West, Ian M. 2014. Durlston Bay - Peveril Point, Durlston Formation, including Upper Purbeck Group: Geology of the Wessex Coast (Jurassic Coast, UNESCO World Heritage Site). Internet geological field guide. http://www.southampton.ac.uk/~imw/Durlston-Bay-Peveril-Point.htm. By Ian West, Romsey and School of Ocean and Earth Science, National Oceanography Centre Southampton, Southampton University. Version: 10th April 2014.
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Durston Bay, Peveril Point, heading

by: Ian West
Romsey, Hampshire,
and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Website hosted by iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.
Website archived at the British Library

Home and List of Webpages |Field Guide Maps and Introduction| |Durlston Bay, Swanage, Middle Purbeck |Durlston Bay - Lower Purbeck |Durlston Bay - Central Zigzag Part and Coast Erosion |Durlston Head - Lower Purbeck Group & Portland Stone |Ridgeway Railway Cutting and Weymouth Relief Road (with Purbeck Group) |Isle of Portland | Lulworth Cove | Purbeck Palaeoenvironments | | Purbeck Bibliography |

Click here for the full LIST OF WEBPAGES

(You can download this educational site to SurfOffline or similar software to keep an offline copy, but note that updating of the live version takes place periodically. SurfOffline)

IMAGES seen at first are only low-resolution version and should be zoomed. Please click on images for full, high resolution versions.

A general view of Peveril Point from the cliff-top in Durlston Bay, Dorset, August 2011

An overview of the Middle to Upper Purbeck section in Durlston Bay, Dorset, from the Cinder Bed Ledge northward towards Peveril Point, 2007

Tidal rapids formed by the spring tide ebbing fast over Peveril Ledge, Peveril Point, Durlston Bay, Swanage, Dorset, 9th November 2007

Sally in the syncline - of Purbeck Marble, at Peveril Point, Durlston Bay, Swanage, Dorset, August 2011

The north limb of the syncline in the Broken Shell Limestone, Upper Purbeck Group, Durlston Bay, Dorset, August 2011

Broken Shell Limestone and two of the Purbeck Marble Beds at Peveril Point, Durlston Bay, Swanage, Dorset, 22 Nov 2007

Durlston Head viewed across Durlston Bay from Peveril Point, in rain, July 2007


Durlston Bay, - Peveril Point, Upper Purbeck Group or Durlston Formation in part
Durlston Bay, Swanage, Middle Purbeck
Durlston Bay - Lower Purbeck Group (Jurassic-Cretaceous)
Durlston Bay - Central Zigzag Part and Coast Erosion
Durlston Head - Lower Purbeck Group & Portland Stone
Durlston Bay - Bibliography

Related Field Guides --- |..Mupe Bay.. |..Worbarrow Bay.. |..Lulworth Cove.. |Purbeck Palaeoenvironments | |Purbeck Group Bibliography |



Durlston Bay (or Durlstone Bay or Durdlestone Bay), Swanage (Dorset, southern England on the English Channel coast) provides the geological type-section of the Purbeck Group (" Purbeckian " - or "Purbeckien ") and the geology has been much studied since it was first described by Thomas Webster in 1816. This classic Purbeck section of lagoonal and lacustrine limestones alternating with shales and marls is mainly of Berriasian (early Cretaceous) age (with some Upper Jurassic near the base). Shelly limestones, the Purbeck Stone and Purbeck Marble, have long been quarried. Fossil remains of dinosaurs, mammals, turtles, pterosaurs, crocodiles, fish, isopods and insects have been found here. In the 19th Century famous discoveries at Beckle's Mammal Pit were even used for creationist versus evolutionary arguments. Ostracods are common throughout and are used to determine palaeosalinities. Charophyte algae occur in low salinity beds. Evaporites, including gypsum and celestite, are common in the lower part. 246 numbered beds have been described in some detail by Clements (1969; 1993). Some of these subdivided. Many of the beds are also named and the ostracod and gastropod content of each is known. Petrographic, magnetostratigraphic, geochemical and clay mineralogical data exists for large parts of the succession. The Jurassic - Cretaceous boundary is somewhere near the base of the Purbeck succession, which is probably mostly Berriasian in age.

This is one of several webpages on Durlston Bay. Also associated with it are bibliographies and sections on Purbeck Facies. The pages are listed below:

Durlston Bay - Peveril Point -Upper Purbecks
Durlston Bay - Middle Purbecks (Lulworth to Durlston Formation) and Building Stones
Durlston Bay - Zigzag Path and Central Part
Durlston Bay - Lower Purbeck Group
Durlston Bay - Bibliography
Purbeck Group - Bibliography
Purbeck Facies and Palaeoenvironments
Purbeck Evaporites

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INTRODUCTION:

Safety

Durlston Bay is an extremely interesting place that is valuable in both educational and scientific terms. Like other Dorset coast sections it can present some significant risks. 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 unwise 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 can be useful here. It is essential to make sensible observation of the state of the cliffs at the time of a visit to be sure that debris is not falling. There is some risk near Peveril Point, with a greater main hazard area being further south where the cliffs are higher. However, as shown in photographs, near Peveril Point the Broken Shell Limestone may fracture and fall to the shore. This is fairly common and can be quite dangerous in places.

Hazard area, southwest of Peveril Point, Durlston Bay, Swanage, Dorset, where the Broken Shell Limestone is unstable and has an open fissure, November 2007

The Broken Shell Limestone to the southwest of Peveril Point was in unstable condition in November 2007 and much material has fallen there since then. Further falls are likely to occur from time to time. If the conditions are bad or the bay seems unsafe try an alternative and safer location.

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INTRODUCTION:

Location and Access

Durlston Bay is situated adjacent to the holiday town of Swanage on the south coast of England. It is a small seaside town with all essential facilities and places to stay. In the summer it is busy with visitors and, at all times of the year it is a popular centre for coastal walks in the Isle of Purbeck. Geologists of various specialisations frequently visit Durlston Bay.

Study of Durlston Bay can commence at Peveril Point at the north end of the bay (an alternative is to start at the Zigzag Path in the middle of the bay and proceed northward. You cannot park at Peveril Point, but you can usually park a car either in the large car park on the hill south of the pier.

If you are arriving with a coach you can drop the party near Swanage Pier. From here a party can walk to Peveril Point and arrangements can be made for them to be collected at a specific time later. Coaches or buses cannot be parked near to Durlston Bay and have to be left in the main coach park, on the northwest side of the town.

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INTRODUCTION:

Location and Historic Maps

Victorian map of the Isle of Purbeck, showing Durlston Bay

Old location map for Durlston Bay, Swanage, Dorset and surrounding area, from the 1950s

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INTRODUCTION:

Aerial Photographs

A composite aerial view of Peveril Point, Durlston Bay, Swanage showing limestone reefs or ledges and some submarine outcrops of Upper Purbeck strata, Channel Coastal Observatory, 7th May 2008, SZ0378NE etc

An aerial view of Durlston Bay, Swanage, Dorset, courtesy of the Channel Coastal Observatory

An aerial view of Peveril Point at the north end of Durlston Bay, Dorset, photograph courtesy of the Channel Coastal Observatory

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INTRODUCTION:

Geological Maps - General

The 2000 edition of the 1:50,000 British Geological Survey Map, Swanage, Sheet 343 and part of 342, Solid and Drift - including the Isle of Purbeck and Lulworth Cove

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.

Part of the geological map for Swanage, 2000 edition, redrawn and showing the Durlston Bay and Anvil Point area

The old 1895 geological map of Swanage, Dorset, sheet 343, - see also the new 2000 edition BGS geological map of Swanage, sheets 342 and part of 343

Geological map, 1907

Geological map of the Swanage area, 1890

Enlarged geological map, 1890

In addition to an old location map, above, some old geological maps from books are provided here to show the geological setting. One is from Woodward (1907), another is a hand-painted one from Braye (1890). This is also shown enlarged in part with details of Durlston Bay. Although the details of the town and roads have changed the geology shown here is basically correct. It is strongly recommended that you obtain the current geological map of the British Geological Survey, Geological Map - Swanage, Sheet 342 east and part of 343 (2000 edition. Solid and Drift Edition. 1:50,000. There is a much older accompanying explanatory memoir by Arkell (1947). The present map - sheet 343 and 342 is broadly similar but the details of the faults are corrected and some additional faults are shown.

Durlston Bay is at the eastern end of the onshore outcrop of the Purbeck Group, an upland with stone wall and quarries. To the north the Wealden strata crop out, and, further north the Cretaceous Chalk forms an east-west trending ridge, the Purbeck Hills.

Melville and Freshney (1982) have provided a brief introduction to the area. The old British Geological Survey memoir by Arkell (1947) is still useful but is, of course, badly outdated now and so much on Durlston Bay has been published since. The field guide by House (1993) is recommended. Absolutely essential for any serious study of the Purbeck strata in Durlston Bay is Clements' Log (1969; 1993), a detailed listing of the strata by Dr. Roy Clements. There are numerous other publications relating to Durlston Bay, many of which are given in the Durlston Bay bibliography.

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GEOLOGICAL MAPS:

Peveril Point - Details

Geological map of Peveril Point, Swanage, 2007

At Peveril Point there are present examples of the Lulworth Crumples, but in relatively low-dipping Purbeck strata. Important evidence might be provided regarding the question of northward or southward movement of the Wealden strata over Purbeck strata (and old and familiar dispute with regard to Stair Hole and Lulworth Cove). The detailed, large-scale, geological map of the Peveril Point area, shown above, is modified and redrawn but mainly based on Cosgrove and Hearn (1966) (following initiation of the mapping by Ian West). Because of some simplification and particularly because proven and speculative bed boundaries are not separated on this version, it is necessary to refer to the original if detailed study is to take place. Note that the beach changes and new exposures may occur and these may lead to improvement in the map. Careful observation of the details of the shore at low tide at Peveril Point is required.

The small syncline in the Blue Marble at Peveril Point seen in relation to the south limb of the syncline in the Broken Shell Limestone, 2011

The photograph above, at mid-tide, is given to facilitate understanding of the large-scale geological map and, more specifically, to introduce the reader to some interesting problems of the synclinal structures (and associated anticline). Observe from map and photograph, that the axial plane of the syncline is not vertical, but is steeply north-dipping. If the mapping of anticline to the north (with a Chief Beef Member core) is studied then it will be seen that the axial plane of this is south dipping (the fold is is locally overturned towards the north). However, to really understand the structures here a good north-section cross-section is needed. (This is set as an exercise further below)

An example part of the the geological map and section of northern Durlston Bay, Dorset, prepared by Dr. John Nunn in 1991

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.

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GEOLOGICAL MAPS continued:

Exercises for Students (re Peveril Point)

Part 1. (elementary - 45 minutes)

Draw a simplified north-south, cross-section through the Peveril Point structure showing both syncline and anticline. Use typical dips for the limbs and close the folds above sea-level and at depth in a speculative but reasonable manner. Use the same vertical and horizontal scale. For thicknesses of units use data in Cosgrove and Hearn (1966) and in Clements (1993).

Part 2. (more advanced essay - 120 minutes)

Discuss the origin of this structure in the light of studies of the Lulworth Crumples and the general structure of the Wessex Basin (see Underhill etc and earlier papers)

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INTRODUCTION:

Chronostratigraphy

Purbeck strata at Mount Saleve, Geneva, Switzerland

The type-section of the Purbeck Group is at Durlston Bay, Dorset, and that section is described in this and the related webpages. The Purbeck ("Purbeckien") is, however, an Upper Jurassic to Early Cretaceous Formation that is well-known in France, Switzerland and elsewhere. It is very well-exposed in the Jura Mountains.

In the past the Purbeck Group has sometimes been placed in the Jurassic and sometimes in the Cretaceous System. The position since the 1950s has been briefly discussed by Rasnitsyn, Jarzembowski and Ross (1998). Casey (1963) put the Jurassic/Cretaceous (Tithonian/Berriasian) boundary at the base of the Cinder Member on the basis of recognising transgressions and making a correlation with northeast England and the Russian area. It was then realised that correlation to the Jurassic/Cretaceous boundary in the south of France indicated a much lower level. On the basis of palynomorphs, it was suggested that the boundary is just above the base of the "Cypris" Freestones (Hunt, 1987). This is supported by charophyte evidence which suggests that the boundary occurs at the base of this member (Feist et al., 1995). Allen and Wimbledon (1991) indicated that the boundary lies right at the base of the Purbeck, using palynomorphs, whereas Ogg et al. (1994) suggested that it occurs between the lower "Cypris" Freestones and the base of the Cinder Member based on magnetostratigraphy. It seems sensible to support the view of Rasnitsyn, Jarzembowski and Ross (1998) that Feist et al. (1995) is right in using the base of the Cypris Freestones as the boundary, and that procedure is adopted here.

The Berriasian/Valanginian boundary was considered to lie at the very top of the Purbeck Group based on palynomorphs (Allen and Wimbledon, 1991). However, Ogg et al. (1994) suggested that it occurs just below the top based on magnetostratigraphy. Again the view of Rasnitsyn, Jarzembowski and Ross (1998) is followed and they take Allen and Wimbledon's scheme, so that the top of the Berriasian Stage corresponds to the top of the Purbeck Group in Durlston Bay and the top of the Cypridea setina ostracod zone.

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PURBECK GROUP (LULWORTH AND DURLSTON FORMATIONS)

Introduction - Isopach Map

An isopach (isopachyte) map for the Purbeck Group of southern England, based on, and modified after, Howitt (1964), with contours in feet, and with additional interpretation

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INTRODUCTION:

Durlston Bay Cliff Section

A simplified cliff section of Durlston Bay, Dorsetj, showing the Purbeck type-section, Upper Tithonian to Berriasian, with double vertical exaggeration

Part of Durlston Bay, Dorset, with the Purbeck Group dipping northward, seen from the sea in an old photograph taken on the 23rd September 1997 from the Waverley

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)

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INTRODUCTION:

General Lithology of the Purbeck Group

Thin-bedded lagoonal limestones of bivalves such as Neomiodon alternate with shales. This part of the succession is high in the Middle Purbeck Group. Evaporites are rare here and plant debris and pyrite are more abundant (the iron-staining is the result of oxidation of pyrite in the Chief Beef Member with its high organic content ). Kaolinite also occurs here amongst the clay minerals in this sub-humid part of the succession. Some of the biosparrudite limestones were cemented early with early loss of aragonite and these are particularly prone to preserve dinosaur footprints. Other thin shell beds remained at or just below the water-table and when buried preserved aragonite (and consequently high strontium). See El-Shahat and West for details.

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PALAEONTOLOGY:

Some Purbeck Fossils

Fossils from Durlston Bay and Other Dorset Localities

Purbeck Fossils - modified from Arkell

Purbeck Fossils - old illustration

These images show some common Purbeck fossils. One is modified after Arkell (1947) and the other after Woodward (1907b).

Further details: The illustrations of Planorbis, Valvata and Hydrobia are enlarged to twice the size in relation to the others. The vertical length of the the sketch of the oyster Praeexogyra distorta in the centre is 4 cm which provides a scale for the others. This species is shown bluish because in the Cinder Bed and adjacent strata it is often blue to black. The Viviparus shells in the Purbeck marble are often bluish or green-stained with glauconite.

The list of species with more detailed information follows:
Hemicidaris purbeckensis Forbes (after de Loriol), the echinoid of the Cinder Bed and indicating marine conditions, spines more common than the test;
Physa bristovii (Forbes M.S.) Phillips, freshwater pondsnail of the Cherty Freshwater Member, Poxwell road cutting, east of Weymouth;
Viviparus inflatus (Sandberger), a common species of the Purbeck Marble; Viviparus cariniferus (J. de C. Sowerby), cast, common in the Purbeck marble;
Viviparus subangulatus (Roemer), pondsnail of the Cherty Freshwater Member;
Ptychostylus philippii (Dunker), freshwater pondsnail;
Ptychostylus harpaeformis (Koch and Dunker), a Cherty Freshwater Member pondsnail;
Ptychostylus intermediate form;
Planorbis fisheri Arkell, a freshwater pondsnail;
Valvata helicoides , a euryhaline mollusc not to be confused with Planorbis and often found in the basal Purbecks;
Hydrobia chopardiana (de Loriol), a common but small gastropod rather like the modern
Hydrobia abundant on salt-marshes and probably euryhaline;
Promathildia microbinaria Arkell, a marine gastropod of the Corbula Member;
Pachychilus manselli (de Loriol), another marine gastropod of the Corbula Member;
Serpula coacervata Blumenbach (after Dunker), small worm tubes common in the Soft Cockle Member and in the Serpulite of Germany which may be of about the same age;
Corbula durlstonensis Maillard, a small marine or near-marine bivalve from the Corbula Member;
Corbula alata J. de C. Sowerby, the most common Corbula of the near-marine Corbula Member;
Praeexogyra distorta (J. de C. Sowerby), a small lagoonal oyster of the Cinder Bed and Intermarine Member, formerly known as Ostrea distorta and superficially resembling Crassostrea virginica of the Texas coastal lagoons and of a similarly sheltered, but more extensive near-marine habitat;
Neomiodon medium (J. de C. Sowerby), a euryhaline lagoonal bivalve, common in the Middle Purbecks and forming the shell beds which are now the Purbeck Building Stones;
Protocardia purbeckensis (de Loriol), a small euryhaline bivalve able to tolerate hypersaline conditions like the modern dwarf Cardium or Cerastoderma of northern Egyptian coastal lagoons today, it is the cockle of the Hard and the Soft Cockle Members;
Unio porrectus J. de C. Sowerby, the river and lake mussel that occurs in the Upper Purbeck where the conditions are approaching the fluvial environments of the Wealden above;
Archaeoniscus brodiei Milne-Edwards (restoration after Haack), an isopod like a sea-slater that lived on the lagoon margins and is preserved as impressions in fine-grained argillaceous limestones at several horizons, in some cases with insect remains.

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Purbeck Fossils - Vertebrates

Jaws and teeth from the  Mammal Pit


Purbeck Fossils - Arthropods

Archaeoniscus brodiei



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Purbeck Fossils - Echinoids

Hemicidaris purbeckensis

It was said by Arkell (1947) that the only Purbeckian echinoderm is Hemicidaris purbeckensis Forbes, shown here, which is indistinguishable from specimens found in the "Portlandien inferieur" (Upper Kimmeridgian) of the Boulonnais (de Loriol, 1866) . In England it is confined to the Cinder Bed of Durlston Bay where is usually very rare. Spines of this echinoid can be found, though, with a little luck, during a brief field trip to the bay. The central to lower part of the Cinder Bed should be searched. They can be found without difficulty at the southern Cinder Bed promontory.

After a gap of 75 years in the discovery of a complete test, Professor Hawkins of Reading University, found 38 crushed tests in the Cinder Bed at Durlston Bay in only a few days of searching. He also found that there is another echinoid present. He did not give a specific name but referred to it as "Pseudodiadema" sensu latissimo.

The original discovery of the Jurassic echinoid Hemicidaris purbeckensis in the Purbeck Group by Professor Edward Forbes led to the unit being placed in the Jurassic System. It is now, however, regarded as mainly Cretaceous (Berriasian).

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Purbeck Fossils - Gastropods

Purbeck gastropods

Physa bristovii

Ptychostylus

Viviparus subangulatus

Purbeck gastropods of Arkell (1940), including Lymnaea cf. physoides from the Lower Purbeck strata of the Ridgeway Cutting, near Upwey, Dorset Viviparus, Physa and Planorbis were freshwater lake-snails. Perhaps Viviparus could tolerate a small salt content in the water, but it is unlikely that the other two could, by comparison with modern analogues. Promathildia and Pachychilus occur in association with brackish to marine faunas. Some species of Hydrobia at the present day are euryhaline and specimens of this genus are often very abundant at the margins of estuaries or lagoons.

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Purbeck Fossils - Bivalves

Unio in limestone, Upper Purbeck

Modern Unio and Viviparus

Above: Purbeck "Unio's " in Upper Purbeck, freshwater, glauconitic limestone. Shown also, a modern Unio and Viviparus with some algae and other plant material; a close view of the Purbeck lake would have looked like this.

Fossils from the Corbula Member, Durlston Bay

Corbula, Neomiodon and other fossils from the Corbula Member, Durlston Bay. The limestone slab was not in situ but was found adjacent to the outcrop. These molluscs came from near-marine conditions; Corbula is commonly found in marine sedimentary sequences such as the Barton Clay of the local Eocene. The lagoon had clearly received an influx of seawater, but it was not sufficiently marine for echinoids or ammonites.

Praeexogyra distorta

The small Purbeck oyster Praeexogyra distorta (formerly Ostrea distorta) is abundant in the Middle Purbeck Group in strata of marine or near-marine origin.

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Purbeck Fossils - Charophytes

Please click on Charophytes
to go to the charophyte section.

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Purbeck Fossils - Foliage of Trees

Foliage from the Upper Purbeck

Foliage from the Upper Purbeck - detail

Plant remains found by Mr A.J. Holmes on 6th December, 1998 in a rock fall under the Coastguard Station at Peveril Point, Swanage. Tony Holmes reports that the bed appears to be DB 221 in the Unio Member. Further comments are given in the Unio Member section .

Left: The foliage appears to be of Araucaria type, that is with similarities to the foliage of the "Monkey Puzzle Tree " or Chile Pine. Above it is piece of foliage of Araucaria excelsa , the Norfolk Island Pine. Below is modern Cupressus foliage, to which previous finds of Purbeck fossil foliage have closer resemblance ( see illustrations of Cupressinocladus valdensis in Francis, 1983). This is in natural colours.

Right: An image of the same foliage with higher resolution, for detail.

Foliage from the Upper Purbeck in false colours

The foliage and sediment shown in false colours, to give an indication of the original appearance of the small piece of branch embedded in the shelly lake mud.

Purbeck trees and dinosaurs

Trees of a type (Araucaria) that might have been the source of the foliage shown above, are illustrated here at the margin of the Purbeck lagoon.

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THE PURBECK GROUP SUCCESSION AT DURLSTON BAY
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UPPER PURBECK STRATA (UPPER PART OF DURLSTON FORMATION, PURBECK GROUP)

Peveril Point, Swanage

The Upper Purbeck strata are only seen in the vicinity of Peveril Point. The exposure is good here, but the structure is a little complicated by minor folds of the Lulworth Crumple type.

Peveril Point from the adjacent cliffs, north end of Durlston Bay, Swanage, Dorset

Peveril Point and part of Durlston Bay, Dorset, photographed from the sea by Clive Boulter, 1997

From Peveril Point it is usually convenient to carry out the field trip southward along the beach. If there is then any problem with the tide or state of the cliffs you can easily retreat to Peveril Point. Of course, it is more systematic geologically to start at the base and work up, and that is possible, but not quite so practical for a party.

Because a southward route is used here, the succession is described in a downward sequence, at least in terms of members, even though this is not conventional in geology.

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STRATIGRAPHY:

Upper Ostracod Shales Member - Description

(formerly " Upper Cypris Clays and Shales ")

These grey shales, often pyritic and with some siderite contain numerous examples of the small ostracod, Cypridea. Notable features are three beds of " Purbeck Marble " . These are fine-grained limestones with numerous shells of pondsnails, or in more technical terms - Viviparus biomicrudites (Viviparus - a genus of pondsnails; bio - material of organic origin such as shells, mic - micrite or fine calcite matrix, rudite - coarse allochems or grains, ie the shells). These limestones are coloured by iron minerals. Pyrite is a common component in dispersed form and darkening the colour of the rock. Disseminated and granular glauconite (or a glauconite-type mineral) is responsible for greenish colours (in combination with other minerals it can produce a blue colour). Oxidised iron as limonite (including goethite) and hematite can produce brown and red respectively.

Syncline, part of a Lulworth Crumple at Peveril Point, Swanage, Dorset

Here at Peveril Point the Upper Ostracod Clays and Shales are well exposed. A small plunging syncline - the Blue Marble Syncline, can be seen. This structure is one of many minor folds within the Purbeck Monocline. Notice incidently the relationship of the jointing to the axis.

Notice the reef of Broken Shell Limestone (a bivalve biosparrudite), dipping parallel with one limb of the Blue Marble Syncline. Broken Shell Limestone ledges run out to sea for a considerable distance and 120 Danish ships were wrecked during a violent storm here in the year 877.

A small asymmetrical anticline in the Upper Cypris Clays and Shales at Peveril Point, Durlston Bay, Swanage, Dorset

Vertical Purbeck Marble at Peveril Point

The syncline, which we have seen, is part of some local but quite intense folding and faulting at Peveril Point. A small asymmetrical anticline is shown above. The structures are similar to and related to the Lulworth Crumples at Stair Hole and Lulworth Cove . As a result of the folding this bed of Purbeck Marble at Peveril Point, above which I am standing, is vertical in orientation, even though the general dip here is only about 4 degrees in a northward direction. The bed is more than a metre in total thickness but splits into several layers. You can easily distinguish Purbeck Marble from the other limestones around here because it consists of gastropod shells are almost circular in cross-section in a fine-grained calcite matrix, and the rock has a greenish, bluish or reddish tinge. Other limestones consist of crushed bivalve shells and are quite different in appearance.

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STRATIGRAPHY:

Upper Ostracod Shales Member - Purbeck Marble

Purbeck Marble, the Red Marble, Peveril Point, Durlston Bay, Dorset, 2011

Purbeck Marble at Peveril Point, Swanage

Purbeck marble fragment from Stone Point, Hampshire

A thin section of Purbeck Marble, a biomicrudite with Viviparus gastropods, seen at low magnification

An image above shows a close view of Purbeck marble. This is a weathered slab on the beach. The gastropods are visible. This rock was originally quarried from the beach here and later inland further west. It has been extensively used for carvings in churches before alabaster was used as a replacement. The Viviparus pond-snails lived in an almost freshwater lake. Their shells are in a matrix of lime-mud (now micrite) and they are darkened by some pyrite and, in some cases, by authigenic glauconite. This particular Purbeck Marble specimen, which was not in situ , has an overall bluish-green colour.

Amongst the photographs above is a fragment of Purbeck Marble from an old shipwrecked slab at Stone Point near Lepe Beach, , Hampshire, south of the New Forest. This is shown as though through a hand-lens. It is typical of Purbeck Marble. The rock is a fine-grained and easily carved limestone, a type of freestone without internal bedding planes on which it might split. It consists of shells of the lake-snail Viviparus. Arkell (1947) identified the particular gastropod as Viviparus cariniferus (J. de C. Sowerby). The gastropod diameters in the sample here range from 2mm to about 3.5 mm. In addition to the complete gastropods there is a small proportion of broken shell material and some "nesting". Glauconite is of the dispersed type, rather than as obvious grains, and is frequently within gastropod shells. The presence of authigenic glauconite in a low-salinity, almost freshwater, bed has for a long time been a topic of interest to sedimentologists. Some reworked Portland glauconite does occur in the Upper Purbeck strata but there is little doubt that in addition, as in this case, glauconite originated in the nearly freshwater lake, perhaps partly because of the the significant iron input near the Purbeck-Wealden junction. The presence of allochems larger than 1mm in a pale buff micritic matrix places this rock as a biomicrudite according to Folk's Classification. Students may refer to it as a "biomicrite" using Folk in basic form for simplicity, but in fact it is really in the rudite category. In terms of textural maturity it is a packed biomicrudite. Using Dunham's Classification it is a gastropod packstone. Although it is a feature that is normally more easily seen in thin-section, even with the hand-lens some geopetal fabrics are visible. These consist of partial fillings of the lower part of gastropods with micritic sediment, the remainder, formerly empty part, of the shell interior being occupied by sparry calcite. If it was necessary to do so, it is possible to recognise the original "way-up" of the specimen.

In some cases the Purbeck Marble is green and oxidation can produce reddish and brown colours. Purbeck Marble has been much used for interior ornamental work in churches and cathedrals and has also been used for large triangular stone anchors. An effigy of King John was carved from this in 1240 and in Worcester Cathedral (Drury, 1948). Such effigies were often painted in colour.

Unio, the freshwater mussel, and usually Unio porrectus here, is common in a thin green, sandy limestone, the Unio Bed. North of Weymouth, at Friar Waddon (SY 644857) the probable equivalent of this bed (or possibly the equivalent of part of the underlying Unio Member) contains, not only many teeth of sharks such as Hybodus, but also pebbles of Portland Chert and derived phosphatic ammonites from the Kimmeridge Clay (West and Hooper, 1969; West, 1979).

Do not be confused by the fact that the Unio Bed occurs within the thin unit here, called the Unio Bed and that Unio is also abundant in the thicker Unio Member beneath the Upper Ostracod Shales. The Unio Bed that we are concerned with now is a conspicuous greenish bed with " Unio " occurs in association with pyritic shales. Reworked Portland and Kimmeridge detritus occurs in the region at this level and earthquake activity associated with the Late Kimmerian movements may be responsible for the liquefaction seen at Stair Hole, Lulworth and adjacent areas.The pond-snail Viviparus , the freshwater bivalve Unio ,fish remains( Pycnodus, Lepidotus ) and ostracods such as the beaked freshwater form Cypridea are common in these beds. These beds are of freshwater lacustrine (lake) origin.

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STRATIGRAPHY:

Upper Ostracod Clays and Shales and Purbeck Marble - Interpretation

The origin of these strata are now considered. These clearly originated in lacustrine (lake) environments. Ostracod, gastropod and bivalve faunas suggest oligohaline (brackish water) conditions. The Viviparus limestones have obviously been formed from remains of numerous pond-snails; these animals probably thrived on luxuriant aquatic vegetation which has not been preserved. Compared with similar modern lakes and later ancient ones (as in the Headon Hill Formation of the Upper Eocene of Hordle Cliff, Hampshire and the Isle of Wight) an absentee is the water-lettuce, Stratiotes, an angiosperm (flowering plant) that had not yet evolved. As in earlier Purbeck lakes, crocodiles and turtles were probably common. The Unio is a typically freshwater bivalve, the larval form of which is parasitic on fish (look for the fish teeth and scales; you should find them - they are dark and shiny).

It is strange that there were sharks in this low salinity water. However, at this time certain genera of sharks had taken over almost freshwater habitats (Patterson, 1966).

The relatively high iron content of these beds with the iron-bearing minerals, pyrite, siderite and glauconite is the result of chemical weathering in the subhumid conditions, that contrasted with the semiarid environments in which the Lower Purbeck originated. Not only was it wetter than earlier but basin and swell contrasts were being enhanced in the late Kimmerian movements. All this was a precursor the influx of Wealden clastics which was soon to come.

The Unio Bed of Friar Waddon, referred to above, contains in its clasts significant evidence for uplift of the basin margins during these Cretaceous extensional tectonics to the extent that the Portland Group and part at least of the Kimmeridge Clay was eroded through. There is detritus of this type in the Isle of Purbeck and the local source is most likely the Wytch Farm or South Dorset High, now well-known from oil industry exploration.

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STRATIGRAPHY:

Unio Member

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

Paired valves of Unio in a Purbeck Marble type of matrix, Durlston Bay, Dorset

Unio shells can occur in a Purbeck Marble type of matrix, as shown above. This is from an ex situ block and it is not clear as to whether it is from a Purbeck Marble Bed with some Unios, or whether it is from the Unio Beds with a Purbeck Marble matrix. The paired valves are gaping and are filled with Viviparus shells and carbonate mud like that of the general matrix. Notice that the Viviparus shells were empty of carbonate mud, presumably with the dead animals inside and later filled with sparry calcite. The rock would be classified as biomicrite or, more strictly, a biomicrudite.

Unio in limestone, Upper Purbeck

Modern Unio and Viviparus

These image of Purbeck and modern Unio and Viviparus are repeated here to give an Unio Bed (and Purbeck Marble) environments. As noted on the image, this specimen of Unio Beds is from the east side of Lulworth Cove, but similar material is present at Durston Bay.

Foliage from the Upper Purbeck

Plant remains found by Mr A.J. Holmes on 6th December, 1998 in a rock fall under the Coastguard Station at Peveril Point, Swanage. Tony Holmes reports that the bed appears to be DB 221 in the Unio Member. The foliage appears to be of Araucaria type, that is with similarities to the foliage of the "Monkey Puzzle Tree " or Chile Pine. Above it is piece of foliage of Araucaria excelsa , the Norfolk Island Pine. Below is modern Cupressus foliage, to which previous finds of Purbeck fossil foliage have closer resemblance ( see illustrations of Cupressinocladus valdensis in Francis, 1983).

Tony Holmes further commented that the bed has an association of carbonised vegetation, fish teeth, fish scales and broken shell material (shells of Neomiodon type are visible in the photograph). When broken the bed has a sulphurous odour and has layers of soft sandstone (?) and harder limestone.

Clements (1992) described the bed as follows:
" DB 221, 0.38 - 0.53 m. A complex bed of rough limestones, clays and shales. Richly fossiliferous with "Unio" Viviparus bones, teeth and plant remains (concentrated towards the top) At the top there is a rough, light grey biosparite ( the Crocodile Bed), with much brown staining, grey shell fragments in a creamy matrix, Viviparus very abundant; cavities lined with calcite, and containing hydrocarbons; plant remains abundant. Towards the middle, the bed has rather thin, dicontinuous, irregular calcareous clay, shale and limestone bands. The lowest limestone is persistant, firmly cemented to the bed below and is crowded with " Unio's" . Samples (summarised from 3 samples - see Clements for full details): Gastropoda: Viviparus, (dominant) Theodoxus (?) fisheri, Planorbis fisheri. Ostracoda: Cypridea menevensis, C. setina, Darwinula oblonga, D. leguminella, Theriosynoecum striata, Rhinocypris jurassica, Timiriasevia punctata.

This bed probably represents the marginal deposits of a lake with pond snails, freshwater bivalves and ostracods, where plant debris and remains of fish and crocodiles were washed into the shallow marginal areas. Bones, teeth and scales were presumbably concentrated in the absence of coarse clastics. It is interesting that the trees on the nearby land were Araucarian rather than an early Cupressus type, like those of the basal Purbecks and some of the Wealden Group above.

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BROKEN SHELL LIMESTONE:

Circular Depressions at the Top of the Bed

"the beds had been forced in by a violent blow" (Fitton, 1824)

Members of the Dorset DIGS group search for the Fitton pits in the top of the Broken Shell Limestone, Swanage, Dorset, 9th November 2007

One the curious circular depressions in the top of the Broken Shell Limestone, Upper Purbeck Group, Lower Cretaceous, South of Swanage Pier, near Peveril Point, Dorset, 9th November 2007

Details of the fracture pattern around one of the strange pits at the top of the Broken Shell Limestone, South of Swanage Pier, Dorset, 9th November 2007

Asymmetry of pit in the top of the Broken Shell Limestone, Swanage, Dorset, possibly suggesting impact, 9th November 2007

Unusual circular depression are present at the top of the Broken Shell Limestone on the sloping ledge south of Swanage Pier. I have puzzled at these from time to time over the last half century. The one shown above is a little more about 1 metre in diameter (as shown by the 1m rule adjacent to fossil sniffer dog!). They tempt speculation about meteorite impacts, dinosaur footprints etc, but the real origin may rather less exciting. The depressions are too iregular in size for dinosaur footprints and too perfectly circular. Unfortunately the central part of the only good one remaining is covered with concrete but I do not think that they show any central bulge or hummock associated with an impact event.

A more mundane theory is that they are just circular collapse feature, with the characteristic circular jointing. One puzzle is whether there was some type of cavity underneath, but, in fact, even this may not have been necessary.

Here are the comments of the original discoverer - Dr. William Henry Fitton, M.D., F.R.S., P.G.S. etc.:

"A very remarkable appearance, which I am at a loss to explain, was observed by Mr. Babbage and myself in 1824, on the back, or top of the Purbeck strata, at that time exposed on the shore between the town of Swanage and Peverell Point. The surface there, which dipped at an angle of 7 or 8 towards the north, was depressed in some places into nearly circular pits or cavities, from 4 to 7 feet in diameter, and about a foot deep in the middle,-as if the beds had been forced in by a violent blow; the depressed surface being divided by irregular but nearly concentric cracks, which were filled with white sparry carbonate of lime. Three of these depressions were visible; two of them about 6 feet apart, one of which was 7 and a half feet [in diameter], the other about 4 and a half feet in diameter at the outer edge. A third, about 10 paces to the west of these, was of smaller dimensions."

At the moment the only good example which I can see and which has associated semi-concentric cracks filled with calcite is the one shown in the photographs. This is closest in size to the one listed as four and a half feet (1.4m.) in diameter. The large one of 7.5 feet (2.29m.) would be easily seen if it was still uncovered. I cannot find it now. From the description above it must have been on roughly the same north-south line as the existing example. A high sea wall has been constructed and undoubtedly much of the original exposure has been obscured.

Strahan (1898) commented that: "The depressions are due apparently to hollows having formed beneath the limestone, and its having collapsed after consolidation but it is not possible to see what caused the formation of the hollows."

A collapse origin seems most likely, but there are no obvious large hollows in the Broken Shell Limestone (modern sea erosion produces small hollows). The origin of the circular depressions probably relates to the compaction history of the Purbeck limestones of Dorset. Some of the Purbeck biosparrudites are of early cemented type and others were compacted and then cemented under burial. Certain biosparrudites, and this may be one, can have both some early cementation and some subsequent compaction. The top of the Broken Shell Limestone may have been cemented and then, later, there was burial compaction. The rock may have partially failed and these circular structures are the locations of non-linear failure. This hypothesis has not been proven by detailed study and further investigation would be useful. The Broken Shell Limestone was not included in the petrographic and geochemical study of Adam El-Shahat, and therefore data is limited.

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BROKEN SHELL LIMESTONE continued:

The Base (Contact with underlying Chief Beef Member)

Broken Shell Limestone above Chief Beef Member, Durlston Bay, Dorset, 8th October 2012

Increase of beef in the uppermost Chief Beef Member towards the base of the Broken Shell Limestone Member, Durlston Formation, Purbeck Group, Durlston Bay, Dorset, with Dr Peter Cobbald, 8th October 2012

The thick, Broken Shell Limestone Member lies above the shales of the Chief Beef Member. This can cause a potentially dangerous situation with regard to rock fall. In certain conditions, particularly wet weather the limestone may fall out to the beach. It is necessary to be aware that there is a potential hazard here. The remains of major rock falls are usually visible at the foot of the cliff.

Of particular interest here is the increase in the proportion of beef in the Chief Beef Member as the Broken Shell Limestone Member is approached. This is shown in a photograph above. The Chief Beef Member is discussed in more detail in the webpage: the Middle Purbeck strata - Lulworth to Durlston Formations.

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Middle Purbeck Strata (i.e. Lulworth Formation to Durlston Formation)

Continue to the Middle Purbecks in the Middle Purbeck webpage



ACKNOWLEDGEMENTS

The support of the Head of the School of Ocean and Earth Science, Southampton University is gratefully acknowledged for making it possible to me to continue to run this website. Professor Adam El-Shahat has made a detailed study of Durlston Bay and his major contribution to the sedimentology of the Purbeck strata is acknowledged. The Durlston Bay succession could not be studied successfully now without reference to the essential log of Dr. Roy Clements. There is much dependence on this excellent and very detailed work. I thank Dr Clive Boulter for use of photographs taken from the paddle steamer Waverley. I very much appreciate the support and help of the Channel Coastal Observatory , National Oceanography Centre, Southampton, in supplying excellent aerial photographs. I am grateful for discussion with members of the Dorset DIGS group, including Alan Holliday and Richard Edmonds. Other people have kindly helped with disussion in the field.

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Associated Webpages


Durlston Bay - Middle Purbecks and Building Stones
Durlston Bay - Lower Purbecks & Miscellaneous
Durlston Bay - Central Zizag Path area
Durlston Bay - Bibliography
Purbeck Group - Bibliography



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

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

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Dr Ian West, author of these webpages

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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