extensive calcitisation produced porous unfossiliferous limestones. Ostracodal limestones above probably originated in shallow, only moderately hypersaline water. All the basal Purbeck strata were formed in and around a large shallow gulf with extensive tidal flats and with water of varying but predominantly high salinities. At times of uplift, thin soils developed on the former margins of the gulf. Forests were able to exist there because, although the area was within the semi-arid zone, it was probably very near to the boundary of the warm-temperate zone. End of Abstract.

West, I.M. 1979. Sedimentary Environments and Diagenesis of Purbeck Strata (Upper Jurassic - Lower Cretaceous) of Dorset, U.K. Unpublished Ph.D. Thesis, Southampton University, 181 p.

West, I.M., 1979. Review of evaporite diagenesis in the Purbeck Formation of southern England. In: Symposium "Sedimentation Jurassique W. Europeen." A.S.F. Publication Speciale, No.1, March, 1979. 407-416.

.

Woodward , H.B.1895. The Jurassic Rocks of Britain. Vol 5. The Middle and Upper Oolitic Rocks of England (Yorkshire excepted). Memoirs of the Geological Survey of the United Kingdom. 499pp.

Wilson, V., Welch, F. B. A., Robbie, J. A. and Green, G.W. 1958. Geology of the Country around Bridport and Yeovil. Memoir of the Geological Survey of Great Britain, Explanation of sheets 327 and 312. 118-129. With contributions on: The Purbeck Beds by F.W. Anderson, D.Sc.; Palaeontology by R.V. Melville, M.Sc.; and Ground Water by S. Buchan, B.Sc., Ph.D. [See particularly pp. 118 et seq - Purbeck Beds [by F.W. Anderson. Purbeck ostracod zones of that date are given with thicknesses of lithological subdivisions.].
.
Weston, C.H. 1849. Further Observations on the Geology of Ridgway near Weymouth. Proceedings of the Geological Society in: Quarterly Journal of the Geological Society; 1849; vol. 5; issue.1-2; p. 317-319. By Charles H. Weston, B.A., F.G.S.
In his former paper on this subject Mr. Weston endeavoured to show the existence of the Hastings sand at Ridgway. He has since visited the various sections of the Wealden between Hastings and Lulworth, and then re-examined the railway cutting at Ridgway, and the result has been to confirm his former views. He finds that the variegated clays, loams and sands exhibited in the latter locality are by no means local, but occur also in Kent, in the south of Sussex, in the Isle of Wight and in Dorset; and he has recently observed them on the Brighton and London Railway near Balcombe. In Sussex these variegated clays form a very subordinate part of the formation, but are more developed in the counties to the west.
In his concise but masterly Geological Sketch of the Vicinity of Hastings, Dr. Fitton notices the "greenish and purplish variegated clay" and sand visible at Leaness Point, between Hastings and Winchelsea. They lie beneath a stratum which Mr. Webster describes as a sandstone intersected by numerous veins of argillaceous iron ore, and rest on a dark-coloured shale also containing several layers of rich iron ore, formerly much worked in Sussex. These lowest shales are placed by Dr. Mantell in the upper part of the Ashburnham beds. Dr. Fitton also points out the anticlinal axis passing from the shore near Leaness through the highest point of Fairlight Down to Battle, the strata dipping away from it on both sides.
Mr. Weston has himself found similar ferruginous and variegated clays and sands in a hill beyond Ham Street on the Rye and Ashford Railway, and near Hastings at Bopeep, west of St. Leonard's, and at Bexhill. He next found them at Sandown Bay in the Isle of Wight, and also between Atherfield and Afton Downs. The clays contain no fossils but the Cypris valdensis and Paludina in some associated beds.
The next appearance of the Hastings sand is at Swanage Bay in Dorset, emerging from under the very steep escarpment of Ballard Downs. The entire group was in this place more varied, and consisted of a greater number of alternations of sands and clays than I found in the Isle of Wight. It appeared in consequence to combine in miniature the more extensively-developed arenaceous deposits of Hastings and the almost exclusive argillaceous strata of the southwest coast of the Isle of Wight.
The variegated clays are indentical with those of the latter and of Ridgway. I could however discover no fossils in them.
The Chalk Downs (of which Ballard Down forms the southeastern extremity) run across the Isle of Purbeck and terminate in the fine bluff cliff of Purbeck Hill on the east of Lulworth. The sections below Purbeck Hill are those of Worbarrow Bay on the east of the chalk and Mewp Bay on the west, of which I only visited the former.
The Hastings sands of Worbarrow Bay consist of a considerable admixture of clays and sandstones. The latter appear to abound here more than at Swanage Bay. The clays posses the peculiar character and colours of those at Ridgway, but some of their colours are rather more vivid. The colour of the sandstones, from the great abudance of the ferruginous base, is in many places intense. I could not discover any organized remains in these clays. I think no one who has examined this part of Dorset, and has traced the base of the chalk escarpment from Ballard Down to Purbeck Hill, can fail to be convinced of the correctness of Dr. Fitton's view, and to feel satisfied of the continuity of the Wealden formation right across the peninsula of Purbeck.
He next visited Lulworth Cove, where the general appearance of the Hastings sand is similar to the localities we have already described. Not far from this place is the last coast exhibition of the Wealden formation in Man-of-War and Durdle Coves. These are separated from each other by a short isthmus, which has been protected by a rock of greatly-inclined strata of Purbeck stone, and is composed of the Wealden very condensely and vertically developed. From this isthmus we see the eastern side of Man-of-War Cove, which appeared evidently to consist of Hastings sand. That point and the isthmus are clearly the remnants of a once continuous mass. The west side of Durdle Cove is composed of chalk which here abuts upon the sea, and runs uninterruptedly along the coast to the high point of White Nore, whence it trends inland.
In all these sections the variegated clays, loams and sands were identical in character with the Ridgway deposit. In this section, which he visited the following day, Mr. Weston "traced the Purbeck beds to their first uninterrupted termination, which consisted of calc grit with Purbeck fossils. Beyond this were clays, and then alternations of the calc grit and clays, and ultimately the Hastings bed exclusively. This section therefore exhibits the same features which Dr. Fitton has observed respecting other sections of the Wealden, showing, 1st, the continuity and sequence in the deposition; and 2ndly, the quiet process of such deposition. Hence we have the most satisfactory evidence that the beds overlying the Purbeck followed in regular succession, and were in fact rather a continuation of them.
On the whole Mr. Weston concludes not only that the variegated clays of Ridgway Hill really belong to the Hastings sand formation, but that their geognostic position is in the lowest part of the Worth and Tilgate group, separating it from the inferior Ashburnham beds.
Mr. Weston also mentions that he found the Purbeck deposits to extend as far west as the end of the Corton Range, and therefore spreading co-extensively with the Portland oolite to the vicinity of Portisham.
In regard to the attempt to explain the singular interposition of the Oxford clay between the Wealden and the cretaceous series as resulting from a drift, he remarks:—
"1st. That I could not perceive in the fossils those marks of abrasion which would indicate their having been drifted.
2ndly. That the Oxford clay has considerable depth. It has already been penetrated to the depth of about sixty feet without reaching its termination.
3rdly. That the vertical surface of the wall of chalk is hardly consistent with the natural results of previous diluvial action in that locality. And,
4thly. In many places the theory of a drift may involve no physical difficulties. But at Ridgway this idea would involve serious objections. Whence, it might be asked, could the Oxford clay have been drifted? The Oxford clay of Ridgway is about 200 feet above that of Weymouth; and the next exhibition of that bed on a higher level would be at Little Bredy near Abbotsbury Common, at a distance of between five and six miles, and at a level, I apprehend, certainly far below that of Ridgway. Whatever difficulties may therefore be supposed to attach to the theory which I have ventured to propose, will not, I think, be diminished by the suggested explication of a drift. I have also satisfied myself by repeated and careful examination that the section contains no double fault.
.
Westwood , J.O. 1854. Contributions to fossil entomology. Proceedings of the Geological Society, in: Quarterly Journal of the Geological Society, 1854, vol. 10, issue 1-2, pp. 378-396. By J. O. Westwood, F.L.S. etc. Available online from the Lyell Collection, Geological Society of London.
The microscopical examination which I have been compelled to make of so many hundreds of fossil insect-remains, for the most part in a fragmentary condition, from the Lower Purbecks of Dorset, although beyond measure tedious from the unsatisfactory results afforded by the nature of the specimens, has still enabled me to arrive at some results, and to form a general comparison of these insect-deposits with those which I similarly investigated whilst preparing the plates of Mr. P. B. Brodie's work on the fossil insects of the Wiltshire Purbecks, etc. If we take into consideration the small, and even minute size of the great majority of the insects, and indeed of the whole of the Coleoptera, which have been passed under review, the idea, that we have before us the wreck of an Insect Fauna of a temperate region, is at once raised; for although it would be rash to assert that a mass of remains of the existing tropical insects might not be accumulated in which a large quantity of minute beetles and flies would not be present, yet I cannot conceive any process, either arising from currents of water, or chemical dissolution of insect matter, which would carry off or destroy the many gigantic forms of insect life always occurring in the tropics.
.
Wilson , V., Welch, F. B. A., Robbie, J. A. and Green, G.W. 1958. Geology of the Country around Bridport and Yeovil. Memoir of the Geological Survey of Great Britain, Explanation of sheets 327 and 312. 118-129. With contributions on: The Purbeck Beds by F.W. Anderson, D.Sc.; Palaeontology by R.V. Melville, M.Sc.; and Ground Water by S. Buchan, B.Sc., Ph.D. [See particularly pp. 118 et seq - Purbeck Beds [by F.W. Anderson. Purbeck ostracod zones of that date are given with thicknesses of lithological subdivisions.].
.
Woods, M.A.. 2009 Weymouth Relief Road : Temporary Excavations in Jurassic and Cretaceous strata (May 2009). Nottingham, UK, British Geological Survey, 17pp. (OR/09/035) (Unpublished). By Mark Woods.
Available online at:
Weymouth Relief Road: Temporary Excavations in Jurassic and Cretaceous Strata. Mark Woods, BGS.
Abstract/Summary
This report provides a stratigraphical overview of Corallian, Purbeck and Chalk Group temporary exposures, created in April and May 2009, in connection with the construction of the new Weymouth Relief Road. At the time of report compilation, the key exposures seen are in the upper part of the Corallian Group (Clavellata Formation), lower to middle Purbeck Group (Worbarrow Tout Member & basal Stair Hole Member) and White Chalk Subgroup (including Seaford Chalk Formation and Newhaven Chalk Formation).

.

In addition, please see the Purbeck Bibliography webpage for more related publications on the Purbeck Formation.

Copyright © 2013 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

Disclaimer: Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea or other causes. Not all places need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancel part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.

Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.

.

at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.

.