Worbarrow Bay (also spelt "Worthbarrow" or "Warbarrow") provides one of the best coastal sections of Cretaceous strata in Europe. At the southern part of the bay the Jurassic-Cretaceous boundary is near the base of the limestones and shales of the Purbeck Formation. Wealden fluvial clastics, with lignite, represent much of the Lower Cretaceous above the Purbecks. There is a small amount of Lower Greensand, then Gault and Upper Greensand with Chalk stratigraphically above, but dipping steeply to the north. This is seen in the cliffs at Cow Corner in the northern part of Worbarrow Bay.

The Purbeck lagoonal limestones and shales are exposed in the cliffs of the promontory of Worbarrow Tout (tout = a lookout) and are particularly interesting. They contain dinosaur footprints, and have abundant brackish water bivalves, gastropods and ostracods. Sedimentary features at Worbarrow include nodular and enterolithic gypsum, satin-spar, calcitised evaporites, dolomite, celestite, cone-in-cone calcite and freshwater chert. An unusual feature in the Upper Purbecks is authigenic glauconite occurring within freshwater gastropod shells.

INTRODUCTION:

Safety

Worbarrow Bay is within the Army Ranges. It is important to follow the regulations of the ranges and particularly not to interfere with or pick up any metallic object that might be an unexploded shell or weapon of some sort. Stay within the official range walks or on the beach.

The western side of Worbarrow Tout is important in terms of geology but has higher risk than most parts of the Worbarrow Bay area. There is significant risk of falling rock debris here and the state of the cliffs near examination and consideration at the time. Do not loiter here if there are signs of material having fallen recently, and if any is seen or heard to fall, move away to safety immediately. Note particularly that there are dangerous overhangs in the Purbeck Formation which should be avoided (they are similar to the overhang on the east side of Lulworth Cove where there have been fatalities) . Safety helmets need to be worn in this area. Scrambling on the rocks of Worbarrow Tout can be hazardous and cliff-climbing is not recommended; the westward-projecting promontory of Portland Stone has some features of geological interest at the landward end, but if one proceeds further there is some risk of sliding on the rocks or falling into the sea. Although the tidal range is not great it is frequently possible to be cut off by the tide at the western side of Worbarrow Tout. The danger is greater if the high tide coincides with a rough sea.

Treacherous mud can occur in part of the Wealden outcrop. The steep Chalk cliffs at the northern part of the bay are dangerous for climbing, and there may be falls of chalk here so care should be taken. Pondfield Cove has some risk of falling rock from the Purbeck Formation at the eastern and western sides and some caves and overhangs which not entirely safe. Note that you cannot climb round from Pondfield Cove to the foot of Gad Cliff. There is possibility of falling from cliffs particularly at Worbarrow Tout, but the risk is not high if the walker is careful to stay on the footpath and does not proceed west along the arrete at the tout.

Individual geological, geographical and environmental visitors, field leaders and project students should make their own risk assessments and no liability is accepted.

INTRODUCTION:

Location and Access

Worbarrow Bay (SY 867802) and Worbarrow Tout (see maps above) are within the Army firing ranges, which are used for tanks. This is not the problem to visitors that it might appear to be. The bay is open at most weekends and bank holidays and usually for a longer period during August. The Army authorities are very cooperative to geologists and public in general and take care of conservation of the area. The place has a certain remoteness which is a pleasant result of it being in the ranges. There are inevitably certain rules and there are range wardens in the area who are concerned with safety and conservation but there is no feeling of severe supervision and little sign of military activity.

The road down to Tyneham car-park (from which there is an easy walk to the bay) is narrow and coaches cannot visit it without special arrangements and permission because they would block the road. There is straitforward access for cars and mini-buses and the car-park at Tyneham is large (map reference - SY 882802), and there are toilet facilities. Please contact the Royal Armoured Corp range office if you have enquiries. From the car-park there is a easy and level 1.4 km walk along a track to the shore. You cannot normally drive to the beach.

For the walker there are excellent coastal "Range Walks" to Gad Cliff, to Kimmeridge and also to Arish Mell and Mupe Bay (and on to Lulworth Cove). These are signposted from Tyneham and Worbarrow Bay.

INTRODUCTION:

Geological and Bathymetric Maps

A simplified geological map of the area is provided above. In addition part of an old geological map of the coast and a White Chalk zone map of Rowe and Sherborn (1901) is also here is shown. As a matter of interest, compare the position of the cliffs just northeast of Worbarrow Tout with the position shown on the current Ordnance Survey map and notice how rapid is the erosion of the Wealden strata here. Unless you are already familiar with the area to some extent, you will probably need the Purbeck 1:25,000 Ordnance Survey Map (Outdoor Leisure Series, no. 15) and the British Geological Survey Map, Sheet 342 or the Swanage Sheet, 342 (East) and part of 343.

The multibeam bathymetry image of Worbarrow Bay, shown above should be compared to the BGS map, a small modified section of which is shown further above. The multibeam bathymetry image confirms that that the outer Portand and Purbeck stone outcrop is not broken by any major fault, although the BGS map shows one at Arish Mell. The bathymetry image reveals that the Kimmeridge Clay outcrop contains a number of north-south trending faults (as elsewhere). The westward overstep of the Wealden by the Albian in Worbarrow Bay is clearly shown. The Wealden is much thinner at Mupe Bay. Note that the continuation of the Coarse Quartz Grit of the Wealden across the bay is very well-shown on the bathymetry image. This confirms the discussion of this topic by Arkell (1947), pp. 158-159. He noted that the Wealden strata are of about 229 metres thick at Mupe Bay in contrast to 425 metres at Worbarrow Bay cliff section. The reduction has been to almost half. The Coarse Quartz Grit at Mupe Bay is 30 metres above the middle of the Wealden at Mupe Bay, whereas it is almost at the middle in the cliffs at Worbarrow Bay. Arkell commented that some 30 percent of the upper division of the Wealden was removed before the deposition of the (locally thin) Lower Greensand. (note that in the illustrations, and in some cases in the text, distinction is not always made between sub-Albian and sub-Aptian unconformities. They are related and closely adjacent phases of the Late Kimmerian unconformity. The well-known, sub-Albian is the major one.)

INTRODUCTION:

Strata at Worbarrow Bay and Worbarrow Tout - Introduction

This diagram shows the simplified succession of strata in the Lulworth Cove region. It applies in general to Worbarrow Bay and Worbarrow Tout, but the thicknesses are greater than those shown, which apply to Lulworth Cove. The Wealden Group, in particular, is much thicker. The Portland Sand is not accessible, but is present in Gad Cliff. The Kimmeridge Clay is not seen in the Worbarrow cliffs and lies offshore to the south. It is well-exposed at the type-locality of Kimmeridge, a short distance away to the southeast and on the other side of Gad Cliff. FF - Fossil Forest horizon, LGS - Lower Greensand (thin here), PL - plenus marl, a grey marl marking the boundary between Lower and Middle Chalk.

INTRODUCTION

Worbarrow Bay - From the Sea

Left: Worbarrow Bay, seen from the sea to the southwest, has a reddish cliff exposure of the Wealden Group between the Chalk (with thinner Gault and Upper Greensand) to the north and the Purbeck and Portland strata to the south. The Portland and Purbeck clearly show the northerly dip and continue eastward in the spectacular and overhanging Gad Cliff. The lower part of this cliff consists of Portland Sands (in fact largely marls with some dolomite and some thin sandstone beds) and the top of the Kimmeridge Clay beneath.

Right: Worbarrow Tout with north-dipping thin-bedded Purbeck limestones and shales, and with Gad Cliff rising away to the east. Gad Cliff has precipices of Portland Stone above marly Portland Sand. Kimmeridge Clay is beneath but fallen debris obscures the lower slopes.

INTRODUCTION:

Worbarrow Bay - Cliff Section

Above left is an old diagram after Davies (1935) . It shows Worbarrow Bay in relation to adjacent parts of the coast. Above right is a cross section from Strahan (1898). It is a useful old diagram showing some more detail, including the positions of the main stratigraphical units in the cliffs of the bay.

GEOMORPHOLOGY - Worbarrow Bay Beach

The beach at Worbarrow Bay is a shingle beach of about one kilometre in length and in general trending from the southeast to the northwest, but with curvature, especially in the southern part. In general it faces the prevailing southwesterly winds and southwesterly waves which result. In this respect and in others it resembles the Chesil Beach on a smaller scale. It is a beach of dominantly flint pebbles, quite well-rounded almost like those of the Chesil Beach (except that there is much more Upper Greensand chert there) and unlike the subangular flint pebbles which are ubiquitous on the beaches of Hampshire, further east.

The beach pebbles show some gradation in size from smaller pebbles in the northwestern part of the beach to larger pebbles at the southeastern end. The grading is not as perfect or as well-developed as on the Chesil Beach but there is a similar phenomenon on a smaller scale. A major difference is that the beach material here is mostly flint which can be supplied by the local Chalk, whereas the Chesil Beach has an abundance of Upper Greensand chert in addition to some flint and also some exotic pebbles.

GEOMORPHOLOGY - Tyneham Gwyle and the Origin of Pondfield Gap

The gap at the narrow isthmus at the landward end of Worbarrow Tout poses some interesting problems. What was the origin of Pondfield Gap? Was it formed by a northward flowing stream now destroyed by the formation of the cove, or was it formed by the Tyneham Gwyle (brook) when it once flowed southward to the sea at this point? Bury in 1936 discussed this interesting problem. His view was that the Tyneham Gwyle originally turned southward and flowed through the gap into Pondfield Cove. He considered that it has since been effectively captured by the sea as a result of coastal erosion and retreat of Worbarrow Bay towards the east. The matter, however, is not clear-cut and is open to discussion and individual assessments.

The conical hill of Worbarrow Tout is separated from Gad Cliff by the little inlet of Pondfield Cove. At the head of the cove there is a narrow neck of nearly level ground, about 11 m above O.D., falling away abruptly on its northern edge, but sloping more gradually to the south. This narrow isthmus clearly marks the floor of an old river-valley, and it is interesting as the only valley which cuts at all deeply into the long range of Jurassic hills, other than the valley that once continued south through Lulworth Cove (and, of course, a stream flowing south from Arish Mell and continuing the valley there should have cut the ridge in the past). Bury noted that while the little valley at Pondfield might at first sight be expected to be the small relic of a consequent stream flowing northward down the dip, the origin may in fact be less simple. The small longitudinal stream, the Tyneham Gwyle enters Worbarrow Bay a little to the north and runs across the sandy beach . This gwyle, which is little more than one and half kilometres in length, rises near Tyneham and flows in a steep-sided ravine. On either side of this ravine there is the broad flat floor of a mature valley which is followed by the gravel track from Tyneham to the sea. The difference in level between the ravine floor and the broader valley floor is about 11 m to 12 m near the mouth and diminishes upstream. Clearly there has been some relatively recent rejuvenation of this stream which has adjusted the bottom of its valley to a new level, but has not yet had time to spread laterally. As in the case of the Bournemouth and Isle of Wight chines and other places in Dorset this is probably due to coastal retreat moving the location of base-level landwards. This type of double valley pattern is very common in the region and can be seen at Kimmeridge , Chapmans Pool, Lulworth Cove, Anvil Point and many other places.

According to Bury (1936) there is likely to be some relationship between this old valley-floor, and the Pondfield isthmus, but there are two alternative ways of explaining it:-

Northward Flowing Stream at Pondfield. - The Pondfield stream may have flowed north, and joined the Tyneham brook, which then flowed as now into Worbarrow Bay, (or perhaps at an earlier stage, before the bay was well-developed, into a southward continuation of the Arish Mell stream); or

Tyneham Stream Originally Turned Southward. - The Tyneham brook may have originally turned sharply southward and flowed into the sea through the site of Pondfield Cove. In this cases its present channel into Worbarrow Bay would have been the result of a later capture or diversion.

Henry Bury (1936) put forward three argument for this second explanation - that the Tyneham stream once flowed southward at Pondfield:

1. Pondfield lies only a few hundred metres to the north of the axis of the Purbeck Anticline and the Portland and Purbeck stone ridge is generally unbroken elsewhere, for example Gad Cliff. It is improbable that a such a short consequent stream could have cut down here through the hard Portland Stone and the Purbeck limestones and shales to such a low level.

2. The remnant of the old stream-bed at Pondfield, although too short for accurate measurement, has a definate fall towards the south.

3. If the Pondfield stream had flowed north then there should be a trace of a ravine corresponding to the Tyneham Gywle. In other words there should be a clearer sign of the join of the streams.

These, then are Bury's arguments. The present writer is least convinced by the third argument. The Tyneham stream has been rejuvenated recently as a result of rapid coast erosion and retreat of the point at which it enter the sea at sea-level. If a Pondfield stream flowed from the south it may have well have done so before the deepening of the ravine. The southward slope of the Pondfield remnant may be due to recent erosion by flood water after heavy rain flowing down into the cove. All in all the arguments are not totally convincing but the original diversion to the south of the Tyneham Gywle proposed by Henry Bury (1936) seems quite possible, but not firmly proven. What seems likely from some comparison of maps is that the rate of coast erosion of the Wealden strata here is like that of the similar, but more extensive, Wealden coast of the southwestern Isle of Wight. It may be of the order of about half a metre per annum, and was perhaps even more in the past before the pebble beach was so well-developed. When the Flower's Barrow hill-fort (now partly cut away by the cliff) was built on Rings Hill the Wealden cliffs would have been far to the southwest, and Worbarrow Bay would have had a different and more curved shape like a large Lulworth Cove. Perhaps the reader would like to consider these matters futher, making further comparisions between present and old maps. In any discussion bear in mind the prevailing southwesterly wind and erosion by waves from this direction. An important complication, not well-understood is that (as indicated by Prestwich) there was an Ipswichian shoreline and perhaps raised beach like that of Portland somewhere to the south of the present coastline. There also exists the possibility of faulting south of Arish Mell. All these factors need to be considered.

The Arish Mell gap in the Chalk Ridge seems to indicate a stream flowing southward across the Worbarrow-Mupe Bay area in the past. The Pondfield gap was considered by Bury (1936) to have been too narrow to have accomodated the Arish Mell stream. He suggested that this pursued a separate course and probably provided the gateway by which, during a phase of marine transgression, the sea entered the region of soft Wealden strata and so initiated Worbarrow Bay. Thus two stream gaps close to each other are suggested and there is a resemblance between this model and the present situation at Corfe Castle where two streams pass through the Chalk ridge a short distance apart. At Corfe, however, the streams are flowing in the direction of dip, not against it.

GEOMORPHOLOGY - Did the Corfe River once flow through the Worbarrow-Mupe Gap?

Arkell (1947) did not agree with Bury's (1936) hypothesis for the origin of Worbarrow Bay. He considered the situation at the 100 feet (about 30m) local river terrace level. This has not been accurately dated in this area but is clearly pre-Ipswichian (because of the raised beach) and probably from about the Wolstonian or Hoxnian part of the Pleistocene. For the purposes of this discussion the exact date does not matter; what is significant is that there is evidence for the late ice-age character of the Corfe River. This now flows from a short distance east of Tyneham eastward to Corfe Castle and then north to Poole Harbour (examine the Ordnance Survey map). Arkell (1947, pp. 330-331) described a considerable deposit of current-bedded gravel at Bucknowle, about 1.5 km southwest of Corfe Castle.

"In Purbeck only two small patches of Plateau Gravel [periglacial river gravel on terraces] are known. The more important, a couples of acres in extent, caps a small hill on the Wealden Beds between Bucknowle House and West Bucknowle, one mile south-west of Corfe Castle. The level top of the hill is 100 ft. [30m] above the flood plain of the adjoining Steeple Brook or Corfe River. A gravel pit shows 14 ft. (4.26m) of well-bedded, medium-grained, subangular flint gravel with seams of rusty-coloured coarse sand and loam. The finer gravel and loam are false-bedded [cross-bedded] towards the east, in the direction of flow of the present stream. There are occasional larger flints, and rare pebbles of quartz, Wealden grit, Upper Greensand, Purbeck Beds, and quartzite (one found, 1.5 inches [3.8 cm] in diameter). Coarse flint flakes believed to be of human making are not uncommon [it is interesting to note that this is about the same height as the Boxgrove-Slindon raised beach of East Sussex and Hampshire which has human remains]. It was the river of this stage that cut the platform in the Chalk north-west of Corfe Castle. The gravel suggests a river much larger than could be supplied by the catchment of Steeple Brook [or Corfe River]."

On the basis of this discovery, Arkell (1947, p. 320) made the very sensible point that "at the 100-foot stage a volume of water was passing eastwards in the direction of the Corfe gaps too great to have been supplied by the catchments of Steeple Brook and Tyneham Brook combined, and

probably derived from a much larger area of Kimmeridge terrain via the Worbarrow-Mupe gap in the Portland escarpment.

Do you, the reader, agree with this? Does it demolish the Bury argument? Note that Arkell's theory may well indicate the Hoxnian or Wolstonian situation, but bear in mind that the geography was different by the time of the Ipswichian transgression and formation of the now raised beaches. Bury was probably discussing the Flandrian Stage (Holocene) or at least the very late Pleistocene and two hypotheses may not be totally incompatible. Arkell's idea about the initiation of the old river gap is a sound one, but this fluvial break in the Portland Stone ridge need never have been much lower than about 40 m until the Devensian. What do you think has really happened here (possible clues - the breach in the Portland Stone ridge between St. Aldhelm's Head and Portland, the origin of the Weymouth rivers)?

STRATIGRAPHY - Chalk at Worbarrow Bay

It is convenient in this particular guide to commence with the younger strata and proceed down through the Upper Greensand and Gault, Lower Greensand, Wealden Group, Purbeck and finish with the Portland Stone. The geographical direction is thus north to south, from Cow Corner to Worbarrow Tout. No Tertiary strata is seen in the bay, unless there is a small amount of Tertiary debris is solution pipes under Flower's Barrow (for archaeological information on Flowers Barrow hill-fort and an aerial photograph see Francesca Radcliffe's aerial archaeology website ). We commence the geology of the chalk section with views of the impressive Chalk cliffs in the north of the bay.

At the northwestern end of the beach at Worbarrow Bay, although the Chalk is exposed in the cliffs only parts can be reached. There is also much fallen chalk debris here, and, it should be noted, risk of more falling! The Lower Chalk (Cenomanian Stage) and some of the Middle Chalk (Turonian Stage) is accessible. In general in Dorset the Lower Chalk is cyclical and rather grey and marly. It is about 27.4 m thick at Worbarrow Bay Arkell (1938a) . The Middle Chalk is usually nodular. Neither of these contain much flint (although there can be a small amount in places), unlike the very flinty Upper Chalk. Marking the boundary between the Lower Chalk and the Middle Chalk is a grey marl, softer than the surrounding chalk. This is known as the Plenus Marl (or Marls). The names Lower and Middle Chalk have been widely used in the older literature but most of the Chalk now has new member and formation names, based on subtle lithological differences (see for example: Mortimore and Pomerol, 1987) . The intention for this and associated webpages is to use both old and new names so that the units can be easily recognised.

Worbarrow Bay is a notable place for unusual facies and thickness changes in a relatively short distance in the Late Jurassic (West, 1975) and in the Cretaceous (Drummond, 1970) , (Nowell, 1996; 1997a, 1998, 2001) . This has probably been caused by penecontemporaneous faulting, of the phase known as Late Kimmerian ("intra-Cretaceous" in the old literature). Worbarrow Bay is a complicated locality through which runs some branch of the major structural boundary between the English Channel Basin to the south and the South Dorset or Wytch Farm High to the north. The main result of this is the variation in thickness of most stratal units exposed between Worbarrow Bay on the east side and Mupe Bay on the west where there are usually thinner successions. However, even at Worbarrow Bay there are differences between the sequence of Chalk seen high in the cliffs somewhat to the east and the sequence seen effectively a little further west but down at near sea level at Cow Corner (the northwestern limit of the beach).

Eastward, in the cliffs the Chalk commences as is usual in this area with a Basement Bed. This is distinctive bed, 1.2 m thick, of rather soft, brownish, sandy chalk with scattered dark green grains of glauconite. It lies on a pitted and phosphatised surface of the topmost Upper Greensand (C.W. Wright in Arkell, 1947) . In the pockets of this many white, clayey casts of small ammonites have been found. They includeCalycoceras newboldi (Kossmat), Mantelliceras mantelli (J. Sowerby), Calycoceras gentoni (Brongniart), Scaphites aequalis J. Sowerby and Scaphites obliquus J. de C. Sowerby.

Above the Basement Bed the Lower Chalk becomes harder, less sandy and grey in colour. Wright found that at 1.8 m from the base (base of the Lower Chalk?) a number of large worn ammonites, the nuclei of which were determinable as Mantelliceras cantianum Spath according to 0 C.W. Wright in Arkell (1947) . The specimen shown above may be one of these but this is not proven.

The next 6 m or so consists of hard grey chalk with scattered black flints, some with a brown cortex, some passing gradually into the matrix. Fossils are rare. There then follows 15 or 18 m of blocky chalk with a little marl and few fossils. The Plenus Marl (high Cenomanian) is a good marker bed, 1.8 m in thickness and greyer and softer than the strata above or below.

In the northwestern part of the Bay at beach level near Cow Corner, the base of the Cenomanian is different. It consists of dark glauconitic sand passing down into similar sand at the top of the Albian (C.W. Wright in Arkell, 1947) . Only a few ammonites have been found in the bottom 1.5 m of the Cenomanian and the junction has been drawn at the upper limit of Albian forms, 0.6 m above the nodule bed. Thus near the base of the Chalk at this locality the lithostratigraphic unit the Lower Chalk does not exactly correspond to the chronostratigraphic unit - the Cenomanian Stage. It is not abnormal that these do not coincide. It is abnormal that such a lithological change should take place in such a short distance.

The very top of the Cenomanian and the lower part of the Turonian Stage is represented here by the Ranscombe Chalk of Mortimore and Pomerol (1987). The Cenomanian part is the thickest exposed highest-Cenomanian in southern England and the thickening is probably again a result of local penecontemporaneous tectonics (at specific localities there were movements both up and down to a limited extent).

Nothing higher than part of the Middle Chalk, the Ranscombe Chalk, is accessible at Cow Corner. Chalk belonging to the "Inoceramus Labiatus Zone" and the "Terebratulina lata Zone" (these are part of the old "zonal" scheme of Rowe for dividing the Chalk and widely used in the past) can be examined on the wave-battered ledges at the point. They are not very fossiliferous but characteristic species have been found. Note that similar strata form the back of Lulworth Cove. On the beach there are boulders from higher parts of the Chalk, particularly the "Holaster Planus Zone". This is an old name for the uppermost part of the Turonian and is characteristed by rather softer chalk than that beneath and particularly by the presence of flints. This was classified at the lowest part of the Upper Chalk (although note that it isTuronian like most of the Middle Chalk beneath).

In terms of structure the Lower Chalk dips at 35° N. at the top of the cliff and bends down to 50° in a northward direction where it passes under the beach. Arkell (1947) mentioned that at the top of the Plenus Marl there is a mild Group 5 (of Arkell) fault, above which the basal beds of the Middle Chalk show the most remarkable development of fracture cleavage to be seen on this coast. The close-set cleavage planes, roughly vertical, have been etched out to perfection on the sea- and spray-washed cliff at the farthest accessible point of the bay (they have been figured by Arkell in Proceedings of Dorset Natural History and Archaeological Society for 1940).

STRATIGRAPHY:

Upper Greensand and Gault

The Gault Clay and the Upper Greensand are mostly Albian in age, and have a total thickness here of 57 m House (1993) . Slumped gault is shown in the left-hand photograph above, with perhaps some slumped Upper Greensand. The base of the Gault above the Lower Greensand is marked by a 0.3 m thick pebble bed of quartz and lydite (Palaeozoic black chert). The thin pebble bed may or may not be visible in the cliffs because of sliding of the Gault and associated strata. Furthermore because the area is within a firing range you may not be allowed to explore the upper cliff (where there could be tank shells). These rounded pebbles mark a plane of unconformity and represent the relics of an ancient pebble beach. On the basal pebble bed is a ferruginous, argillaceous stone band, 0.3 m thick (C.W. Wright in Arkell, 1947) , equivalent to the Benettianus Beds of Osmington. The main part of the Gault is a black and dark-green, glauconitic silty clay or loam. This dark-coloured, soft unit is responsible for many landslides at various places, and on the Isle of Wight it is known as the "Blue Slipper". It has caused landslides in Worbarrow Bay in the high cliff to the east of Cow Corner and fallen debris tends to obscure the exposure. The Gault contains Hoplites devisensis Spath and other ammonites. In the nodules ("doggers") of the landslips the echinoid Cardiaster latissimus has been found (C.W. Wright in Arkell, 1947) .

The Upper Greensand consists of more massive sandstones and the exposure of this in the cliff is dependent on the the state of landslides. In the upper part is the Exogyra Rock, which at most localities on the Dorset coast is easy to find. This is a hard, rather nodular, sandstone, 1.8 m in thickness and it is full of the small brown bivalves - Exogyra conica, E. columba and other fossils. The familiar Upper Greensand, calcareous worm-tube - Rotularia concava (J. Sowerby) [Serpula concava in the older literature] is common throughout and also occurs in the Gault (see photograph above).

There is remarkable local variation in lithologies and thicknesses of the the Albian in this area. See Drummond (1970) for more information and for graphic logs.

STRATIGRAPHY

Lower Greensand

The Lower Greensand (Aptian) is 25 m thick. It consists of grey and black sandstones with some ironstone and horizons of marginal marine bivalves (House, 1993) . The Punfield Marine Bed takes its name from Punfield Cove, near Ballard Cliff, Swanage. This is a fossiliferous ironstone with the bivalves Eomiodon and Cuneocorbicula (Casey, 1961) , and the gastropod Cassiope.

WEALDEN:

Wealden Group - General

The Wealden strata are reddish, yellow with some cream and purple coloured sediments. They are Lower Cretaceous in age, late Berriasian, Valanginian, Hauterivian and Barremian. The facies is fluvial with coarse channel sandstones, overbank silts and clays and with some lignite beds. The latter sometimes occur in abandoned river channels. In terms of fossils there are some carbonised impressions of Equisetites (a horsetail) and some other foliage, and large logs of lignite that are common near the levels of the sand channels. Dinosaur bones have been found here but are relatively rare. Penecontemporaneous oxidation in the vadose zone (i.e. above the water-table) is responsible for some of the red and yellow colours. The water table was close to the sediment surface, though, and gley-type palaeosols are present.

WEALDEN:

Sequence of Strata

A generalised succession of the Wealden strata of Worbarrow Bay is given below. This is modified and updated to some extent from a log of W.J. Arkell and C.W. Wright, originally measured in 1938, and published in Arkell (1947). The 35 units listed are useful, but there are not really sufficient subdivisions for the number and variety of beds actually present in the Wealden Group of Worbarrow Bay. However, it is a useful starting point for future, more detailed studies. The bed numbers of Arkell are preceded by "Ark". See also the vertical succession for the Wealden of Worbarrow Bay given by Strahan (1898), pp. 127-128.

WEALDEN GROUP, WORBARROW BAY

(top - beneath the Lower Greensand)

Ark. 35. Sandstone, ferruginous, bright ochre and orange coloured, with an irregular surface. 0.25 - 2 ft, 0.076 - 0.6 metres.

Ark. 34. Sand, white with some white clay. 6.5 ft, 1.98 metres.

Ark. 33. Ironstone band, thin, blood red and purple, botryoidal, 2 to 3 inches, 5 - 7.6 centimetres.

Ark. 32. Sandstone, red, purple and orange. 18 ft., 5.5 metres.

Ark. 31. Sand, white and clays. 68 ft., 20.1 metres.

Ark. 30. Sands, red with blood-red and purple ferruginous bands. 33 ft., 10 metres.

Ark. 29. Gritstone bed, coarse. 1 ft to 2 ft. 0.3 to 0.6 metres.

Ark. 28. Clays, mainly slipped (a thick sequence that should be subdivided if possible). 214 ft., 65.2 metres.

Ark. 27. Sand, yellow. Approx. 10 ft., Approx. 3 metres.

Ark. 26. Clays, partly dark. 70 ft., 21.3 metres.

Ark. 25. Sand, yellow, cross-bedded. 6 ft., 1.8 metres. Ark. 24. Gritstone bed, coarse, ferruginous. 1.5 ft., 0.46 metres.

Ark. 23. Clay, sand and sandstone. 60 ft., 18.3 metres.

Ark. 22. Gritstone bed, dark, ferruginous and impersistent. 1 foot, 0.3 metres.

Ark. 21. Clays, mottled (palaeosols?). 30 ft., 9.1 metres.

Ark. 20. Gritstone, coarse, ferruginous ("rusty"), with lignite. 1 foot, 0.3 metres.

Ark. 19. Clays, mottled (palaeosols?). 30 ft., 9.1 metres

Ark. 18. Conglomerate, ferruginous, hard, impersistent. 2 ft., 0.6 metres.

Ark. 17. Clays, sandy towards the base. 60 ft., 18.3 metres.

Ark. 16. Ironstone Band. 1 foot, 0.3 metres.

Ark. 15. Clays, sandy, with jarosite, blackish to pale, passing irregularly into cross-bedded, concretionary sandstone masses.

Ark. 14. THE COARSE QUARTZ GRIT. Cross-bedded, conglomeratic ironstone, sand and interbedded clay seams. Maximum 20 ft., 6.1 metres.
(Total from the top of the Wealden to the base of the Coarse Quartz Grit - 701 ft., 213.7 metres)

Ark. 13. Sandstones and sands, pink, white and orange, some clay. 36 ft., 11 metres.

Ark. 12. Clay, sand and thin sandstones, with a thin impersistant grit in the middle (with lignite? - IMW). 115 ft. 35 metres.

Ark. 11. Ironstone, banded, conspicuous. 3 ft. 0.9 metres.

Ark. 10. Clay, largely red. 47 ft., 14.3 metres.

Ark. 9. Sand and grit, white and yellow. 24 ft., 7.3 metres.

Ark. 8. Clay, red, purple and mottled (palaeosol). 59 ft. 18 metres.

Ark. 7. Ironstone, hard, purple. 0.5 ft., 0.15 metres. Ark. 6. Sand and sandstone, coarse yellow and brown. 14 ft., 4.3 metres.

Ark. 5. Ironstone, hard brown, in places 1 foot, generally -. 0.5 ft, 0.15 metres.

Ark. 4. Clays, jaositic (yellow) towards the base. 63 ft., 19 metres.

Ark. 3. Lignite beds. 5 ft., 1.5 metres.

Ark. 2. Clays and subordinate beds of sand to end of exposure. 190 ft., 58 metres.

Ark. 1. Gap, unexposed at the stream, then Wealden clays. 135 ft., 41 metres.

Total below the Coarse Quartz Grit - 692 ft., 211 metres.

Total Wealden strata in Worbarrow Bay - 1393 ft., 425 metres (Strahan (1898) gave 1097 ft., 7 inches, 334.5 metres).

Notes on the section log
by Arkell (1947)

"The section printed above was measured in 1938 with the help of Mr. C. W. Wright. After careful corrections for the dip and angles of measurement, the beds above the base of the Coarse Quartz Grit (701 ft.) emerge as only 30 ft. thinner than in Strahan's section (allowing for the transference of the highest 6 ft. of Strahan's section to the Lower Greensand; see below, p. 176). Below the Coarse Quartz Grit beds 9 to 13 (225 ft.) correspond to Strahan's beds 6 to 11 (253 ft.). Below this, however, Strahan has only 108 ft, of beds seen plus 140 ft. not seen (248 ft. total), whereas it is possible to measure in continuous section 332 ft. of beds, to which have to be added at least 135 ft. of beds not seen, making 465 ft. total. Hence the total thickness of the Wealden Beds emerges as 1,393 ft., some 250 ft. thicker than stated by Strahan."

WEALDEN:

Sequence from the Coarse Quartz Grit Downward

I measured up this succession from the Coarse Quartz Grit downward in about 1980. I have since redrawn it with some modifications. The cliff exposure will have retreated to some extent since the logging took place and, because of great lateral variation, there may be some minor differences now.

The succession shown is entirely fluvial with some overbank deposits, and notable lignite beds. Some of the latter contain whole trunks of coniferous trees.

WEALDEN:

Wealden Sandstones - General

Many of the Wealden sandstones show channel-like features and are lenticular and laterally discontinuous. Coarse lag deposits may occur at the base, sometimes with lignitic logs. Lignite beds sometimes occur above the sandstones where they are probably fills of abandoned channels, perhaps oxbow lakes, with transported logs and plant debris. The most important sandstone in the Wealden sequence of Worbarrow Bay is the Coarse Quartz Grit, shown in the photographs. This is thicker and coarser than the others, is multistorey and is sufficiently coarse to be a pebble bed in parts. The cross-bedding of this body of sandstones is very obvious in the cliff.

This is a very interesting pebble bed, a part of the Coarse Quartz Grit. You will notice that the pebbles are subangular to subrounded and mostly of vein quartz, with some fracture pits. Some of the quartz is stained by dispersed hematite, giving it a pink colour. Compared to later pebbles beds, like those in the Eocene or Pleistocene strata this is a strange deposit. It is notable that there is no flint. This is because flint has been formed in later strata, the Chalk. Now there is some black chert which is black. The chert is not obviously shelly and therefore does not resemble Portland Chert, although small quantities of the latter have been found in the Wealden. It is probably mostly a Palaeozoic radiolarian chert of a type which occurs in southwest England. Some black pebbles are not chert but tourmalinised quartzite or slate, also from southwest England (Arkell, 1947).

Thus much of the debris, and this may include a substantial proportion of the vein quartz, has probably come from the hard, Palaeozoic strata of southwest England. This is compatible with the fact that the Wealden deposits become coarser westward in Dorset. Not all the sediment may have come from the southwest England peninsula, though. Allen has found a sand grain in the Coarse Quartz Grit of the Lulworth area which has been dated as about 600 million years old . Thus, some of these hard, resistant debris may be as old as Precambrian. The nearest area of extensive Precambrian rocks in Brittany, northwest France, and, indeed, Wealden sediments have been found in the English Channel, offshore to the north of Brittany. It seems a likely source area, although the Grand Banks of Newfoundland has been suggested by Professor Allen.

WEALDEN:

Isolated Channel Sandstones

The Coarse Quartz Grit is a multistorey accumulation of lens or wedges of coarse river channel sandstones. In contrast, isolated channel sandstones, wedging out into finer argillaceous sand or into clay are also common in the Wealden cliff section of Worbarrow Bay. Some of these are shown below.

These are the deposits of small anastomosing channels of a river entering a large lake like Lake Eyre in Australia. The Wealden lake was of about the same size as Lake Eyre, but at a higher latitude, probably 37 degrees North. Whereas Lake Eyre is relatively arid with a annual rainfall of 125mm per annum and has evaporites, the Wealden lake was in a wetter but still very seasonal environment, and the rainfall was probably above 400mm per annum. The small, isolated channels have probably been formed in brief flood events, which were sometimes strong enough to bring large tree logs into the area. Avulsion was probably common.

This photograph shows the purple channel sandstone, one of the lesser sandstones beneath the Coarse Quartz Grit in the Wealden of Worbarrow Bay. This particular one is clearly recognisable by its prominance and colour in the general cliff photographs above. Notice the channel architecture, with the obvious feature of wedging-out.

WEALDEN:

Lower Energy, Fluvial Sandstones

Above the Coarse Quartz Grit, in the northwestern part of Worbarrow Bay there is a fairly continuous sandstone of much finer grain size than the grit. This is seen when walking towards Cow Corner and it is here, informally called the Cow Sandstone, for convenience of recognition. It was hoped that this fairly even and continuous bed might be like the Barnes High Sandstone of the Vectis Formation (upper Wealden and lagoonal) of the Isle of Wight. However, closer examination soon showed that it has a cross-bedded and channeled architecture, rather like that of the CQG, but not so obvious. Presumably this is another fluvial braided river channel deposit, but formed in conditions of lower energy.

Note the iron cementation at the top and bottom of the bed. The base, incidently has a thin coarser band, presumably a channel lag deposit.

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WEALDEN:

Lignite Beds in Wealden Strata of Worbarrow Bay
(with a hollow trees in the lake filled with multiphases of sediment - i.e. slow, multiphase, sedimentation)

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Remains of fossil logs, branches and twigs are preserved in carbonised form as lignite in the Wealden. Some of this is non-porous and breaks with a shining, black, conchoidal fracture. This type of lignite is rather similar to jet. The Wealden lignite does not normally occur as beds of brown coal, as occur in the Eocene strata, but is dispersed in silt, fine sand or clay or a mixture of these. At Worbarrow Bay the most obvious lignite beds are in the succession below the Coarse Quartz Grit. In one of these impressions of horse tails and plant foliage have been found.

The photograph above shows another cross-section through a hollow log preserved as lignite. This example is in a lignite bed a short distance below the Coarse Quartz Grit. The coniferous log seems to have rotted on the forest floor much like the beech log of the present-day New Forest, shown alongside. It is interesting that although there are numerous examples of silificified coniferous trees from the Purbeck Formation, mostly from the Isle of Portland, I know of no record of a hollow Purbeck silicified tree. There are indications of decomposition and erosion of tree stumps but no true hollow log. This may be some result of the semi-arid conditions of the early Purbeck compared to the sub-humid conditions of the Wealden (there is much evidence for this, including differences in clay-mineralogy and the absence of evaporites in the Wealden). Logs rotted in the damp Wealden forest in a manner like those in England today, but the behaviour of fallen trees in the dry Purbeck forests with associated salt-lakes was probably different.

STRATIGRAPHY:

Wealden Group - Dinosaur Print?

See also:
Brighstone Bay, Isle of Wight - Wealden with dinosaur footprints.

Near the base of the Coarse Quartz Grit of the Wealden Group in Worbarrow Bay is a conspicuous depression into underlying muddy sand and clay with lignite. The depression is filled with well-sorted, medium sand, and there has been post-depositional goethite (iron) cementation within it. The structure might be a load cast but there is not a thick, unbroken sequence of sand above it. Instead there is a reactivation surface just a few centimetres above the top of the structure. However, this does not eliminate a load cast because a thick unit of sand above could have been eroded away, shortly after deposition. Other load casts were not seen nearby.

Probably a more likely explanation is that this is a cross-section through a large and deeply indented dinosaur footprint like those in the Wealden Group at Hanover Point, Brightstone Bay, Isle of Wight, although I cannot be certain. Such a depression into the muddy sand and clay with lignite is quite likely to have been formed by a passing dinosaur, and would not be unusual for the Wealden strata. (After the above notes were written, a short paper was published by Paul Ensom (2009) recording an actual tridactly dinosaur track on the a sandstone bed. He considered that it had probably come from Stuart's (1978) bed no. 7.)

Incidently notice how the lignite occurs as distribued lenticles within the sandy clay. It is very compacted and this is normal within the Wealden sequence. Most of the Wealden sediments have suffered major compaction, perhaps even up to 10 times in some cases. This contrasts with much of the underlying Purbeck Formation, where, although there is compaction in some beds, there is also much early cementation. Compaction of the Wealden will have taken place over a significant period of time, with some, of course, having been very early. The Wealden deposits may have been at least twice the total thickness until deeply buried under the Chalk. The Worbarrow Bay, basinal depression in the mid-Cretaceous probably contained at least 800 metres of Wealden, many times more than the small thicknesses at Lulworth Cove and Durdle Door. (It should be appreciated that Worbarrow Bay is in a more "basinal" facies than Lulworth, in the same way that nearby Kimmeridge idge has "basinal" facies of Kimmeridge Clay, much thicker than at Ringstead; this topic will be discussed more fully elsewhere - in simple terms Mupe Bay is on the intra-Cretaceous high and Worbarrow is in the intra-Cretaceous basin).

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The Purbeck Strata (Jurassic-Cretaceous), Lulworth and Durlston Formations - Succession -

The cliffs of Worbarrow Tout provide the best exposures here of the Purbeck limestones and shales. Much, but not all of the sequence is also visible in Pondfield Cove, nearby. The Purbeck sequence at the Tout has been carefully logged by Ensom (1985). His vertical log (see image above) is essential for any detailed study of the strata here. It is comparable and in the same style as the well-known Clements Log of the Purbecks of Durlston Bay. Ensom's diagrams provide not only provide considerable detail, but also are drawn in a manner that makes recognition in the field of individual beds quite easy. The casual geological visitor may obtain enough introductory information from this website, but the specialist will certainly need the log, and then should use these Ensom bed numbers (with subdivisions if required).

In addition, basal Purbeck details, particularly of the evaporitic Broken Beds etc., will be found in West (1975) and earlier West papers. These evaporite papers also deal with the Soft Cockle gypsum. See West (1965) on the enterolithic veins and nodules of the calcium sulphate. The older Portland Stone projects seaward at the southwestern end of Worbarrow Tout. Northeastwards from here there is an excellent exposure of the Purbecks seen as a sequence of steeply dipping beds. At low tide most of this is accessible, with a little scrambling over rocks. At high tide the lowest part of the succession may be difficult or impossible to reach and, in addition, the sea comes up to the foot of the northern slope, seen in the photograph here and access requires wading (this is not recommended especially when there are significant waves - this is a dangerous coast).

Upper Purbeck

Upper Ostracod Shales Member

(formerly " Upper Cypris Clays and Shales ")

Unio Member (Unio Beds)

Soft green, glauconitic shale full of Unio porrectus and Viviparus . Bands of hard limestone. " Beef " , shelly limestone and dark grey shales. These strata originated in low salinity, lacustrine conditions. The bones of turtles and crocodiles from this lake are preserved at Swanage.

Middle Purbeck

Broken Shell Limestone Member

Limestone, a biosparrudite probably mainly consisting of Neomiodon debris, with fish teeth, palates and scales (Pyncnodus). Gastropods include Viviparus . This is probably mostly of brackish-water origin but the gastropods indicate some very low salinities at times. Limestone (the Burr) from this horizon has been used for the construction of Corfe Castle.

Chief Beef Member

Dark soft shale with bands of hard shell-limestone (biosparrudite) with Neomiodon. Beef is diagenetic fibrous calcite formed under burial. As usual it is associated with lines of white, disintegrating, aragonitic Neomiodon shells. The organic rich shale here has preserved the aragonite to some extent, although elsewhere in the Purbeck Formation it has mostly been lost quite early. This is a brackish-water lagoon sediment.

Corbula Member

Shells include Neomiodon , a common bivalve of the Middle Purbeck Formation. The oyster of the Cinder Bed, Praeexogyra distorta also occurs in this unit as does a " Pecten ". This originated in nearly marine salinities.

Scallop Member Hard shell limestone (biosparrudite?). Usually sandy with Chlamys , a bivalve which probably suggests near-marine salinities. This unit indicates a significant marine incursion. Seawater entered the lagoon on a scale almost like that at the time of deposition of the Cinder Bed.

Intermarine Member

Beds of hard shell-debris (biosparrudite) limestones with about 1 metre of pyritic shales with some beef and jarosite in the central part (" mid-intermarine shales "). The clays are smectite-rich. This is one of the main parts of the Purbeck Formation where dinosaur footprints are preserved in extensive damp lagoon-margin flats of shell sediments. In the Swanage area the biosparrudite limestones (El-Shahat and West, 1983) of this form most of the Upper Building Stones, although they do not seem to have been much quarried at Lulworth where they are thinner. The shell beds are composed of debris from the brackish water bivalve Neomiodon . The shells were originally aragonite but have been replaced by calcite now.

This unit originated in a lagoon that had some influx of freshwater but generally remained brackish. The climate was less arid than that in which beds lower in the sequence were formed. This accounts for generally more sand, kaolinite clay, pyrite, plant debris and lower salinities than those indicated for the lower members. The subhumid conditions seem to have been favourable to dinosaurs, crocodiles and turtles.

INTERMARINE MEMBER:

Dinosaur Footprints

See also the:
Portland Dinosaur Footprint webpage.

[Note: As shown in an image above, the dinosaur footprints of Worbarrow Tout, Dorset might in some respects be similar to the footprints of a rhinoceras, which also seem to be three-toed. Unfortunately, although I have seen numerous dinosaur footprints in the Purbeck strata of Dorset, I have never studied the footprints of a rhinoceras or an elephant and am only speculating. However, there may be potential for comparative research here.]

Tridactyl dinosaur footprints in a brackish-water shelly limestone, or biosparrudite, a short distance above the Cinder Bed have been demonstrated on field trips to Worbarrow Tout for about the last 35 years, by myself and, no doubt, by others. They are in the top of bed WB 121 of Ensom (1985), a relatively thick limestone at the base of the Intermarine Member. I recorded them briefly in a Geologists' Association field trip report West, Shearman and Pugh, (1969), which gave an account of a field trip in 1966 with Professor Douglas Shearman and Mary Pugh. They were shown to Professor Kirkaldy, who used a photograph of them in a popular geology book.

They were recorded again, this time with a little more detail, by Delair and Brown (1975). Unfortunately, the authors, who were unaware that they were already quite well-known, listed them as from the basal Purbeck strata. Even though I drew attention to this, the error lasted through to the paper of Delair (1982), until finally being corrected by a precise record in the graphic log of Ensom (1985). The footprints are now quite worn and were in better condition originally. I have included some photographs below, some of these quite old, and, in addition, there is a sketch in the paper by Delair and Brown (1975), although I think that one print might be missing in that diagram. Delair and Brown considered the prints to be of Iguanodont type. They are interesting in representing multiple trackways.

Left: Footprints in shelly limestone above the Cinder Bed at Worbarrow Tout. A recent photograph (1995), slightly retouched to reduce the visual effect of a joint crossing the footprint.

Centre: A sketch from a photograph of the footprints in less eroded condition in January, 1978.

Right: A monochrome photograph taken in January, 1978, with Dr Yehia Ali, from Cairo. The footprints have been emphasised artificially by placing wet mud in them, but following as far as possible the natural outline.

Left: An oblique view of the footprints in January, 1978.

Right: Details of one of the footprints.

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Intermarine Member - Buildings Stones - Origin of the Shell Deposits

The Purbeck shell limestones (left) probably originated in shell beaches like those at the margin of the Coorong Lagoon in South Australia (right). Species of Neomiodon are probably the main components of the Middle Purbeck building stone, but in the Lower Purbeck Formation there are shell-beds that consist of the high-salinity tolerant bivalve - Protocardia purbeckensis. The Middle Purbeck Building Stones usually have the mollusc shells in crushed condition so the left part of the illustration is based on a Corbula Member shell bed from Durlston Bay, which also has some euryhaline gastropods.

Visiting bipedal herbivores such as kangeroos or Iguanodons may leave footprints in favourable circumstances.