by Dr. Ian West,

Romsey, Hampshire
and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,

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 |Field Guide Maps and Introduction| |Durlston Bay - Peveril Point, Upper Purbeck Group | Durlston Bay -Lower Purbeck |Durlston Bay - Central Zigzag Part and Coast Erosion |Durlston Bay Bibliography | Purbeck Group Bibliography | |Purbeck Palaeoenvironments | |Purbeck Dinosaur Footprints |
Click here for the full LIST OF WEBPAGES

Selected external link: |Jurassic Coast

Durlston Bay, - Upper Purbeck Group
Durlston Bay - Middle Purbeck (this webpage)
Durlston Bay - Lower Purbeck (Jurassic-Cretaceous)
Durlston Bay - Central Zigzag Part & Coast Erosion
Durlston Head - Lower Purbeck Group & Portland Stone
Durlston Bay - Bibliography

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Related Field Guides --- |..Mupe Bay.. |..Worbarrow Bay.. |..Lulworth Cove - Introduction | | Lulworth Cove continued; Purbeck Group, west side of cove
Lulworth Cove continued: Purbeck Group, east side of cove

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INTRODUCTION:

Safety

Durlston Bay is a beautiful, extremely interesting and educationally and scientifically valuable place. Like other Dorset coast sections it can present some natural hazards. At times, it is rather more hazardous than most places on the Dorset coast. In 1975 a schoolboy was seriously injured by a rockfall at Swanage and the following year another was killed by a falling rock (Lee, 1992 ). It is particularly important not to go under the cliffs (except for the low parts) in wet weather or after heavy rain or when there is melting ice. There is a major risk of encountering falling rocks here in those conditions. Never loiter close to the cliffs or hammer or search under overhangs. Safety helmets are recommended for field parties here, although of course they will not save you from the fall of a large rock. There must constant observation of the state of the cliffs to be sure that debris is not falling. Care must be taken before proceeding south of Peveril Point. At times it may be safe as far as the Cinder Bed but hazardous beyond. Conditions vary. Often Durlston Bay can be studied without much problem, at other times it can be an unsafe place. Choose whether you go there or whether you go elsewhere according to the conditions. If you go there, choose just which parts you consider safe for study.

The Broken Shell Limestone to the southwest of Peveril Point has been in unstable condition and rock falls have occurs. Conditions can be unsafe south of the Cinder Bed Ledge in the northern part of the bay. Here the lower part of the cliff is vertical and there is loose debris above. Falling fragments achieve dangerously high velocities here. This is definately a place to avoid in late winter and early spring, particularly after or during heavy rainfall.

The area to the south of the northern Cinder Bed Ledge is dangerous at times from risk of rock fall. In 2007 it was noted that the cliff has been undercut in the area of the Mammal Bed and has an open fissure has developed above. By 12th June 2010 this part had fallen as shown in a lower photograph. In 2010 there is still much sign of instability and there are fragments of split fallen rock on the shore. It is hazardous to approach to cliff in this area, and plainly foolish to linger there or sit on the beach near the cliffs. Anyone walking into this part of the bay should be watching the cliffs and the shore for indications of danger such as unstable and recently fallen rock. Further rock falls may occur. If you are there, keep down near the sea and keep moving on fast.

With regard to other points, there is some small problem regarding the tide. The section is best studied at low tide, but the tidal range is not great and unlikely to cause anyone to be trapped by rising tide. You may, however, be obstructed at high tide at the north Cinder Bed Ledge. If you are obstructed south of it than return by the Zigzag Path (middle of the bay). Ascent of the cliffs is dangerous except where there is a well-defined path and do not try cliff climbing. At low tide beware of slipping on slimy rocks and injuring yourself (be careful on the Upper Building Stones north of the Cinder Bed Ledge). If the conditions are bad or the bay seems unsafe try an alternative and safer location.

INTRODUCTION:

Geological Maps

The British Geological Survey map, 1:50,000, Solid and Drift, 2000 Edition, Swanage Sheet, 343 and part of 342, is well worth purchasing. It can be obtained from the British Geological Survey website and is very inexpensive, costing only 12 pounds sterling. The map shown above is the new edition of the year 2000. It is different in some respects from older editions.

For study of the northern part of Durlston Bay the detailed work of Nunn should be consulted. Dr. John F. Nunn MD, DSc. PhD, FRCS, FRCA, a well-known research aneasthetist and Egyptologist (author of Ancient Egyptian Medicine) retired to Swanage. He made a very detailed geological survey of the Peveril Point area and continued his studies southward to the Zigzag Path area. He also excavated and published on the poorly-known Laning Vein (of the Middle Purbecks). See: Nunn, J.R. (1991). A geological map of Purbeck Beds in the northern part of Durlston Bay. Proceedings of the Dorset Natural History and Archaeological Society, vol. 113 for 1991, pp. 145-148. This work is best used in conjunction with the well-known log of Dr. Roy Clements, essential for any study of Durlston Bay.

INTRODUCTION:

Durlston Bay Cliff Section

Above is a simplified cliff section of Durlston Bay with vertical exaggeration twice the horizontal scale. A photograph of the part of the cliff from the sea is provided for comparison. The gently dipping northward succession is repeated by normal faults in the centre of the Bay. Most field parties study the northern end from Peveril Point southward. The Cinder Bed forms a ledge which can usually be passed but on occasions may be difficult or impossible at high tide. Safety helmets are advised except at Peveril Point. Some of the cliffs, especially south of the Cinder Bed ledge can discharge falling rocks in certain conditions. Wet wintry weather can make them hazardous and in these conditions it is wiser for parties not to proceed far south of Peveril Point. If the conditions are good, and it is safe to do so, then the gypsum and other interesting features can be seen on the shore further to the south. The cliff can then be ascended at the Zigzag Path. (Specialists may wish to walk over the beach boulders southward to the evaporite features at Durlston Head)

PURBECK FORMATION -INTRODUCTION

General

The Purbeck Group (or Group) is a well-known lagoonal unit of thin-bedded limestones and shales that crosses the Jurassic-Cretaceous boundary (Upper Tithonian to Berriasian). It has a remarkabe fossil content of vertebrates which includes mammal remains, crocodiles, turtles, fish and dinosaur remains and tracks. There are abundant fossil insects at certain horizons and impressive tree and other plant remains. The Middle Purbeck part, dealt with here, is Cretaceous in age and belongs to the Berriasian Stage. See associated webpages for Upper Purbeck and Lower Purbeck strata.

PURBECK FORMATION - INTRODUCTION

Isopach Map

The known distribution and thickness of the formation in the south of England and English Channel is shown here. The formation is also extensively developed in Germany, Poland, France, Switzerland Tunisia, Denmarks and elsewhere. The Purbeck Limestone Formation (or Purbeck Group, according some people) is often known abroad as Purbeckian or Purbeckien.

INTRODUCTION:

Sequence of Lithologies

INTRODUCTION-PETROGRAPHY:

Introduction - Limestone Classification

Middle Purbeck limestones can be identified in general descriptive terms in the field with the aid of a hand lens and dilute hydrochloric acid. Bivalve, shelly limestones are common and obvious in the Middle Purbeck Group, particularly in the Intermarine Member. The Cinder Bed is clearly an oyster shelly limestone. With a hand lens it is possible to recognise some ostracodal limestones. Some limestones are more difficult to identify and all the limestones provide much more information in thin-section. Because of the possible presence of dolomite, siderite, aragonite, quartz, gypsum, celestite, pyrite etc in various Purbeck strata it is recommended for the researcher to have X-ray diffraction data available if possible, and trace element data such as the content of Sr etc is very useful. Some tabulated XRD and geochemical data from an old but good thesis by El-Shahat (1977) is given in the sections on Members. Some introductory explanation of the petrography of the Middle Purbeck is now given.

Thin-section petrography of the Purbeck carbonates is not dealt with in detail here. It is convenient, though, to refer to a few limestone types. Most geologists probably use the well-known Folk's Classification, although some, particularly in the oil industry, prefer Dunham's Classification.

For reference, a simplified version of Robert Folk's (1962) scheme is given here. It is only satisfactorily used with good thin-sections, but with experience you can often make an intelligent guess at the Folk's term using a hand-lens. It is important to note that in the full scheme Folk specified that coarser carbonates, >2mm., should have the ending "rudite" (being rudaceous). Thus the coarser equivalents of biosparites are biosparrudites and the coarser equivalents of biomicrites are biomicrudites.

INTRODUCTION-PETROGRAPHY:

Carbonate Shell Sands (Limestones)

(text to be added)

INTRODUCTION-PETROGRAPHY:

Carbonate Diagenesis - Biosparrudite Lithification

(text to be added)

MIDDLE PURBECK MEMBERS (PART - LULWORTH FORMATION TO DURSTON FORMATION) - INTRODUCTION

These are discussed from the top down. This is because visits to Durlston Bay usually commence at Peveril Point, near to the public car park, and the geologists usually then proceed southward passing from younger strata to older.

The Upper Purbeck strata at Peveril Point are described in a separate webpage:

|Durlston Bay - Peveril Point, Upper Purbeck Group.

Before starting on the Middle Purbecks the geologist will probably have already descended through the Upper Purbeck:

Viviparus Clays, 3.3 metres plus,
Purbeck Marble Beds and Ostracod Shales, 14 m.,
Unio Beds, 1.5 m.
Broken Shell Limestone, 3 m. (very obvious and thick limestone just south of Peveril Point).

Under the Broken Shell Limestone look for the obvious, soft grey shales with beef - fibrous calcite in layers. Once you see this you have reached the Chief Beef Member, and the section seen is then described below in descending order.

CHIEF BEEF MEMBER:

Introduction

Beneath the Broken Shell Limestone is the Chief Beef Member. The accompanying vertical section, based on part of Clement's (1969; 1993) classic log, shows the Chief Beef Member succession at Durlston Bay. It is easily found in the cliffs. If you are unsure where it is, look for the thick (about 3m) Broken Shell Limestone which forms the southernmost of the large Peveril Ledges and which ascends the cliff just south of Peveril Point. The Chief Beef Member is the dark shaly unit with beef that lies underneath this, and well above the Upper Building Stones (or Intermarine Member). The sequences consists mainly of organic-rich dark soft shale with bands of hard shell-limestone (biosparrudite) with Neomiodon. Beef is the form of diagenetic fibrous calcite that develops by diagenesis in organic-rich shales under burial. It may show cone-in-cone structures. The name is an old quarryman's term for a material which can occur as fairly thin layers with a fibrous fabric resembling slices of the herbivore muscle. As usual it is associated with aragonite which is usually the source of the carbonate for the diagenesis. Here, there are lines of white, disintegrating, aragonitic Neomiodon shells. The organic rich shale here has preserved the aragonite to some extent, although elsewhere in the Purbeck Group it has mostly been lost quite early. This is a brackish-water lagoon sediment.

For other beef occurrences in Dorset see the Lyme Regis Field Guide . There beef occurs mainly in the Shales-with-Beef of the Lower Lias which are oil source rocks for the Wessex Basin.

CHIEF BEEF MEMBER continued:

Origin of Beef

Diagenetic fibrous calcite (" cone-in-cone " and "beef ") veins are common features in shales and marls throughout the stratigraphic column; their morphology has been studied for more 130 years (Sorby, 1860). Controversy remains over the time and mechanism of emplacement and the chemical environment in which precipitation took place (Marshall, 1982). Vein growth is commonly shown to be displacive; impressions of a fossil can be found on the upper and lower surfaces of the same vein and shale inclusions may be ruptured into crude conical form (Marshall, 1982 and references therein). Fibrous calcite of this type is normally associated with organic matter and aragonite. Although some suggestions have been made about an early origin, many veins in other formations postdate concretions whose curged internal laminae reveal syncompactional growth and some veins have been shown to postdate hydrocarbon emplacement (it is not possible to add full references here - so for these see Marshall, 1982)

Marshall (1982) commented that the symmetrical fibrous calcite veins (" beef ") occur throughout the Chief Beef Member at Durlston Bay. Beef of maximum thickness (about 2 cm.) occurs here where asymmetric downward growth has taken place in bed DB 219 beneath an overlying molluscan biosparrudite (the Broken Shell Limestone, Bed 220). The veins are predominantly ferroan like the late (burial) cements of the Middle Purbeck (El-Shahat and West, 1983). In terms of isotopic composition del ¹³C is near 0. del ¹⁸0 is round about -6.5. The adjacent limestone has a whole rock isotopic composition for del ¹⁸0 of -5.7. Similar results have been reported by Allen and Keith (1965). They reported 8 Purbeck samples having oxygen isotope values near -5 and carbon values near -2. Marshall (1982) regarded these results as not reflecting purely primary conditions but a mixture of primary components and later cements. Shell aragonite (El-Shahat and West, 1983) has clearly played a role in supplying carbonate. Its relative instability means that it could have passed into solution (very locally) and the carbonate has been largely reprecipitated as fibrous calcite with a special fabric with maximum resistance to overburden pressure. Clearly, the matter is more complex than this, though, and some lateral migration of carbonate in solution may also be involved. More detail is provided in an investigation of Purbeck geochemistry and stable isotopes by Quest (1985).

Even if the isotope problem is not fully resolved most authors seem agreed that the beef formed diagenetically under deep burial, involving material at least partly from pre-existing carbonate sediment. Exactly how the organic matter is involved is a more complex problem (although it should be noted that it is partly responsible for preservation of aragonite to the burial stage). Future research on Purbeck beef could usefully involve more study of the very thick beef seams at Lulworth Cove where both Purbeck sediments and the structures have special features.

CORBULA MEMBER:

Introduction

This overview photograph is provided again to show the location of the Corbula Member. It is very easy to reach by descending the slumped part of the cliff shown or by walking round over the boulders from Peveril Point. You cannot miss the Corbula Beach Bed and by looking closely at associated slabs you will soon see numerous examples of the small bivalve Corbula.

These limestones and shales, for which a log based on part of Clements' Log is provided here, are notable for a varied content of gastropods and bivalves. Apart from the small Corbula alata bivalves include Neomiodon , a brackish water genus which is a major constituent of limestones in the Middle Purbeck Group. The oyster of the Cinder Bed, Praeexogyra distorta also occurs in this unit as does a " Pecten ". This originated in nearly marine salinities.

The symbol " I " on the right indicates that fossil insects have been found at this level. Bed DB 175 contains fossil wasps (Rasnitsyn, Jarzembowski and Ross, 1998).

Corbula, Neomiodon and other fossils from the Corbula Member, Durlston Bay. Not in situ.

SCALLOP MEMBER:

Introduction:

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. A log of these strata is shown with the Corbula Member log above.

INTERMARINE MEMBER:

Introduction

The Intermarine Member consists of thin-bedded, lagoonal and lacustrine limestones and shales above the conspicuous oyster bed - the Cinder Bed. Many of the limestones have been quarried for Purbeck Stone. These are the main limestones of the Upper Building Stones. The old traditional name, the "Intermarine Beds" presumably applies well to the upper part which is dominantly of brackish water origin. The unit lies beneath the near-marine Scallop Member with "Chlamys" and other more typically marine molluscs.

The general features of the succession can be seen from the diagrams above, which include a modified extract of part the classic log of Clements ,1969; 1993, and a modified correlation diagrams of El-Shahat, 1977. It consists of beds of hard shell-debris (biosparrudite) limestones with some pyritic shales in the central part (" mid-intermarine shales ").

The lower part (Downs Vein etc, just above the Cinder Bed), though, is of almost freshwater origin and has similarities to the Cherty Freshwater Member in containing pond-snails like Physa bristovii, which is in beds DB 115 and 116, and Ptychostylus , abundant in DB 112, and charophyte algae. It, thus, starts as freshwater lake deposits but marine water gradually gained access to what then became a large lagoon. Austen (1852) and Fisher (1856) referred to the unit as the "Turtle Beds" which may, perhaps, be more appropriate than the " Intermarine term "used in Bristow and Fishers' (1857) vertical section and reproduced in Damon (1884). Nevertheless, the name " Intermarine " is so well-established and well-known that it is preferable to retain it.

The shelly limestones are mostly coarse enough to be termed "biosparrudites "(coarse shell debris with a sparry cement) in accordance with Folk's (1962) classification, but sometimes referred to for simplicity as " biosparites " (shell debris sands) as in Clements' (1969; 1973; 1992) log. They represent debris mostly from the brackish water bivalve Neomiodon . The shells were originally aragonite but have been replaced by calcite now. They have been accumulated by storms with easterly winds at the western margin of the Purbeck basinal area. This basinal region which extended eastward to the Isle of Wight and part of the Weald of Sussex was in basinal in relative terms with thicker sequences and less subaerial exposure - but actually still lagoonal and lacustrine and very shallow). The shell beds originated as extensive white shell sediments. Some of which were usually at or just below water-level and others formed shell beaches, above water-level for much of the time (El-Shahat and West, 1983).

Water-levels fluctuated, probably seasonally and over longer periods in the seasonal subhumid climate, and dinosaurs left their footprints in the damp shell deposits which were sometimes cemented by carbonate early, thus favouring preservation. Some footprints may have been preserved by shell debris being washed in by storms over dried muds with footprints and desiccation cracks, as happened in the case of the Cherty Freshwater Member. The apparent abundance of dinosaur footprints in the Intermarine Member in Isle of Purbeck is the result of several factors. The extensive shell beaches and marginal mud-flats were adjacent to a vegetated peninsula (the Wytch Farm or South Dorset High) and thus likely to be traversed by dinosaurs, especially during drought. The quarrying in the area has revealed footprints frequently, although they are also found commonly at Durlston and Worbarrow Bays. (For dinosaur footprints in the Lower Purbeck of Portland see the Portland Dinosaur Footprint webpage).The climatic and environmental factors are more important than an apparent abundance because of quarrying. The Intermarine Member originated in a climate 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, and almost no evaporites. The subhumid conditions seem to have been particularly favourable to turtles, crocodiles and dinosaurs.

Some of the limestones continue onto the shelf area of Lulworth but the main deposits are east of Worbarrow Bay. There were many quarries in the Swanage area, but later quarrying has moved westward to near Langton Matravers and Worth Matravers. The quarrying in the past has resulted in the naming of many of the individual beds.

INTERMARINE MEMBER continued:

Inorganic Geochemistry

INTERMARINE MEMBER:

The Red Rag (DB133)

(a very obvious ferruginous bed resulting from a palaeoclimate change)

Certain notable beds of the Intermarine Member in Durlston Bay are shown in the above images. Some of these beds will be discussed in more detail.

The Red Rag - DB 133 has been described by Clements (1993). This is a medium grey, mottled with creamy brown, coarse, bivalve biosparrudite. It is rather rough and massive. There are varying contents of sand grains throughout, with more more clayey and sandy laminae, and discontinuous shale beds. It is cross-laminated in part. " Neomiodon " is particularly well preserved in the shaly beds. Other bivalves include " Unio " , Praeexogyra (?), Isognomon, " Chlamys ", etc. The top surface and the top 20-30 mm. yields bones, coprolites and plant debris including leaves. The basal surface is covered with hemispherical pockets into the bed below. Gastropods include Viviparus and Ptychostylus harpaeformis . Ostracods include Cypridea ? menevensis gr. ?

El-Shahat and West (1983) reported the Red Rag to be a late-cemented limestone, compacted prior to diagenesis and with total loss of aragonite under burial. In mineral composition it is mostly calcite with 15% clay and a little quartz. A notable feature of this bed (El-Shahat, 1977) is the high manganese content of 2283 ppm. Although parts of the bed are brownish and ironstained (hence the name) the geochemistry does not show an abnormal Fe content. In fact, the rest of the geochemistry is unremarkable.

Ensom (1994) noted the presence of pebbly detritus on the upper surface. This is of Garden's (1991) " Assemblage 1 " type. The implications of the coarse material here are interesting. Perhaps this bed indicates a brief phase of local uplift in the Late Kimmerian movements which affected this area during basin extension in the Cretaceous (and earlier). However, it was deposited at a time of increasing kaolinite influx amongst the clay minerals which probably relates to a rather more humid (sub-humid?) climate than previously.

A interesting feature of the Red Rag was found by Ensom (1994). A substantial groove across the upper surface was observed. He described this feature, part of which is shown in the image here, as being gently arcuate, with a width of 0.1 m. and visible for a distance of more than 3 m. Along the length of the groove were a series of crude chevron-like markings, suggesting the disruption of a sediment with a plastic constituency. The chevrons indicate movement from WSW to ENE. He found the direction to be the same as the current direction within the Red Rag as shown by the cross-lamination. Ensom considered it to be a tool mark, produced by an object swept along by currents. He suggested that it was formed by a tree-stump with roots, or a branch. He mentioned that it could have been formed, though, by a drifting carcass coming into contact with the sediment surface, but that other explanations are possible (note that these are part of the Turtle Beds of Fisher, 1856).

The drifting carcass theory is supported by the discovery in 2007 of an excellent crocodile skull (Goniopholis simus) not far below this horizon at the junction of the Shingle and the Shed Bed. This was discovered by the Earth Science Manager of the Dorset and East Devon World Heritage Coast - Richard Edmonds. A photograph of it is shown above. Dead crocodiles, certainly drifted through the lagoon in which these strata were deposited, and the Red Rag marks may well be of carcass origin.

Notice also in the image the brownish rusty staining from oxidised pyrite, a feature which is common in this bed.

INTERMARINE MEMBER continued:

Under Rag (DB131)

The Under Rag - DB 131 - has also been described by Clements (1993). He referred to it as rough, medium grey, very sandy, muddy limestone, which grade into very sandy, calcareous mudstones full of grit-sized shell fragments, and into sandy biosparites with brown sparite cement. Abundant bivalves and gastropods (notably abundant Viviparus ). It has a pocketed base, probably load-casted, which is visible in the image, similar to DB 133 - the Red Rag. The top surface seems to be a leaf bed with numerous moulds of elongate leaves, perhaps of cycadophytes.

It contains the gastropods - Viviparus and Theodoxus fisheri . Ostracods include Cypridea sp. , Darwinula leguminella and other species (see Clements, 1993 for further details).

Petrographically the Under Rag shell limestone is a biosparrudite of the late-cemented type of El-Shahat and West (1983). The aragonitic shell sediment has been compacted under burial. It is still characterised by a moderate strontium content (486 ppm) which relates to its original aragonite content (some still remains - 2 percent). It has a high phosphate content (829 ppm.) which is explained by its content of vertebrate debris (geochemical data selected from El-Shahat, 1977). X-Ray Diffraction of a sample seems to suggest that the sand content is not high (in spite of the description above) unlike the underlying bed.

The clay mineralogy of this part of the sequence is generally that of illite dominant over smectite and mixed layer clay minerals. The Intermarine Member is the first unit to have a small but significant proportion of kaolinite. This latter mineral is the result of subhumid conditions which contrast with the semi-arid environment of the Lower Purbecks. It may relate too to the relative increase in sand influx which is seen in the Intermarine Member, particularly in beds beneath the Under Rag.

INTERMARINE MEMBER

Shed Bed and Lead Bed (DB129b)

(shaley, sandy limestones with plants)

The Shed Bed and Lead Bed (a ponderous bed - that seemed as heavy as lead?) are relatively soft laminated grey beds. They appear sandy but are actually of high carbonate but with about 12% clay and only about 5% quartz sand. They presumably consisted in a large part of carbonate sand because they are vaguely rippled and have some small-scale cross-bedding and small channels. There are some repeating cyclical units each of a about a decimetre. These commence with the rippled carbonate sand and fine-up into carbonate and clay mudstone. They are probably storm deposits and the bed is a variety of tempestite.

Some parts of the bed are high in organic carbon, up to 6.65% but other parts only have 0.42%. It is interesting that the bed is notable for a bituminous smell and perhaps it should be investigated for oil content.

The top surface of the Shed Bed has some unusual features. It appears to be a hardground, because it has numerous small boring (although they might be burrows) filled with aragonitic shell debris, and scattered attached oysters, Praeexogyra distorta . It is not common to find these lagoonal oysters in a life position. They are mostly present in the form of thanatocoenoses (death assemblages).

INTERMARINE MEMBER continued:

Shed Bed - Shingle - Crocodile

At the base of the Shed Bed and the junction with the Shingle, Richard Edmonds (Earth Science Manager of the Dorset and East Devon World Heritage Coast - Jurassic Coast) has discovered a fine specimen of a crocodile skull. This is of Goniopholis simus and a photograph of it is shown above.

In addition some interesting plant remains have been discovered in the upper part of the Lead Bed, again discovered by Richard Edmonds. More information will be given on these later. Charophytes occur in the upper part ( Clements (1993).

INTERMARINE MEMBER continued:

The Roach and Thornback (DB125)

(important building stones)

The Roach and Thornback is a notable, high-quality, early-cemented biosparrudite. The upper part of the bed is the Roach and most of the lower part is the Thornback. The basal part is sometimes known as the Pink Bed (see a table below for more details). It is a low-salinity, C-phase, shell beach deposit, worked over extensively by moderate lagoonal waves (probably about 1 or 2m in height) coming from the east. The low-angle beach was quite clean and light- coloured. The dry sand was probably almost white. Dinosaurs, mostly of Iguanodon-type (Ornithopods) walked on it from time and their footprints have been preserved in the quarries. In places it has a thin slightly reddened top, which is the result of oxidation and incipient soil-forming processes.

Benfield (1948) thought that the Roach took its name from the contained fossil fish. However, the name may have just come from the French "roche", just it was good rock to use, back in historic times.

INTERMARINE MEMBER continued:

The Roach (DB125) and its Dinosaur Footprints

Although dinosaur footprints occur in various Purbeck limestones which originated as early-cemented, beach deposits, the beach-bed, the Roach is well-known as the main dinosaur footprint bed. There follow the comments of Mr. Ernest Oppe (1965) of Worth Matravers, a local enthusiast and collector back in the 1960s. Because of a discovery of a parallel track, at one stage, Mr. Oppe had entered into a local discussion about whether the dinosaurs walked with feet apart. Later, however, he observed that the dinosaurs of Dorset had the balance and sense of orientation to do a "dancing deportment-master walk". He noted that they were able to walk in a line as straight as that which was once required by the police when checking a for suspected drunken driver!

"In their productive Middle Purbeck - Lower and Upper Building Stone Series (50 feet and 34 feet respectively) between which runs the celebrated Cinder Bed, one special stratification in the Upper series, the "Purbeck Roach" must be given precedence for its astounding presentation of dinosaur footprints. Much in demand now for crazy paving, Purbeck Roach is composed of several layered slabs up to a total of, say, 2 feet including a bottom slab sometimes called "Pink Bed". The slabs vary in thickness and quality and are usually separated by thin "joints" of "dirt" (fossil soil [not proven as palaeosols]) and crusts of "scurf" [vein calcite?], or may be easily parted along lines of cleavage [actually bedding] by a skilful operator. ..."

"Although the present compilation is concerned only with dinosaur trackways, it should never be left unsaid that one and the same tread mark commonly shows through several of the superimposed slabs indicated above.

The tiers so surviving display originally a depth of mud trodden by the monsters and then covered over by more of the same material, even to the extent of providing an infilling or natural cast of the imprint for posterity.

When quarried. however. the stones become scattered in the ordinary course at once and so multiply the number of specimens in circulation to an illusory degree. To Messrs. G. and R. Cobb. Worth Matravers. a preservation of vertically associated slabs from their Acton quarry is due, all exhibiting the same two footprints one close behind the other through and upon (a) a "crazy paving" slab (b) a thinner type slab (c) a lowest impression upon an undermost sound Pink Bed slab. No natural cast was secured, nor need be in any such operation but the bulges on the under sides of the specimens with the exception of the basic Pink Bed stone are noteworthy..."

"In general, the old time quarriers' comment on finding a footprint - "he has been this way" - (aptly quoted by studious successors) remains significant. Large questions have been narrowed instructively, but not closed and insistence upon tangible evidence is now of wider concern."

Here are some records of just a few of the many dinosaur footprint finds from the Roach (Oppe and West, 1962):

Locks Quarry, Acton, Langton Matravers - Roach (Pink Bed) - two prints 11 inches apart, 18 inches but probably the lowest slab of a superimposed series, in Mr Oppe's collection;

Cobbs Quarry, Acton, Langton Matravers - Roach - two prints each 9 inches, about 9 inches apart;

Cobbs Quarry - Roach - medium sized, at Worth Matravers.

Cobbs Quarry - Roach - 18 inch cast in the County Museum, Dorchester;

Suttle's Quarry - Roach - (Pink Bed) (Anon. 1962, Suttle et al. 1962). At 25 feet of uninterrupted track consisting of two parallel rows of prints two feet apart. Width of individual prints 1 foot. Length of stride two feet. Track headed in direction 240 degrees. Other prints are present (Swaine, 1962).

Roach, above, and also in Pink Bed. (Calkin, 1933) 14 prints in two tracks like those of Suttles Quarry. Prints 11 inches wide.

Roach (Benfield, 1948, p.32), 3 footprints.

The reason why the Roach seems to have most dinosaur footprints lies in its origin as the topmost of a thick sequence of exposed carbonate beach sands (The Freestone, Sugar and Roach have been all early-cemented into beachrock). This broad, lagoon beach surface must have remained exposed for a long period. This would explain the reddening of the top. Later it was submerged under the next freshwater deposits (DB126 with Pychostylus).

INTERMARINE MEMBER continued:

Freestone and Sugar (DB121-DB123)

Beds DB121 and DB123 are massive biosparrudites (shelly limestones) of the early-cemented, beach-origin, type, similar to the Roach and Thornback (DB125). The table above may be of use not only to petrographers and general geologists but also to archaeologists studying the historic use of Purbeck Stone. Bed DB122 between these good quality limestones is a slightly impure variety of bivalve biosparrudite (2% quartz and 4% clay). DB121 and DB123 are similar to the Roach. The quarrymen have their own names and may not always be entirely consistent regarding the nomenclature. In any case there is likely to be some lateral variation, and the quarry section may not be exactly the same in thickness or sequence as at Durlston Bay.

It is interesting to note that the three main quarried biosparrudites in the lower part of the Intermarine Member are all low-salinity type with the distinctive C-phase ostracod Cypridea fasciculata granulosa. Dinosaur footprints occur in all three beds.

INTERMARINE MEMBER continued:

Lias Rag (DB 116a and DB116b) - with Silicified Tree Remains

Silicified tree remains, or petrified wood, are well-known to occur at the two horizons in the basal Purbeck Group in Dorset. These are just above the Lower Dirt Bed and just above the Great Dirt Bed. Both of these horizons are probably within the uppermost strata of the Jurassic System. Silicified tree remains also occur higher in the Purbeck Group in the base of the Lias Rag of the Intermarine Member in Durlston Bay, Swanage. This is in Cretaceous strata - Berriasian. A piece of silicified trunk, 17cm width by 7.5 depth was found lying parallel to bedding in a slightly collapsed piece of the Lias Rag by a student making a field study with me in 1994. This was subsequently cut by a diamond saw.

A sawn and polished cross section shows the heartwood and the sapwood of this conifer trunk. An unusual feature for the Purbeck Group is the preservation of some bark on the outside. It is not continuous but there are patches of it remaining. Notice the wider spacing of the tree rings in the heartwood compared to the sapwood. Notice also the medullary rays. The trunk has been compacted and mineralised tension gashes are present. For more information on Purbeck tree rings, with regard to basal Purbeck trees, see Francis (1984) - The Seasonal Environments of the Purbeck (Upper Jurassic) Fossil Forests.

Silicified tree remains have also been found in the same bed, the Lias Rag, by Mr. Trev. Haysom in his Purbeck stone quarry at Acton near Langton Matravers. Paul Ensom is studying the occurrence of silicified wood in the Middle Purbeck Group at this and other horizons and will publish on the subject later in the year.

A very good quality, but ex situ tree trunk with good tree rings was found by the author about half a century ago at Connor Cove (Fisherman's Ledge), east of Blackers Hole. It was thought at the time to have come from the basal Purbeck strata. No well-defined dirt bed with trees has been found in that particular area, which is in Purbeck basinal evaporite facies. Some stromatolites are present in the basal Purbecks of the cliffs and these might contain tree holes (like those on Portland) but clear and obvious examples have not been seen. An alternative possibility is that this tree trunk could have slumped down from the Middle Purbeck strata higher up the hill. However, the very close-spacing of the tree rings ( Francis, 1984) is more compatible with a basal (i.e. evaporitic) Purbeck environment. Another factor is that that tree trunk is uncompressed laterally and thus was probably petrified in an upright position, like the trees of the basal Purbecks. Some uncertainty remains, though (Ensom discoveries may be relevant to the problem).

Note that silicified wood is also common in the basal Purbecks of the Vale of Wardour. Silicified wood pieces (and jet) occur at the top of No. 3 Gypsum Seam (bed) in the Mountfield Inlier (Howitt, 1964). This Sussex occurrence is at about the horizon of the Broken Beds in Dorset.

For more on silicified trees see:

Fossil Forest webpage.
Isle of Portland - Geological Introduction (with Portland fossil trees)
Portesham Rocket Quarry (with fossil trees).

INTERMARINE MEMBER continued:

Laper and Underpicking (DB114)

These are argillaceous ostracod biomicrites, with desiccation cracks. Dinosaur footprints have been found in this. The bed has shaly partings and there are a few seams of beef in the lower part. Bristow and Forbes in Damon (1884) recorded the presence of green matter (glauconie?) in the impure sandy limestone of this unit. It is interesting that the fauna contains the Purbeck lagoonal serpulid in addition to Neomiodon and fish teeth. The gastropods include Hydrobia and Valvata helicelloides according to Clements (1973). Ostracods are C-phase with Cypridea granulosa fasciculata, Darwinula leguminella and others. It probably is, of course, a good bed for collecting ostracods.

Sr is quite high at 953 and 1246ppm. and there is a trace of aragonite in the bed. Clay attains 11 and 30% in samples studied by El-Shahat (1977).

INTERMARINE MEMBER CONTINUED:

Downs Vein (DB 113)

(This is a bed formerly worked on the Downs south of Swanage and Langton Matravers)

The Downs Vein is a late-cemented biosparrudite limestone 0.86 m in thickness, usually about 2 feet, 6 inches in the older papers. It was quarried on the Downs (Round Down etc) to the south of the main quarries. It was much worked in the past for paving and flooring until replaced by cement floors and tarmac. It is generally easy to identify because of its position a short distance above the Cinder Bed.

(There are some complications, though, in the old literature. Fisher (1856) listed this as the "Rotten Grey Bed" referring to the Ptychostylus Bed beneath as the Down Vein. The quarry lists like those below suggest that the Downs Vein refers to the quarried stone. Bristow and Forbes in Damon (1884) referred to this as the "Upper Tombstone, Brassy Bed and Lower Tombstone Bed" and gave an erroneous summary thickness of 13 feet, 6 inches which should have read 3 feet, 6 inches.)

Clements described the Downs Vein as "Massive limestone, with a thin lenticular chert seam near the top. Crowded with small lamellibranchs." El-Shahat and West (1983) showed this in a diagram as DB 113 (but, wrongly used Fisher's name - "Rotten Grey Bed" instead of Downs Vein). It is a compacted biosparrudite, buried as an aragonitic shell deposit before lithification. Geochemically, it has quite high Sr and P but is not remarkable. It is mostly low-Mg calcite now but there is small amount of aragonite left.

The brief description given by Bristow and Forbes (in Damon 1884) is interesting.

"Thick bed of Shell Limestone. [the Downs Vein]
The surface of the bed covered with scratches in directions 15 degrees W. of North, and 5 degrees E. of North [slickensides related to beddding plane slip]. Melanopsis. Fish (Microdon radiatus, Hybodus strictus, Lepidotus minor). Turtle (Pleurosternum concinnum). Pterodactyle, Crocodile (Macrorhynchus) etc. - 2 feet, 9 inches.

[the next 3 beds comprise, DB 112 the "Pytchostylus Bed"]

Pale-blue Marl - 3 inches,
Shaly Shell-Limestone - 3 inches,
Soft Black Shales - 3 inches

Next below comes the Soft Cinder [DB111c]

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INTERMARINE MEMBER continued:

Downs Vein - Palaeoenvironment and Diagenesis

The Downs Vein and associated strata are interesting. The basal unit, Bed 112, the Ptychostylus Bed, is of freshwater, lacustrine origin. These beds occur in the "basin" (i.e. downwarping area, but not deep water). They represent an offlap, lowstand unit, as shown by the correlation diagram above, and are not properly developed in the Lulworth shelf area. The Ptychostylus Bed, at least, represent a brief post-Cinder return to Cherty Freshwater facies within the basinal area. The palaeosalinity of the Downs Vein itself is not definately known, but the presence of chert might, might possibly indicate an early phase of high pH interstitial water as was likely in the case of the Flint Bed of the Cherty Freshwater Limestone.

The vertebrate content of the Downs Vein is interesting. Considering the faunas of the beds above and below it seems to be the product of a freshwater or nearly freshwater lake. Presumably the fish are freshwater forms. The turtles and crocodiles seem appropriate to this environment and perhaps, pterosaurs were numerous over the lake. At an excavation just south of Downshay Wood (SY 979 792), the Downs Vein (DB 113) contains vertebrate trackways (Ensom, 1994) which have been made by a small reptile.

The petrographic and geochemical features of the Downs Vein indicate, as mentioned above, that it was buried as an aragonite shell bed, not lithified as a shell beach above the water-table, as were some beds. Association with footprints and the occurrence of desiccation cracks in the Laper above show that it originated as shelly flats which were, like most of the Purbeck strata, at just about water-level.

The depositional palaeosalinity of the Downs Vein is, at the moment, an open question. The possible Mg²⁺ activity deserves consideration since aragonite survived to deep burial.

If the water was not saline there would normally be no significant quantity of Mg²⁺ ions to inhibit the wet transformation from aragonite to calcite (cf. Fyfe and Bischof, 1965). Rapid transformation and a low proportion of Mg²⁺in the resulting calcitic rock would be expected. In fact, although MgO is not of a very high value, it is signicant at 1.05% in the Down Veins, and more than would be expected. Considering that the rock is 91% calcite now, and if the original sediment was mostly aragonite, then the Mg²⁺content is orders of magnitude higher than the normal figure for molluscs, at least according to Friedman (1968). He found 0.1 part per thousand in mollusc shells from the Gulf of Aquaba. The horizons with the highest Mg in the Middle Purbecks are the argillaceous lake deposits with mammals (the Mammal Bed and the Sly Bed and similar units). It is clear that the lakes were Mg²⁺ rich, although this was decreasing upwards. (The peak of high Mg²⁺ is in the palygorskite-containing dolomites of the Soft Cockle Member. These, however, originated in hypersaline conditions and will be discussed in more detail later.)

Many of the Purbeck lakes come into the category of "carbonate lakes" with significant Mg²⁺ , and at times, in the Lower Purbeck, with Mg²⁺ > Ca²⁺. It is less conclusive that the Mg²⁺ retarded aragonite transformation, but it is quite probable.

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INTERMARINE MEMBER continued:

Downs Vein (DB 113) - The Downs Vein Men

The following is an extract from Benfield (1948, p.112 et seq.) explaining how the Downs Vein was quarried in the past:

"Beneath the freestone seam comes the downs-vein, another seam which has been so popular with quarrymen during many generations that very little of it remains. Like the lannen-vein it is a thin seam, and putting up legs is therefore almost child's play compared with the high freestone and new-vein quarries. Usually it also has an advantage over lannen-vein in being very easy to underpick; in places the dirt can be got out easily "like soap" some people say, but when they say like soap they mean something softer than half stone and half clay.

Not only have all the best downs-vein quarries been worked out, but the market for that class of stone has died out too. The average thickness of its beds was about three inches; it was thus easy to handle, and this helped, too, to make it popular - no great thick heavy blocks to move and haul. A downs-vein man need never lift anything heavier than a three-inch bed and even that could be cut comparatively small. Indeed they were bound to cut it small to - get through the lanes with it, because the ceiling in the downs-vein was seldom good, and the legs had to be close together. Sometimes they used a smaller and narrower cart because the lanes were pinched ini so close, and even then they had to bring the tops of the legs nearer - a lane three feet wide would not have stood in many places. Often it was less than three feet high before it began to sag. Where it sagged and the sides bulged it meant that a visitor must crawl on hands and knees to visit a downs-vein. The men who worked there did not crawl on hands and knees, they squatted on haunches with knees up under chins and shuffled forward like that, and it was surprising how fast they could go with the knuckles pressed into the floor. People will exclaim with surprise when they see the chimps and gorillas in a zoo using the knuckle$ thus instead of the flat of the hand, so when they see a man using his hands in that way they straightway feel superior. But if you take any man, and not only a quarryman, who for years has been handling heavy tools and pulling and twisting until his hand is a mass of muscle, you will find that his hand is not very flexible and will certainly not bend backwards anywhere near right angles. Such a man will prefer to press down with his knuckles when he crawls - watch him when he pushes himself up from the floor or when he helps himself round the corner of a table.

Anyway a downs-vein man learns to travel along his lane on haunches and knuckles where an ordinary man would go on hands and knees. To turn, the downs-vein man simply swings his head from one side to the other and pivots, but the crawler finds three feet (and it is often less) is not enough to let him turn; most men need that for their bodies and heads alone, and there are still trailing legs below the knees; in short, long practice proves that the downs-vein posture is the only one that is really satisfactory.

A downs-vein is often a wet quarry, so to protect their shoulders, which were always rubbing on the ceiling, some men used to wear a sheepskin protector. The ceiling is always wet in the best of quarries, as there is enough condensation of breath to keep it so even if the floor is dry, but a downs-vein ceiling always seems to be running with water.

Amongst quarrymen, who are not themselves ordinary men perhaps, the downs-vein men were a race apart; it used to be said that some of them never came to. the surface except to attend a meeting in times of dispute. Some years ago, when a count of the trade was made, some downs-vein men were left out; they were seldom seen outside their family from one year to another. There are very few of them left now, but I have seen them with red-rimmed eyes blinking in the sun and with little round balls of clay stuck in their whiskers; that they were up and out on view was enough to make neighbouring quarrymen say: "Look, there's old So and So; I haven't seen him for months."

The coming of cement floors and tar paving spoilt the market for downs-vein. As it came out in beds about three inches thick it was just right for flooring and paving stones, and not only was it the right thickness but it cleaved out plainly and often out of winding. The old men sometimes claimed that their stone cleaved out so well that they merely had to tool it over and then joint it round; the proof that they had little waste can be seen to-day-the heap of scars flung out from an old downs-vein quarry is scarcely any size at all, while that from any other may cover acres and be piled in the air as well."

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INTERMARINE MEMBER continued:

Buildings Stones and Quarrying

The quarrying presumably started in the cliffs of Durlston Bay where the rock is directly exposed. Mines which were driven directly into the cliff here have collapsed, been buried by debris or lost by erosion. The remains of some were visible until fairly recently just south of the southern Cinder Bed ledge, south of the Zigzag Path. Even as late as the 1850s when the cliff section was being measured there was still a quarry in the cliff above the Cinder Bed ledge but it is not clear whether it was working then (Bristow and Forbes in Damon, 1884). The "Lias Beds, " part of the Intermarine Member, were measured in this quarry.

Generally, though, after initial extraction in Durlston Bay, much of the quarrying moved westwards, inland, along the outcrop, into the area shown in this 1890 map. This working of the Middle Purbeck Building Stones particular increased after the Great Fire of London. Lewer and Smale (1994) reported that George Manwell wrote in a letter to his sons that " After London was burnt some years and the city began to rebuild and flourishing, there was an uncommon call for Purbeck stone, and paving was sold at so high a price as 30s per cwt. This of course attracted the notice of the neighbourhood round; and numbers of boys from different parishes, at the distance of 20 miles, were apprenticed here to the stone trade and premiums given. "

Daniel Defoe describing Purbeck in 1724 (as quoted by Lewer and Smale, 1994 p. 48) commented that " This part of the County is eminent for Vast Quarreys of Stone, which is cut out Flat, and us'd in London in great quantities for Paving Court Yards, Alleys, Avenues to Houses, Kitchins, Foot-ways on the Sides of High Streets, and the like; and is profitable to the Place, as also is the Number of Shipping employ'd in bringing it to London. There are several Rocks of very good Marble, only that the Veins in the Stone are not Black and White, as the Italian, but Grey, Red and other Colours. "

After much opencast excavation with problem of removing the overburden with manual labour and horse and cart, underground excavation started. This, in due course, became organised into a pattern in which a perimeter wall was built, there were two or three open sheds made for the quarriers to work with shelter against the weather. There was a lock-up shed for a pony or donkey and gear (Lewer and Smale, 1994) . Having dug down a short way, a stone archway was built at the entrance and an inclined slide was made so that a quarr-cart loaded with stone could be hauled up by a chain from a capstan. The capstan was turned by the donkey or pony pulling on a long " spack" or ash pole. Two large " crab " stones (incidently, note the bed name - "Crab ") supported the elm capstan post.

The picture here shows this old method of quarrying on the hills west of Durlston Bay, based on a etching in Robinson (1882) (see also Lewer and Smale (1994) for old photographs of a similar quarry). The view in this scene is northward with the Purbeck Hills of Chalk in the distance. The steeply inclined shaft often went to a depth of about 30 metres or more from which galleries were driven underground along the bedding, in the direction of strike. The one horsepower operation of the capstan is shown (incidently, there is a monument stone to a loyal and hardworking quarry mule on the Priest's Way path west of Durlston Bay according to Pushman, 1987).

In Durlston Country Park, on the road from Durlston Castle to the Anvil Point lighthouse, and near the Information Centre, you can see the restored mouth of an old inland quarry. This quarry is in Middle Purbeck strata.

We can find out more about what this quarrying was like in the past by turning to Robinson (1882).

Robinson tells us
" The quarrymen are generally pleased enough to welcome a stranger in their stony dens, and require no pressing to lead the way thither with lighted tallow-dip in hand. Rough steps dug in the earth of the shaft, next the slide of smooth flat stones up which the capstan drags the truck, afford an awkward, and in wet weather slippery, means of descent. At the bottom the floor of the quarry is found to consist of solid stone, covered in many places with a thick layer of mud, for even in summer the water penetrates in from above and around, while during the winter months pools many feet deep will often collect in the lower workings. Passing under the arch of the shaft, a narrow lane, from which the merchantable stone has been removed, leads gently downwards to the newer passages, where the quarrymen are actually operating. We are in absolute darkness now, but for the dim flickering light of the dip, and there is a damp earthy small; but the air is not deleterious to health. The ceiling of the lane is of a thick but useless rock, here and there intersected by a "joint" , or dropping a few inches at a "fault". Stone or wooden props are placed to shore up these weak spots. Very fragile and decayed many of the wooden supports appear, and it has not unfrequently happened that one has given way, and the superincumbent mass, descending, has crushed the workers in its ruins. Of one such misfortune a strange grim story is related in the addenda to Hutchins' History (of Dorset); -

" A melancholy accident happended, October 27, 1813, in one of the stone quarries of Swanage. Two men of the names of Samuel Phippard and James Summers, went to the quarry in the morning to work as usual; at the hour of dinner a boy that was accustomed to inform them of the time went in, and seeing no light, nor hearing any answer to his call, returned and procured a light for himself, when, upon his re-entering, the first object that presented itself was Phippard dead, with his head and one hand jammed between one of the pillars of the quarry, and a large block of stone that had fallen from the ceiling. At that time the boy heard Summers, from under a quantity of stone and rubbish, exclaim "Is that a light from heaven?" The boy was struck almost senseless with fright, and instantly ran out to procure assistance. On some of the neighbours entering they found Phippard as before described, and Summers confined under two large blocks of stone which had formed a kind of arch over him. The poor fellow was soon released from his awful situation, with two of his fingers nearly severed from his hand and one of his legs broke. He recovered his senses in a short time, but died after about thirty-six hours. He said that at the time of the quarry falling in they both tried to escape, though in different directions, but neither was successful, and when he was under the stone he called several times to Phippard, but not receiving any answer, he concluded that he was dead."

(to continue with Robinson's account - he mentions that the quarry lanes are controlled by bed thicknesses and are sometimes only three feet high)

"------- The mere getting of the stone is not a very difficult task, for the layers are of no great thickness, and are crossed by numerous transverse vents. Crowbars are used to loosen the mass, being inserted into thin layers of clay which divide the beds of stone. -- The haulage to the bottom of the shaft is done by manual labour, and this is probably the hardest exertion the men now have to undergo; though formerly, and even within memory, it said that all the stone used to be conveyed from the quarries down to the seaside on the backs of the quarrymen.

When we emerge again to the light, our guide will probably offer to show some specimen of fossil fish or turtle he has found, and put aside for sale to his visitors."

Nowadays, the quarrying is usually in small open pits worked by the use of machines (see Sunnydown Farm Quarry , below, as an example). Recently, Dr Eric Robinson (1998), a specialist on building stone, has discussed the matter of whether quarrying will continue in the Isle of Purbeck or whether it will in due course be banned by planning judgements. He argued that quarrymen in the past have avoided mess, noise and dust. He says that because of the use of Purbeck Marble and Stone in cathedrals and churches there has to be a means to provide new stone for repair. There have been responses from the Senior Planning Officer and Property Manager of the Purbeck Estates. There seems to be some agreement that small-scale, traditional type quarrying may be acceptable in this environmentally sensitive area, but large-scale opencast working with much heavy machinery is not.
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INTERMARINE MEMBER - CONTINUED

Buildings Stones - More on Names of Beds

To some extent the names of beds are discussed when dealing with particular parts of the Intermarine succession. Some further notes are added here.

Understanding the old quarrying nomenclature of beds is necessary to some extent because old geological accounts, which have fossil records, use these. Various fossil remains found in the 19th century are sometimes recorded in relation to quarrying bed names. It is not always easy to relate old names to modern geological logs, but most of the quarrymens' beds can be recognised.

Clements (1993) linked some old names to his log. Using Bristow and Forbes (in Damon, 1884) and other sources I have attempted to add some more to a version of Clements' Log for the Intermarine Member, provided again here. Problems arise because, as explained by Eric Benfield (1948) "the whole freestone seam (Freestone Vein) may be twelve or more feet thick, of which only between two and three feet are freestone itself, the rest is made up of other beds totally different in texture and working qualities. But of all that thickness we only dug six feet; two beds remained to make a floor, and several were left up as a ceiling. In different quarries different beds are kept up. I have worked in quarries that took more than my father did, and in another which had to keep up an extra bed."

This varied use of stone beds may have contributed to variations in the nomenclature and there is no doubt that geologists did not always interpret the quarrymen's names correctly.

This is an old (1855) handwritten list of stone bed names (modified from Pushman, 1987 - see this publication for more information and prices of stone); it has similarities to the Bristow and Forbes list in Damon (1884) but does not tie in exactly.

Some earlier historic information on the old names of the building stones has been provided by Stevenson (1812) and also by Webster (1826). (I am grateful to Terry Hughes for the reference to Stevenson). Austen (1852) and Fisher (1856) also provide names. The appropriate section of Stevenson's list (with a few locating DB numbers added) follows. The sequence below is in order with the Leper at the top and the Downs Vein at the bottom of the Intermarine Member. To correlate these with the standard Clements' log (1993), the Leper is in the middle of the Laneing Vein - the massive biosparrudite - DB 144 (note that there is also a " Laper " at DB 114). The Downs Vein is just above the Cinder - DB 113

Leper, so called perhaps from its resemblance to the skin of a leprous person; and
white-bed, each ten inches thick; good. [part of DB 114]
The soft and hard bed, about a foot thick; good. Then the
mock hard bed, one foot; of no, use, being full of clefts.
A foot of brown earth mixed with, yellow, and blue.
(backing bed)A stratum of twelve inches, called the backing bed, fit only for the inside of a wall.
Earth two feet, the same as above.
The rial, used for kirbs, or the outside stone in the paved footways; good and durable. [probably the Royal - near DB 140]
Blue light earth two feet.
Topgallant rag, six inches; so very hard, as to merit the appellation of the Devil's Bed. [Topgallant or Devil's Bed near DB 139]
Three or four feet of blue earth.
White rag. [Is this the Red Rag of other accounts - DB 133?]
Freestone rag, about six inches thick,, and the
under rag one foot; each [ Under Rag - DB 131 - easily recognised] good.
Blue earth one foot.,
Lead bed, nine inches, a very ponderous stone; but not of much service. [upper part of DB 129b - does this include the Shed Bed - lower part of DB129b?]
Earth one foot, of a smoke colour.,
Shingle, one foot, not very good, [DB 129a]
Yellow earth, six inches.
Grub, six inches, very good [near DB126]
Freestone roach eighteen inches [0.46m.], very good [upper part of the Roach - DB 125 - total 0.79m.].
Thornback, seven inches [0.17m.]; very good [lower part of the Roach? - DB 125 - total of 0.79m.]
[Is there something missing here - where is the Freestone DB121? Has the Roach and Freestone been linked or confused?]
Lias, one foot; not the best. [not sure - near DB 121?]
Lias rag, two feet, but of no great use. [Lias Rag - DB 116]
Hard rubble earth, ie earth in hard lumps, about five feet, three inches blue, the remainder a light brown. [DB 115 - the Planorbis Bed & DB114]
Down vein, three feet, good. [well-known as the Downs Vein - DB113 - and 112? - but also referred to as Upper Tombstone Bed, Brassy Bed and Lower Tombstone Bed - Bristow in Damon, 1884 ]

INTERMARINE MEMBER continued:

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 Group 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, which also has some euryhaline gastropods.

Visiting kangeroos or Iguanodons may leave footprints in favourable circumstances. (Right hand side of image modified from an photograph by Peccinotti in Dutton, 1980)

CINDER MEMBER (CINDER BEDS):

Introduction

The Cinder Member or Cinder Beds, DB111, is 2.95m. thick, bluish grey and full of small shells of the lagoonal oyster Praeexogyra distorta. It is massive and quite different in character from any other bed in the Purbeck Group. It is well-exposed as a prominant ledge in the northern part of Durlston and easily reached by a walk along the shore from Peveril Point. At high tide the sea may come up to the ledge and prevent access further to the south. The Cinder Bed can also be seen between the two faults in the centre of Durlston Bay, and in the southern shore ledge exposure a short distance to the south of the Zigzag Path (and the faults).

These beds of bluish-grey shelly limestone with small oysters are easily recognised by the blue-grey colour, its relative lack of jointing and a tendency to erode to a roughly rounded surface. The oyster Praeexogyra distorta is of a genus that is ancestral to Exogyra , hence the name. In terms of palaeoecology it was probably similar in habitat to the small oyster Crassostrea which lives in coastal lagoons of the Gulf of Mexico.

Clements (1993) described the Cinder Beds or Cinder Member, DB111 in the following terms:

"Massive Praeexogyra distorta shell bank, set in a matrix of light-grey (weathering creamy-brown) calcareous mudstone and muddy micrite, which contains much Praeexogyra debris, together with scattered sand grains. The bed can be subdivided as follows:
DB111c. Top 0.51m.
DB111b. Middle 0.97m.
DB111a. Bottom 1.47m. - More of limestone than DB111(c) or DB111(b), which are relatively clayey. Praeexogyra smaller. Sample:- Gastropoda: none. Ostracoda: Galliaecytheridea postsinuata 3; indet. 2; one specimen of Theriosynoecum striata was found, but is thought to be a contaminant."

Spines of the echinoid spines Hemicidaris purbeckensis occur in the Hard Cinder which show that conditions were normal marine for a short time. The complete echinoid, shown here, is very rare (see the fossil section for more details). Many bivalve shells which had aragonitic shells are visible in thin section. They have been replaced by neomorphic calcite and are difficult to see in the hand specimen.

This bed has been taken as the Jurassic-Cretaceous boundary but there have been problems with this. It may well correlate with the Jurassic-Cretaceous boundary in the Boreal Realm, that is the northern and Russian region, but not with the (now official) Jurassic-Cretaceous boundary of the south of France. The boundary is now believed to be near the base of the Purbeck Group which is regarded as mostly Cretaceous. Nevertheless, the Cinder Bed is a very extensive and useful marker for the middle part of the Purbeck Group.

This bed indicates an incursion of the sea into the lagoon. It is the result of a brief marine transgression. In the shallow environment the water became just about marine enough for echinoids ( Hemicidaris purbeckensis ), but not for long. The marine conditions were not stable for long enough for ammonites to live in this area. The small oyster could tolerate lower salinities than truely marine and still flourished when the water became rather brackish. This was not the only marine incursion; there were about 37 others at least during deposition of the Purbeck Group.

CHERTY FRESHWATER MEMBER:

Introduction

The succession of limestones and marls of the Cherty Freshwater Member is shown in these diagram (click to enlarge). The left hand one is simplified and completely redrawn (footprint symbols indicate horizons at which dinosaur footprints have been found). The right hand one is based on Clements' Log with modifications (in particular some bed names have been added). To facilitate locating the units the strata immediately above and below is also shown. The Cinder Bed is the obvious marker horizon which can be used to locate the Cherty Freshwater units shown here. The Flint Bed should then be found, and the other beds located from it. For detailed study refer to the original logs of Clements (1969, 1993), which should be taken into the field.

Incidently, with regard to bed names it should be noted that although the Feather Bed is a short distance below the Cinder Bed, the term " Feather Quarry " has been used by Austen (1852) and Fisher (1856) for strata including the New Vein and the Flint Bed. This could lead to confusion regarding fossil records such as Nuthetes destructor . An extract on this topic from an email from myself (3.12.99) to Paul Ensom is as follows:

"The Feather Bed is DB 108 in Clements. The Feather Quarry is the middle Cherty Freshwater Member, from DB 95 to DB 101. Both the Mammal Bed, DB 83 and the Fern Bed etc DB 92,93 were referred to as "Dirt Beds" in Fisher, 1856, and from the illustration both were in the Beccles Mammal Pit working face (see below). Some of the Feather Quarry was in the Mammal Pit but probably not the Feather Bed. Richard Owen in a handwritten note placed "Nothetes destructor, Teleosaurus gra....." and something else, not readable on my xerox copy (of a Natural History Museum paper) in the Feather Quarry with an asterix in Austen's bed 84, about Clements' DB 96."

Most of the strata within this member are lacustrine with low-salinity (freshwater or near-freshwater) gastropods such as Viviparus, Ptychostylus, Planorbis and Physa . Low-salinity ostracods such as Cypridea are common and charophyte algae occur in certain beds. An apparent anomaly is the occurrence of small pseudomorphs after halite in the Flint Bed. This is probably because the lake in which these beds were deposited was not absolutely devoid of sodium and chloride ions and of extreme desiccation in the summer probably dried out and a little salt was precipitated in the lime-mud.

For details of charophyte occurrences at this level and for discussion of palaeoenvironmental implications of charophytes please see the Purbeck facies section

Chert in fine-grained limestone with freshwater molluscs, Cherty Freshwater Member, south of the North Cinder Bed Ledge in the northern part of Durlston Bay. This chert is a replacement of carbonate sediment and shells, as is usually the case in limestones. Most chert in Mesozoic limestones occurs in marine sequences with sponge remains which have provided the silica for diagenetic change. In this bed a freshwater sponge, Spongilla purbeckensis, occurs, but it is not abundant and whether the siliceous spicules of this provided all the silica is not clear. Diatoms in the freshwater lake in which this sediment was formed might have supplied the silica but the early Cretaceous date may be too early for the evolution of diatoms. Note that the nodules are of irregular shape and are not influenced by the jointing which is presumably later. Molluscs in the chert are uncompacted suggesting an early date of diagenetic precipitation of the silica. This is a common feature of chert and flint. Around the chert nodules fossils are selectively silicified so that good specimens are obtained where this bed has been much weathered.

The Flint Bed or Cherty Freshwater Bed, is DB 97 of Clements (1992). It is a biomicrite with charophytes and contains Valvata helicelloides, Hydrobia, Physa bristovii, Planorbis fisheri, Viviparus, Cypridea tumescens, Cypridea granulosa aff. fasciculata, Theriosynoecum forbesii and Darwinula leguminella. The " New Vein ", bed 101, a low-salinity, bivalve biosparrudite (i.e. a coarse-grained biosparite) cemented early, prior to full compaction (El-Shahat and West, 1983). In places it preserves dinosaur footprints because it was a shell-sand at the margin of a lagoon prior to cementation. It was the deepest objective of the Purbeck quarries of the Swanage area. Most of the worked stone was of biosparrudites like this. This and the Cap Bed, a short distance above, was much worked in the Townsend area of Swanage. It was also worked more recently at Sunnydown Farm quarry, Langton Matravers.

In the section at Sunnydown Farm, discussed briefly below, Paul Ensom has made many remarkable discoveries close to the level shown in the image (Ensom, 1988; Ensom, 1996; Kielen-Jaworowska and Ensom, 1992). Dinosaur, amphibian and mammal remains have all been found in the Sly Bed (equivalent of DB 102) just above the New Vein. Here too have been found eggs of dinosaurs, the possible remains of a hatchling and numerous crocodile teeth (Ensom, 1997). Mammal remains occur in the equivalent of DB 98 at about the level of the recess shown between the Flint Bed and the New Vein. At Sunnydown this contains coprolites and has rootlets underneath penetrating down into the limestone. A marshy environment with horsetails seems to be implied.

The gastropods in the Cherty and Marly Freshwater Beds caused some interest in the 19th Century because arguments about special creation rather than evolution were still taking place in Europe. Professor Edward Forbes, a famous geologist, conchologist and botanist and researcher on the Purbecks wrote the following humorous verses:

"Down among the Purbecks deep enough,
A Physa and Planorbis
Were grubbed last year out of fresh-water stuff,
By Bristow and E. Forbes

(Agassiz has just given his bail,
'Twas adverse to creation,
That there should live pulmoniferous snail,
Before the chalk formation.) "

(in Geikie's " Life of Forbes", p 461 as quoted in Damon, 1894)

The shell of the air-breathing, sinistrally (left) coiled pondsnail - Physa bristovii is probably quite common in the Cherty and Marly Freshwater Members. Like the other gastropods it is not easily seen in the limestones, though, except where silicification has occurred and the limestone has subsequently been much weathered (or if an appropriate block of limestone is dissolved in acid). Silicified fossils like this can found in places weathered out on the cliffs and in soils between Anvil Point and St. Alban's Head. At Durlston Bay the limestones are not deeply weathered and gastropods are most easily found in this part of the succession in softer argillaceous horizons. Physa can be found in the Sly Bed - DB 102 and in the Mammal Bed - DB 83. It is not surprising, of course, to find such gastropods with charophyte algae because undoubtedly other non-calcified algae would have been associated and provided them with a good food supply.

Viviparus subangulatus is a species almost confined to the Cherty Freshwater Member (Arkell, 1941). It occurs at all localities from Durlston Bay to Portesham, often silicified. Well-preserved specimens show delicate spiral threads. It also occurs in the Mammal Bed and there is no doubt that it could tolerate freshwater conditions. The type specimen comes from the Serpulite of Nenndorf near Hanover in Germany, where lagoonal strata of about the same age occur. It is also known in the Jura Mountains.

CHERTY FRESHWATER MEMBER continued:

Sunnydown Farm Quarry

Significant discoveries of dinosaur footprints, dinosaur egg remains, mammals, amphibians and other fossils have been made by Paul Ensom (Ensom, 1988; 1994; 1997; Ensom, Evans and Milner, 1991; Kielan-Jaworowska and Ensom, 1992; West, 1988; and other papers) at Sunnydown Farm, near Langton Matravers (SY 9822 7880). This locality is a small shallow quarry about 5 km west of Durlston Bay in the Cherty Freshwater Member.

Some description is given further down of the well-known Beckles' Mammal Pit. The Sunnydown Farm site - "Ensom's Mammal Pit ", though, has produced many of the mammals known from Beckles' Mammal Pit and some even more exciting finds. It clearly provides a valuable picture of Lower Cretaceous life.

Notable features are footprints of sauropods (cf Diplodocus ), herbivorous dinosaurs ( Iguanodon?) and one or more carnivores. The footprints occur on the base of the Cap Bed (equivalent to DB 103 at Durlston Bay). Very small theropod (cf. coelurosaur) teeth have been found in the clay samples in which the footprints were made, and in addition much rarer teeth of the ornithischian fabrosaurid Echinodon becklesii have been recovered (Ensom, 1988). Nineteen sauropod footprints were found. The footprints show 5 toes, the outer hoofed and the inner clawed. There are 9 forelimb and 10 hindlimb casts. The stride of the sauropod was 2.2 metres. Sauropod tracks are also present at the same horizon at Lovells Quarry, about 300 metres to the northwest.

In the clay of the Sly Bed (equivalent to DB 102)mammals belonging to the Multituberculata, the Triconodonta, the Docodonta, the Symmetrodonta and the Eupantotheria. A Rhychocephalian reptile (a Sphenodontid like the modern Tuatara), lizards, crocodiles, salamanders and a frog were found.

The fauna of the Sunnydown Sly Bed can be summarised as follows. This is based, with some simplications and notes, mainly on Ensom (1988) and Ensom et al. (1994) and is intended for only general use. Specialists should refer to the original papers and to later papers of Ensom and co-worker for the full records.

Fish teeth and scales - Hybodus, Lepidotes and others.
Salamanders, and the amphibian - Albanerpeton
Frog (not identified)
Turtles - Tretosternum and ?Pleurosternum . (Common Purbeck turtles)
Lizards - Paramacellodus sp. , Becklesius sp.
Sphenodontid reptile (like the still-living 3-eyed Tuatara of New Zealand of lizard-like appearance)
Crocodiles - Nannosuchus ( a small form - perhaps a juvenile of Goniopholis ), Theriosuchus, Bernissartia, Goniopholis . (Remains of crocodiles have long been found in the Purbecks of the Durlston Bay area in the Middle and Upper Purbeck. Goniopholis is known as the " Swanage Crocodile ". A crocodile bed exists in the Upper Purbecks at Peveril Point)
?Pterosaur teeth (flying reptiles)
Dinosaurs of Saurischian, Theropod type: Nuthetes destructor . This has been referred to in Ensom et al. (1994) as possibly equivalent to Megalosaurus , a common Purbeck dinosaur. Nuthetes has long been known from the Cherty Freshwater Member, from either the Feather Bed or Feather Quarry Bed; the latter is about this level. According to Coram (1988) Nuthetes was a small agile dinosaur between one and two metres long with a fairly small head and long hind legs and tail. It was a small flesh-eater with sharp serrated teeth. It probably prowled the margins of lakes scavenging for dead fish, or scampering through undergrowth in search of lizards and small mammals.
Dinosaurs of Ornithiscian, Fabrosaur type: Echinodon becklesii . This was a very small dinosaur, a plant eater with tiny spiked teeth. It probably looked similar to Nuthetes, with its small head and bipedal stance. It was less than one metre in length and belonged to a fairly primitive group of dinosaurs which had mostly died out in the rest of the world (Coram, 1988). He suggested, based on evidence regarding similar creatures in older rocks in South Africa, that Echinodon was able to survive dry, hot summers by going to ground and sleeping through them, a process known as aestivation.
Mammals - all small forms including: Multituberculates: Bolodon, Plioprion minor . These were small rodents with specialised teeth - rather like desert rats.
Mammals - Triconodonts, Docodonts: Peraiocynodon
Mammals - Symmetrodonts: Spalacotherium
Mammals - Eupantotheria: Phascolestes, Peramus, ?Peraspalax, Amblotherium

Cockroaches and other insects.

Plants - (Ensom et al. 1994) include charophyte algae, ? Equisetum (horsetails) and conifer shoots. Cupressinocladus was like the foliage of the basal Purbeck fossil trees, which were similar to cypress or juniper trees.

Dinosaur footprints are also found from time to time in Durlston Bay. This tridactyl example, photographed many years ago, is from an unknown horizon and was found in a loose block on the beach. Note that this is a natural cast (inverted) of footprint made into the underlying mud and that mud was cracked by drying out in the sun. The footprints at Sunnydown Farm are similarly into mud (smectite-illite- mixed layer clay) with carbonate sediment filling the depressions. At Durlston Bay, at present, another footprint can be seen in the cliff near the Zigzag Path .

MARLY FRESHWATER MEMBER

General

Safety Note - This is a particularly hazardous area in terms of risk of rock fall. It has been rather unsafe since 2007, and a significant fall, predicted here in that year HAS now taken place. Other falls are likely. If it is necessary to go here, keep clear of the foot of the cliff as far as possible, and strictly avoid any place where there is freshly fallen, split rock or where the cliff above is badly fractured and unstable. This part of the bay is not a place that is safe for children, and large parties should not linger close to the cliff. Some other parts of Durlston Bay, are less hazardous.

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MARLY FRESHWATER MEMBER

Stratigraphic Introduction

Once again, the classic log of Roy Clements (1992) should be studied to understand the strata of the Marly Freshwater Member of the Purbeck Group in Durlston Bay. The Cherty Freshwater Member and the top of the Soft Cockle Member are also shown to enable understanding of Marly Freshwater strata in context. In general the Marly Freshwater Member consists of argillaceous, freshwater lake deposits with Hydrobia, Viviparus, Planorbis, Physa and charophytes, and with important vertebrate remains. The Mammal Bed, in this middle of this unit, is very well-known.

This older and smaller image above shows Marly Freshwater strata in general (DB 86 downwards). The upper part here is the basal sequence of the Cherty Freshwater Member. Chert still occurs as low as in DB 87.

MARLY FRESHWATER MEMBER - MAMMAL BED

Introduction

The most notable bed of the Intermarine Member is the the Mammal Bed, DB 83. In this 14 genera of early mammals were discovered. An excavation known as Beccles Mammal Pit was made, south of the beach exposure, at the top of the cliff in the middle of the last century. The remains of mammals are not usually found here at beach level although rare discoveries have been made. The microvertebrate remains are those of minute teeth and jaws of small mammals.

Some features of the Mammal Bed and associated freshwater strata are shown in photographs provided here. Note that the marls and limestones contain a good freshwater fauna, including Planorbis and charophytes. The Mammal Bed marl was a carbonaceous, calcareous mud with freshwater shells. Thus it was a lake margin or swamp deposit, not a well-developed rendzina palaeosol like the Great Dirt Bed of the basal Purbecks. It has not been leached of aragonite shells and does not have limestone pebbles. No trees, growing in place, have been found within it.

MARLY FRESHWATER MEMBER - MAMMAL BED:

Mud Holes - Holes filled with Carbonaceous Mud

An unusual aspect of the Mammal Bed are peculiar holes, pits, channels or downbulges of organic-rich marl, originally mud. These are not understood as yet but are considered in terms of possible theories of origin.

Primary Origin:

These might be primary sedimentary features. They could be some type of palaeokarst features, although they do not appear to contain speleothems. They could be some type of channel but there is too much cantilever (overhang) for a normal channel, even a saltmarsh channel. They are not of appropriate morphology to be giant dinosaur footprints.

Rather more promising is the possibility that they might be "Gator Holes" made by alligators or crocodiles. Such reptiles that were very common in the Purbeck environment. The Swanage Crocodile - Goniopholis crassidens) is well known and there were other species present. See the section on a: crocodile from the Shed Bed - Shingle junction, Intermarine Member, Durlston Bay.. Gator Holes are made at present in the Everglades of Florida. In times of drought alligators dig down into Gator Holes. However, this is merely a suggestion at present, and more study is needed and much better evidence found before such a colourful conclusion could be firmly supported!

Tectonic and/or Compaction Effects:

There is little doubt that the initial formation of the pits was some primary sedimentary or biological process. However, the strata have been heavily compacted, cemented and subject to some tectonism since Purbeck times. Closer examination of the the mud holes shows that they contain small faults or shears. Some tectonic or compaction effects seem to be present. The boundaries between carbonaceous marl and light grey marl, are mostly clear-cut joints or shears. However the fault are small and no large fault was observed.

It seems likely that the pits have been affected at least to some extent by some limited decollement movement at this weak horizon. There seems to have been small translation northward of the overlying strata. (Northward translation and decollement is also in the case in the Purbeck Broken Beds of Durlston Bay, a major decollement horizon. The Broken Beds have a large downbulge at Durlston Head. However, the Durlston Downbulge is associated with major and obvious faults, and these are lacking at the Mammal Bed pits.)

Preliminary Overview:

At present the origin of these holes or pits filled with carbonaceous marl is not satisfactorily understood. Further study is needed. They seem to be mainly of primary origin, with compaction and some minor tectonism superimposed. The mode of primary excavation of the holes or pits is the major problem.

MARLY FRESHWATER - MAMMAL BED continued

Mammal Bed - Fauna

Details of the invertebrate fauna of the Mammal Bed have been described in the classic work of Roy Clements as follows:
DB 83. Dark-grey, shelly, carbonaceous, calcareous clay and shale. Rests on an irregular surface of, and in part grades into the bed below. Gastropods very abundant. Ostracods common. Some silicification. Sample - Gastropods: Valvata helicelloides 386; Hydrobia 164; Planorbis fisheri 82; Physa bristovii 36; Ostracoda: Cypridea tumescens 48; C. dunkeri 17; C. granulosa s.s. 10; Scabriculocypris trapezoides 21; Damonella pygmaea 6; Mantelliana purbeckensis 5; Theriosynoecum forbesii 2; Darwinula leguminella 1; Fabanella boloniensis 1; Dicroryma sp. indet. 1.

In addition to gastropods mentioned above, Arkell (1941) has recorded the presence of Viviparus subangulatus shown here. Generally, Viviparus is not such reliable evidence of freshwater conditions as Planorbis , and can tolerate some small salt content, but in this case a truely freshwater environment seems likely.

Mammals include: the Multituberculate - Plagiaulax becklesii ; the Pantotheres - Amblotherium and five other genera; the carnivorous Triconodonts and Symmetrodonts - Triconodon mordax, Trioracodon ferox, Spalacotherium and Peralestes , according to Arkell (1947).

Chemical composition of this bed has been given by El-Shahat (1977). The details are not repeated here but a few salient points only are mentioned. The clay with carbonate mineral composition means that calcium oxide is lower than in the beds above at 17 per cent. Silica is, not surprisingly, higher at 40 percent. Phosphorus (1043 ppm) is three times that in the beds above, presumably because of the vertebrate content. Strontium is quite high at 987 ppm, probably because of present (now only 5 percent) or former aragonite content. It is interesting to note that magnesium is three times as high as in beds above (magnesium oxide 8.16 percent). Although dolomite has not been recorded it should be noted that beds containing dolomite and palygorskite occur in the Lower Purbeck underneath and this may be almost the last relic of a magnesium-rich lake.

In terms of palaeoenvironment, the Mammal Bed is the deposit of a shallow lake with freshwater molluscs and accumulated vertebrate debris, perhaps washed into the marginal shore region. The continuation of this lake deposit has not been found at the cliff exposures to the west, presumably because at this time these more positive areas, out of the main basin, would have been land. The Mammal Bed Lake presumably had adequate pond weed for the molluscs to live on.

The White Put, near the base of the Cherty Freshwater Member, is a freshwater lake deposit with insects and fish. It consists of about 67 percent calcite and 21 percent clay and has only a little aragonite (2 percent) and very little quartz (1 percent). High strontium at 1870 ppm may suggest that the White Put was originally an aragonitic lake deposit. This bed has two or three times as much magnesium as adjacent beds but has not got high levels (magnesium oxide - 4.95 percent). The White Put Lake still had some magnesium-rich water chemistry, but as the climate was becoming more humid the magnesium-rich lake environments were becoming less well-developed. Planorbis fisheri, Physa bristoviiand Cypridea dunkeri shows that the salinities were often low (although, of course, salinities were variable because of the seasonality - the ostracod listing in Clements may aid understanding of this).

Higher in the section also within the Cherty Freshwater Member a conspicuous horizon higher in the sequence is the shale of the Fern Bed discovered by Mr Coram. DB 92 contains insects and fish. It also has celestite and DB 93 above has high strontium on analysis. Although with some superficial similarities to the Mammal Bed these two argillaceous units do not have the high magnesium or phosphorus of that bed. Silica and aluminium is rather similar because of the clay content.

MARLY FRESHWATER MEMBER continued:

Beccles Mammal Pit - An Excavation for Creationism?

The most significant collection of mammals from the Purbeck Group was made by Mr. S. H. Beccles from his legendary excavation in 1857 on the cliffs of Durlston Bay. He systematically searched the Mammal Bed, DB 83, part of the Marly Freshwater Member, and a carbonaceous marl with a rich gastropod fauna. The excavation was in part due to the encouragement of Sir Richard Owen (the famous vertebrate palaeontology of the Natural History Museum, London) who had studied Purbeck mammal remains found earlier (Owen, 1854). The mammals from Beckles' Mammal Pit were described in detail by Owen (1871) in his monograph - " Fossil Mammalia of the Mesozoic Formations ". A briefer description and interpretation is in Owen (1861) a few comments from which are given in a lower section. The article below gives an account of the excavations at Beckles' Mammal Pit in Durlston Bay , written at the time, and, understandably for that part of that century, with a creationist as opposed to a modern evolutionary viewpoint. For more information on Purbeck mammals see the paper by Kielan-Jawororowska and Ensom (1992).

GEOLOGICAL DISCOVERIES AT SWANAGE

(From a Correspondent)

(The Reverent Charles Kingsley. Illustrated London News, Dec. 26th 1857, pp 637-638)

I went this summer in search of Dr. James Hunt, who is now becoming well know as a curer of stammering, and who is, I am bound to say, the only man who ever seemed to me to possess any scientific or rational understanding of that most distressing malady. I found him at Swanage, a spot unknown, perhaps, to three-fourths of the readers of this paper, but one well worth seeing, and when once seen not easily to be forgotten.

At the east end of the Isle of Purbeck is a little semicircular bay, its northern horn formed by high cliffs of white chalk, ending in white isolated stacks and peaks, round whose feet the blue sea ripples for ever. In the centre of the bay the softer " wealden beds " have been worn away, forming an amphitheatre of low sand and clay cliffs. The southern horn is formed by the dark limestone beds of the Purbeck marble. A quaint old-world village slopes down to the water over the green downs, quarried, like some gigantic rabbit-burrow, with the stone workings of 700 years. Landlocked from every breeze, huge elms flourish on the dry sea beach, and the gayest and tenderest garder flowers bask under the hot stone walls. A pleasanter spot for summer sea-bathing is not to be found eastward of the Devon coast than Swanage, and all which is wanted to make it famous is houses into which visitors can put their heads at night.

As was to be expected from the variety of soils and the sheltered situation, I found the neighbourhood rich in rare plants and insects, the sea-beach strewn with numberless sea-weeds; but the great attraction of Swanage, to those who dabble in science, is the extraordinary number and value of its fossil remains.

The excavations, of which I send you a View, are interesting as the scenes of important geological discoveries. They owe their existence to the energy and acuteness of one man - S. H. Beccles, F.G.S., Esq., who began them in December last, with the view of ascertaining if mammalia, or other air-breathing animals of a high order, existed in any number during the age in which the Secondary rocks were deposited.

A mammal jaw has already been discovered by Mr. Brodie on the shore at the back of Swanage Point. Mr. Beckles's business was to trace the vein from which this jaw had been procured, through its course along the cliff above, and to search it as thoroughly as he could. With that practical sagacity and zeal which distinguishes many of our scientific men, he found the precious vein (a stratum about five inches thick, at the base of the Middle Purbeck beds), and set to work. Before he could lay it barehe had to remove a superincumbent load of fifty-two feet thick, forty feet of which was solid rock; and again and again, after losing the vein in where it was shifted and snapped by earthquake " faults " , to try fresh cuttings at fresh points of the cliff. In nine months he removed many thousands tons of rock, and laid bare an area of nearly seven thousand square feet (the largest cutting ever made for purely scientific purposes), and even more difficult, he educated the more intelligent of his Swanage workmen into trusty and observant fossil hunters.

A gentleman, who thus for the pure sake of science, without a thought of gain, devoted his money and nine months of his life to the drudgery of a common stonebreaker, deserves success. And Mr. Beccles has attained what he deserved. Reptiles (tortoises and lizards) he found in hundreds; but the most important discovery was that of the jaws of at least fourteen different species of mammals. Some of these were herbivorous, some carnivorous, connected (we understand) with our modern shrews, moles, hedgehogs, etc., but all of them perfectly developed and highly organised quadrupeds. The detailed results of Mr. Beckles's labour will shortly be published in the Transactions of the Geological Society.

To which our readers may answer " Cui bono ". All this may be very amusing, curious, but what is its use? Its use is this. It was supposed till very lately that few if any mammals were to be found below the Tertiary rocks, i.e., those above the chalk; and this supposed fact was very comfortable to those who support the doctrine of " progressive development ", and hold with the notorious " Vestiges of Creation " that a fish by mere length of time became a reptile, a lemur, an ape, and finally an ape a man. But here, as in a hundred other cases, facts, when duly investigated, are against their theory. A very ancient bed of the secondary rocks is found full of mammalia, as perfect as most which now walk upon this earth; and Mr. Beckles's discovery gives strength to the theory of best scientific men, that not merely species, but whole orders, were created from time to time by some act of the Almighty mind, as perfect at the first moment of their existence as at any subsequent one. Thus are the conclusions of sound science shown more and more to coincide with those of sound religion; and every man who, like Mr. Beccles, by discovering physical truth helps the cause of spiritual truth, deserves well of his country even though all he visibly brings them be a few jaws of unmarketable vermin.

C. Kingsley

(This article, without the engraving, is also largely reproduced in Brannon, 1860 . This author further commented that the original discovery of remains in this bed was made by Mr. W. R. Brodie. He also pointed out that the Mammal Bed in Beckles' excavation " was covered by a thin and very regular bed of flinty stone, that when bared from the great mass of superincumbent rock, had the appearance of an ancient pavement, being naturally parted into irregular polygonal forms and lying for the most part quite smooth. Not the least intesting circumstances ascertained by these excavations were the number and character of the transverse and oblique faults, the concomitants of the great longitudinal one, and traces of which may still be discerned. " )

MARLY FRESHWATER MEMBER continued

Mammal Bed

Sir Richard Owen - Uniformitarianism, Super-Intelligence and Petty Wars against Cockroaches.

In the middle of the 19th Century, Richard Owen, F.R.S., (later Sir Richard Owen) Superintendent of the Natural History Departments in the British Museum, was probably the most famous palaeontologist and comparative anatomist in Britain at the time (a great man but he could at times be a little sharped tongued; he once greatly insulted a famous palaeontologist - Hugh Falconer - by calling him a " distinguised botanist"!). As mentioned above, he had encouraged the excavation by Mr Beccles, and he wrote many papers relating to Purbeck vertebrate fossils.

He noted that the early mammals including those from the Purbeck (and late Triassic of Frome and Middle Jurassic Stonesfield Slate) were of small species which he believed belonged to the " low organised Marsupalia " or possibly the Insectivorous order (Owen, 1861) . He saw the advance from these and other fossil forms, commenting that "there has been a succession of species illustrating in the main the progressive perfection of the nervous system and concomitant predominance of mind over matter. " he realised that " organic remains, are succeeded, one by another, to the present period, and never re-appear when once lost sight of in the ascending search.".

Owen then considered such evidence as against the doctrine of Uniformitarianism (the, now, generally accepted principle that the present is the key to the past). Of course, if you consider species of organisms you cannot go very far back in comparing the present with the past. Modern versions of Uniformitarianism do not suggest this, but are general principles for interpreting the evidence of ancient environments, ways of life and deposition of sediments. In fact, Owen used uniformitarian principles in this ways to understand the Purbeck land and lagoons.

Richard Owen was against the evolutionary theory, that in his opinion diverted from "the gold centre of truth". He was too knowledgeable, however, to be expecting to find, like the Reverent Kingley, a modern mammalian fauna in the Purbecks, and saw the extinctions and advances through geological time. Owen's explanation for this was that the Great Cause is both active and anticipatory in planning advancement, including that of the mammalian brain. Thus, his comparison of primitive Purbeck and other early mammalian fossils with present creatures resulted in an optimistic look to a future " period when the earth may become the abode of a higher race of intelligences.".

We turn from the philosophical problems posed by the Durlston Bay fossils to the detailed interpretation. Owen (1861) tried to understand the classification and feeding behaviour of the mammals. These are very old views, though, and should be treated with caution. Modern papers should be consulted for a better understanding.

Triconodon was considered a marsupial by Owen. Plagiaulax becklesii and the smaller Plagiaulax minor had large front teeth to pierce, retain and kill. The succeeding teeth are like shears. The premolar has some resemblance to that of the kangeroo-rat. He considered Plagiaulax to be carnivorous marsupial which probably lived on small mammals and lizards.

Spalacotherium with its ten multicuspid molar teeth had jaws adapted to the piercing and crushing of the tough chitinous cases and elytra of insects (there were many insects in the Purbeck environments, especially cockroaches - see Jarzembowski papers). " The particular modification of the pointed cusps, as to number, proportion, and relative position, resembles in some degree that of the Cape Mole (Chrysochlora aurea). He noted the association of insects and cycadophyte plants in the Purbecks. In warmer climates at the present time the enemies of the insect class are bats, small land mammals and lizards which carry on their "petty warfare" on insects simultaneously. He said " no surprise need therefore be felt at the discovery that Mammals and Lizards co-operated simultaneously and in the same locality at the same task of restraining the undue increase of insect life during the period of deposition of the Purbeck beds." He regarded numbers of Purbeck insects as part of an ordained plan.

The relationship of the Purbeck "dwarf" crocodiles to the mammals was discussed by Owen in 1879. Numerous remains of small crocodiles (Nannosuchus and Theriosuchus), resembling broad-faced alligators but only about half a metre in length, were found during the course of Mr Beckles' excavations and were said to come from the "Feather-bed marl" (I wonder what exactly is meant by this. Did these remains come not from the Mammal Bed but from marl near the Feather Bed, that is somewhere near the Sly Bed which has produced so much at Sunnydown?). Owen had considered it likely that the mammals kept down the crocodile numbers by destroying their eggs and young, and quoted a modern analogue for this. His anatomical studies produced no sign that there was a direct air connection from the nose to the windpipe with separation from the mouth. He thought, therefore, that the crocodiles could not kill moderate-sized mammals by drowning because of the problem of water entering the mouth. He thought they had to be confined to small prey. As part of his argument he said " Let any Fellow of the Geological Society, with his head under water, hold his nose and open his mouth, and he will experience some trouble at the glottis. " I am afraid that I am a Fellow who will not try this!

MARLY FRESHWATER MEMBER continued:

Mammals - the Floating Corpses

Mansel-Pleydell (1888)discussed the predominant occurrence of fossil jaws.

'Ten genera and twenty-five species of marsupial mammals, from the size of a mole to that of a polecat, were found associated with these lizards. The majority of the remains consisted of the lower-jaws, and a very few of the upper maxillaries. There has been no instant of the recovery of an entire skeleton nor of several bones in juxaposition to each other; to account for this Dr Buckland suggested the possibility that the corpses of drowned animals, distended by gases during putrification, while floating in the water would lose their hanging lower-jaws, while the rest of the body would drift into the sea.'

MARLY FRESHWATER MEMBER continued:

Beccles Mammal Pit - Strata Present.

A modern photograph from the cliff at the beach (exposure in northen part of the bay) has been added to the 1857 picture from the Illustrated London News. The beds present in the Mammal Pit are easily recognisable. Incidently, both the Mammal Bed, DB 83 and the Fern Bed etc DB 92,93 were referred to as "Dirt Beds" in Fisher (1856), although this is undoubtedly a quarrying description, rather than an interpretation of these as palaeosols like the Great Dirt Bed. They are not the same showing evidence of lake or marsh sedimentation.

Willet (noted in Heap, 1957) made a further extensive but unsucessful excavation near the pit in 1880. He noted that "Mr Beccles, hoping to resume his labours, had purposely cast down a large projecting accumulation of debris." Perhaps it is that pile which is visible in the etching, made from a photograph. Heap in 1957, with the aid of a young mining student, Derek Rendell, found Beccles' pit at the lowest point of the garden of a house known as Durlston Cliff. It was overgrown and partly flagged. Heap made a search of Mammal Bed material from here but did not find any fossils of note.

Miscellaneous - Unidentified Jaw

The jaw with teeth shown above was found by a student in 1991, Heather. It is has not been identified and its whereabouts at present is not known. It was ex situ but found in Durlston Bay and is almost certainly from the Purbeck Group, although the particular part is not known. It may be from the Middle Purbecks or possibly from the Upper Purbeck Broken Shell Limestone. At present it is a mystery.

ACKNOWLEDGEMENTS

The Faculty of Natural and Environmental Sciences, Southampton University is particularly thanked for having hosted this website and supported the website activity of Dr. Ian West for many years. I particularly acknowledge the meticulous field and laboratory studies of Professor Adam El-Shahat of El-Mansuara University, Egypt. He thoroughly investigated the thin-section petrography and geochemistry of the Middle Purbeck strata of Durlston Bay and Purbeck Stone quarries of the Isle of Purbeck. I am very grateful for his permission to publish further aspects of his work, which was undertaken under my supervision. It is very kind of Roy Clements who many years ago gave me permission to use parts of his superb and classic descriptive logs of the Purbeck Group of Durlston Bay. Everyone visiting Durlston Bay should be carrying a copy of Clements' Log. I thank the many people who have discussed the strata in Durlston Bay with me including Professor Jane Francis, Richard Edmonds, Earth Science Manager of the Dorset and East Devon World Heritage Coast, various members of the Dorset DIGS group, Robert Coram and many others. Trelevyn Haysom, the quarry owner and quarrying expert, has been extremely helpful in disussing the quarrying bed names of Purbeck strata and for pointing out many features of geological interest in the Purbeck Group of the Swanage area.

See also for more related information on the central part of Durlston Bay: Durlston Bay - Central Zigzag Part & Coast Erosion

See also:

Durlston Bay - Part 1: Introduction, Fossils & Upper Purbecks
Durlston Bay - Part 3: Lower Purbecks & Miscellaneous
Durlston Bay - Part 4: Bibliography and References
Purbeck Group - Bibliography and References

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Webpage - written and produced by:

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

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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.

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