West, Ian M. 2014. Kimmeridge: East - Rope Lake Head to Freshwater Steps; Geology of the Dorset Coast. Internet site: www.southampton.ac.uk/~imw/Kimmeridge-Rope-Lake-Head-Geology.htm. Romsey. Version: 5th May 2014.
Kimmeridge Field Guide - East - Rope Lake Head to Freshwater Steps
Ian West,
Romsey, Hampshire

and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University, Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory ,
Website archived at the British Library
Home and List of Webpages |Kimmeridge - Kimmeridge Bay | Kimmeridge to Brandy Bay and Gad Cliff |Kimmeridge - East - Hen Cliff, Yellow Ledge and Cuddle | Kimmeridge - Blackstone, Oil Shale at Clavell's Hard |Kimmeridge - Burning Beach, Burning Cliffs| Chapman's Pool, Houns-tout Cliff and Egmont Bight |Kimmeridge Clay Fossils

| Selected external links: | Jurassic Coast (DCC) | Exmouth to Milford-on-Sea 1800-2000, Kimmeridge section - old photographs collected by Doreen Smith
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.)

GO EAST? - CHAPMANS POOL AND HOUNS-TOUT CLIFF;
GO WEST? - CLAVELL'S HARD AND OIL SHALE

Click or double-click on images for full-size high resolution versions!
(Browser zoom will not produce good photographs with sharp captions)

Rope Lake Head, Ian West looks up at the Kimmeridge Clay cliff from the Rope Lake Head ledges, east of  Kimmeridge Bay, Dorset, February 2014

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A general view of the Upper Kimmeridge Clay of Rope Lake Head cliff, seen from a seaward ledge, and photographed by Dr. Ramues Gallois, 2011, East of Kimmeridge Bay, Dorset

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The cliffs of Upper Kimmeridge Clay, east of Rope Lake Head, Kimmeridge

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Clavell's Hard to Rope Lake Head seen from the shore ledges, east of  Kimmeridge, Dorset, Hogg group, 2007

The cliff at Rope Lake Head, east of Kimmeridge Bay, Dorset, with hudlestoni Zone, Upper Kimmeridge Clay, March 2006

Ledges at Rope Lake Head, east of Kimmeridge Bay, Dorset, seen in freezing conditions, March 2006

The Kimmeridge Ledges of dolomite within the Kimmeridge Clay, near Rope Lake Head, east of Kimmeridge Bay, Dorset

Other Kimmeridge Field Guides

Kimmeridge Clay Fossils
Kimmeridge - Kimmeridge Bay and Kimmeridge Clay Introduction
Kimmeridge - westward to Brandy Bay and Gad Cliff
Kimmeridge - East - Hen Cliff, Yellow Ledge and Cuddle
Kimmeridge - Blackstone, Oil Shale at Clavell's Hard
Kimmeridge - Burning Beach, Burning Cliffs and the Lyme Volcano
Kimmeridge - Egmont Bight to Chapman's Pool
Kimmeridge Clay Boreholes at Swanworth Quarry
Kimmeridge - Bibliography - Start
Kimmeridge - Bibliography Continued

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INTRODUCTION:

Location and Geological Maps

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A modified old map,1900, of Kimmeridge Bay, Dorset, and the adjacent Kimmeridge coast, the site of old oil shale workings

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Geology of the Kimmeridge coast, Dorset, partly redrawn after a modern BGS geological map

Map - Geology of the Isle of Purbeck

Geology of the Isle of Purbeck shown on an old map (part of a map modified from Damon, 1884). Modern changes in the geological mapping of the area have only been of detail. Some place names have changed since Victorian times and Clavell's Hard has been added by the present author. Note an older spelling of "Kimeridge" with one "m".

A very simplified location and geological map of the Kimmeridge Bay area and adjacent coast, Dorset, southern England

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[Ledge map to be inserted]

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The lithological succession, using traditional names, in the Kimmeridge area, Dorset, based on old geological survey maps, with a minor correction on the Jurassic-Cretaceous boundary

Old geological map of the Kimmeridge Area, Dorset, based on 1895 and 1904 editions

This map, modified after Cox and Gallois (1981) shows the solid geology of the Kimmeridge area in particular and locations referred to in this and associated Kimmeridge field trip guides.

A multibeam bathymetry image of the sea floor south of Kimmeridge, Dorset, courtesy of the Channel Coastal Observatory

The multibeam bathymetry image and the geological map of the coast between Kimmeridge Bay and St. Aldhelm's Head, Dorset, linked to show sea floor geology

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INTRODUCTION:

Safety; the Hazards of Kimmeridge Cliffs

A small rock fall from Kimmeridge Clay cliffs, Kimmeridge coastline, Dorset, 2011

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The tidal trap just west of Clavell's Hard, Kimmeridge, Dorset

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Note that there is some significant danger with regard to the Kimmeridge cliffs. One danger, the main one, is risk of rock fall; the second hazard, quite significant, is that of being cut off by the tide. This applies particularly to the cliffs east of Kimmeridge and the risk is greatest beyond Rope Lake Head. The localities described here are beyond (eastward of) Clavell's Hard and are all in the tide-hazard zone. It is important to note that they can only be accessed at a very good low tide. It is not safe to remain on this part of the coast (without a boat) for more than an hour or two, carefully timed to coincide with low tide. It is absolutely essential to be able to return westward with adequate time so as to pass around Clavell's Hard to safer coast (you can get cut off at Yellow Ledge at high tide but the risk is much less).

Escape routes to the east have now gone. The old Freshwater Steps do not exist now and there is no way up the cliff there, so there is no route to the east except at very low tide (on the ledges around the Freshwater Steps promontory). At Clavell's Hard the old access by rope and a cliff path has gone as the cliff has cut back. Wading around Clavell's Hard is not recommended because the shale ledges under the water are slippery and you cannot see exactly what your are walking on. It is very easy to slip and fall in the sea. To be safe, allow plenty of time to return and watch the Fucus or brown seaweed zone. If the tide covers this you may be too late. Mobile phone contact may or may not be possible beyond Clavell's Hard or Rope Lake Head, and some people take a whistle for emergency purposes. (Note that if you do not go beyond Rope Lake Head, and therefore you can watch Clavell's Hard regarding the tide level, the risk is much less.)

In general, keep out as far from the cliffs as possible. Some debris falls every day and if you are close to the cliff you are at risk of being hit by a rock-fall.

Cliff collapse at the old No. 2 Level of 1890 Blackstone workings, just west of Clavell's Hard, Kimmeridge, Dorset

Because of the hazard of rock falls and it is a dangerous place to visit unless it is well-understood and proper precautions are undertaken. The cliffs are vertical and high and subject to erosion by the sea at the base. The shale and mudstone is full of joints and fissures and not stable. Small pieces will tumble off from time to time as you walk along.

More serious are substantial falls like the one shown above. These may occur without warning; suddenly there will be a loud crash and a plume of debris and dust. These happen particularly in certain weather conditions, such as when there is frost or rain and sometimes when the shale has dried in the hot sun. If you are out on the low-tide ledges falling debris would not usually reach you but there is no guarantee of safety. The risk is greatest where the cliffs are highest, where there is joint-separated shale above and where there is evidence of a recent fall in the form of shattered debris.

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Kimmeridge Safety Addendum:

Photographs shown on this website have been taken over a period of many years by the author and various other people. Some photographs are from organised field-trips, which may be those of the present author or of other geologists. Many, though are from informal, private coastal walks, or from private, research field-trips. The photographs are for geological purposes only and are there to show rocks, not people or techniques. They are not intended to show safety procedures and no activities shown are necessarily intended to be copied. This website is about geology for geologists. The cliff, sea, tide and weather conditions vary greatly so always make your own assessment of the cliffs and conditions on the day, and arrange your coastal procedures in accordance. Always consult tide tables before field work at or near Kimmeridge. No responsibility at all is taken for any activities of field parties or individuals going to the Kimmeridge coast for their own purposes or objectives. As at other geological sites a risk is present and the possibility of an accident, although a rare occurrence, cannot be eliminated.

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INTRODUCTION:

Aerial Photographs - Introductory
(others are included in specific sections)

Aerial photograph index map for Kimmeridge Bay to Freshwater Steps, Dorset, based on a Channel Coastal Observatory image

Location aerial photograph of  Kimmeridge Bay and the cliffs to the east as far as Freshwater Steps, Dorset

An aerial photograph of the coast from Clavell's Hard to Freshwater Steps with some zonal information, east of  Kimmeridge, Dorset

Kimmeridge Ledges, of dolomite, limestone and bituminous shale, seen in an aerial photograph of the coast east of Rope Lake Head, Dorset, courtesy of the Channel Coastal Observatory, 2003

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STRATIGRAPHY:

Kimmeridge Clay - Stages and Ammonite Zones

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

STAGE CHRONOZONE ZONAL AMMONITE
Bolonian

(Upper part of broader "Kimmeridgian" according to some previous authors. At Kimmeridge, the Upper Kimmeridge Clay, in the cliffs east of Kimmeridge Bay, and also west in part of Brandy Bay, corresponds to this. Includes Yellow Ledge, Kimmeridge oil shale, Rope Lake Head SB, White Band and associated shales etc.)

Fittoni Virgatopavlovia fittoni
Rotunda Pavlovia rotunda
Pallasioides Pavlovia pallasioides
Pectinatus Pectinatites pectinatus
Hudlestoni Pectinatites hudlestoni
Wheatleyensis Pectinatites wheatleyensis
Scitulus Pectinatites scitulus
Elegans Pectinatites elegans
Kimmeridgian

(Lower part of the broader "Kimmeridgian" as used by some previous authors. At Kimmeridge the Lower Kimmeridge Clay, within Kimmeridge Bay, corresponds to this. Includes the Flats , Washing Ledge, Maple Ledge Dolomite Beds and associated shales etc.)

Autissiodorensis Aulacostephanus autissiodorensis
Eudoxus Aulacostephanus eudoxus
Mutabilis Aulacostephanus mutabilis
Cymodoce Rasenia cymodoce
Bayliei Pictonia baylei

The stretch of cliffs from Clavell's Hard though the Rope Lake Head area to Freshwater Steps is mainly in the hudlestoni and pectinatus Zones of the Upper Kimmeridge Clay. This is the best place to see the strata of these zones, but it has the problem of rather difficult access in relation to tides. These zones are also well exposed in Brandy Bay, but that is within an Army firing range.

Generalised sequence of the Upper Kimmeridge Clay in Dorset after Cox and Gallois (1981)

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STRATIGRAPHY:

Cliff Section

Section of the Kimmeridge cliffs from Broad Bench eastward to Houns-tout, near Chapmans Pool,  Dorset

A low-resolution version of one group of a continuous set of photographs taken from the sea by Richard Edmonds of the Jurassic Coast organisation, Dorset. The Kimmeridge cliffs from Clavell's Hard to Chapmans Pool are shown

A cliff section is shown here for a large part of the Kimmeridge Coast. This is based on Arkell (1933), Cox and Gallois (1981) and other information. Amongst other things, it shows the position of the Kimmeridge oil-shale, Clavell's Hard and the location of the mining in the cliffs.

Beneath this diagram is shown a low-resolution version of one of many continuous coastal photographs from sea taken by Richard Edmonds of the Jurassic Coast, Dorset and East Devon World Heritage Coast. The full images are very high quality.

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INTRODUCTION:

General Introduction

These are very interesting cliffs that are only accessible at low tide by walking from Kimmeridge Bay. It is important in showing thin, laminated, coccolith limestones, like the White Band, that are not well-seen at other accessible localities. It also contains the 'Basalt Stone' a dolomite of basalt-like appearance. At the eastern end of the section the Freshwater Steps waterfall can be seen. Attention to safety is very important here. You can be cut off by the tide and there is a significant danger from falling debris from the cliffs if you approach them. It is wise to be there at low tide, staying out from the foot of the cliffs as far as possible, but taking care not to slip on seaweed covered ledges and rocks.

The Kimmeridge Blackstone, oil shale, forms a small shore ledge just west of Rope Lake Head, near Kimmeridge, Dorset

This continues from the field guide for the cliffs to the west. We start at the Blackstone or oil shale just west of Rope Lake Head. For more information on the oil shale please see the Blackstone web pages .

Angel wing pyrite

In this area, between Clavell's Hard and Rope Lake Head, pyrite occurs as flat radiating plates in certain beds of the shale. This type of pyrite has been sold in the past at Lyme Regis as 'angel wings'. The surface is wet with seawater, beneath which it stays unoxidised. On a land surface it will soon decompose to produce iron oxide and sulphuric acid. Such oxidation can be delayed by varnishing a specimen with a plastic resin. The presence of the pyrite shows that there were reducing conditions in the sediment but not necessarily in the Kimmeridge seawater.

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LOCATION:

Rope Lake Head Area

Rope Lake Head seen from the shore on the west side, near Kimmeridge, Dorset, 2010

Upper Kimmeridge Clay strata, faulted in the cliff at Rope Lake Head, east of Kimmeridge, Dorset, as seen in 2006

After passing the oil-shale, which forms a small ledge on the shore, when walking eastwards, the small promontory of Rope Lake Head is encountered. The cliff shows a faulted succession of upper Kimmeridge Clay, ranging over the wheatleyensis-hudlestoni boundary (although note that different authors place the exact boundaries of the zones at slightly different lithological horizons).

On the beach, particularly to the west of the headland, there are many boulders from the Rope Lake Head Dolomite Bed which descends to the beach at this headland; the outcrop of this bed is the reason for the promontory. In addition there are fallen blocks from the Basalt Stone (actually a dolomite bed); these are conspicuous because they tend to become rounded and show a obvious dicey fracture. There are also White Stone Band blocks, which are white and laminated. The thinner, Middle White Stone Band is also present in the upper cliff here, but fallen blocks from this would be thinner.

Kimmeridge ledges at low tide, south of Rope Lake Head, East of Kimmeridge, Dorset, 2006

At low tide and extensive area of Kimmeridge ledges is seen south of Rope Lake Head. This is mainly formed by the Rope Lake Stone Band, even though it is only 50cm thick. A complication is that there is repetition by faulting. Much of the ledge is covered by a species of Fucus seaweed. The stone band ledges like this one are obvious at the shore even at medium tide and in some cases (e.g. Yellow Ledge) at high tide. However, further out the width of stone band ledge decreases in proportion to bituminous shale ledges. The oil shales and bituminous shales form most of the offshore ledges and sea-floor outcrops, as can be seen on the multibeam bathymetry maps. Surprisingly the stone bands are difficult to recognise on these, but groups of bituminous shales are obvious. The calcareous mudstones, or marls, are much more easily eroded.

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STRATIGRAPHY AND LOCATION:

Rope Lake Head Dolostone (Rope Lake Head Stone Band)
- Composition

The Rope Lake Head Stone Band and The Little Stone Band in the hudlestoni Zone, Upper Kimmeridge Clay, above the Kimmeridge Blackstone, east of Kimmeridge, Dorset

The Rope Lake Head Dolomite Bed is a thin stone band, about 0.5 metres thick. It is easily seen because it is only a few metres above the Blackstone. Close examination can be made on the shore at Rope Lake Head, and just to the west of this promontory. It is a unit of the Upper Kimmeridge Clay and belongs to the low Pectinatites hudlestoni Zone (the Blackstone beneath is the top of the Pectinatites wheatleyensis Zone).

Coccoliths, as seen by SEM, in the Rope Lake Head Stone Band, Rope Lake Head, Kimmeridge, Dorset

In composition, the Rope Lake Head Dolomite Bed is an argillaceous/kerogenous ferroan dolomite (Bellamy, 1980). This bed is in close association with both oil-shales and with coccolith beds. The bed contains coccoliths (and may thus have been described as a limestone by Cox and Gallois, 1981). It is of interest as the stone band closest to the Blackstone (about 4.5 metres above it). It may represent a stone band facies intermediate between the grey, blocky, dolomite type and the laminated coccolith limestones of the pectinatus Zone.

With regard to geochemistry the Rope Lake Head Dolomite Bed contains 33.9%% CO 2 , 6.2%% organic matter. There are the following trace elements in ppm: V - 110; Cr - 45; P - 470; Zr - 9; Ni - 8, Cu - 20, Mo - 5, Ga - 3, Mn -240, Li - 20, Rb - 50, Cs - 10, Sc - 3. This data is from Dunn (1973). The composition is not unusual for the Kimmeridge Clay. The organic matter is less than typical for adjacent shales. V and Cr are normal for the adjacent strata but P is rather lower than in the shales (although note that it is quite a significant component of Kimmeridge strata). Zr is as low as would be expected for a rock with low clastics and Ni and Cu are lower than in the shales. Mo and Ga are as low as is usually the case. Mn, however, is four or five times higher than in the shales but this would be expected in view of the carbonate composition of the rock. Li is low while , Rb, as would be expected, is lower than in the shales (it normally substitutes for K in illite). Cs and Sc are low as in associated strata. Generally then, because of the partly diagenetic origin of the rock, trace elements are low compared with the adjacent bituminous shales. Only Mn is appreciably higher.

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STRATIGRAPHY AND LOCATION:

Rope Lake Head Dolostone (Rope Lake Head Stone Band)
- Trace Fossils - Rhizocorallium

A Rhizocorallium burrow, Rope Lake Head Dolomite bed, Upper Kimmeridge Clay, west of Rope Lake Head, Kimmeridge, Dorset, 2010

The Rope Lake Head Stone Band does not contain abundant macrofossils but it is notable for many large examples of the U-shaped, trace fossil Rhizocorallium. They are very clearly shown on the smooth, sea-washed surface of the bed. As shown in the photograph above they seem to contain faecal pellets.

Burrowing crustacean faunas were able to live at times in the bottom conditions of the Kimmeridge Clay sea. Wignall (1991) has studied the ichnofacies of Kimmeridge Clay that originated in dysaerobic (oxygen deficient) conditions. See his paper for a discussion of Rhizocorallium irregulare and other trace fossils.

The Rope Lake Head Dolomite Bed is only about 4 and half metres above the Blackstone, the main Kimmeridge oil shale. Thus anoxic conditions did not necessarily persist on the sea floor when the Upper Kimmeridge Clay was deposited. Nevertheless the organism which made this burrows must have been tolerant of poor oxygen conditions.

Rhizocorallium in the Rope Lake Head Dolomite Bed

Another Rhizocorallium ichnofossil in the same bed is shown above (the wet surface shown here shows some bluish reflected light that is not the natural colour of the bed). The pen, shown for scale, is 14.9cm in length and the whole trace fossil is 38.5cm in length. This is very large for Rhizocorallium .

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STRATIGRAPHY AND LOCATION

The Little Stone Band and the Short Joint Coal

(K43/3, low Hudlestoni Zone, Upper Kimmeridge Clay)

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The Short Joint Coal or oil shale at Rope Lake Head, east of  Kimmeridge Bay, Dorset, 2010

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When you arrive at Rope Lake Head, you will probably be standing on the Rope Lake Head Stone Band, the resistance of which is, of course, the reason for the existance of a headland. Look around you and notice the tide levels, because, as mentioned elsewhere, you have to get back and around Clavell's Hard before the tide rises significantly. You can see Clavell's Hard in the distance from here, but if you go beyond it is not visible and your level of risk increases.

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Standing on the Rope Lake Head Stone Band notice a thin stone band within oil shale, about 3m above the RLHSB. This is the Little Stone Band, within the Short Joint Coal. It has some peculiar characteristics.

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The Short Joint Coal and The Little Stone Bed is about 3 metres above the Rope Lake Head Stone Band on which I am standing to photograph, Rope Lake Head, east of  Kimmeridge Bay, Dorset

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The hudlestoni sequence above the Rope Lake Head dolomite bed at Rope Lake Head, east of Kimmeridge Bay, Dorset, 2010

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The Little Stone Band and the Short Joint Coal at Rope Lake Head, Kimmeridge, Dorset

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The Short Joint Coal at Rope Lake Head, east of Kimmeridge Bay, Dorset, showing a central calcite rhomb limestone with oil shale above and below, 2010

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The top surface of the Little Stone Band shows square jointing, with an appearance like that of close-spaced paving slabs, seen here just east of Rope Lake Head, Kimmeridge, Dorset, 31st February 2014

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Early calcite rhombs in the limestone of the Short Joint Coal, Rope Lake Head, east of Kimmeridge Bay, Dorset, seen in SEM

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An unusual bed of particular interest is the Short Joint Coal, a thin oil shale, and the Little Stone Band which overlies it. The Short Joint Coal is is bed 43/3 of the hudlestoni Zone and the Little Stone Bed is 43/4. These two beds are higher than the Blackstone and are easily found almost 3 metres above the very obvious, Rope Lake Head Stone Band. It can be seen in the cliffs elsewhere but is most accessible at and around Rope Lake Head. The oil shale bed is less than 30 cm thick and was not economical to mine for oil shale. The organic content is probably quite high.

The limestone is unusual for the Kimmeridge Clay of the type section. This Little Limestone Bed was named by Coe et al. (1999) in the detailed graphic log of the RGGE Project. The Short Joint Coal is a much older name, that probably comes from local use of the oil shale by the villagers as a fuel The Little Limestone Bed (although not under this name) was described as coccolith-rich limestone within oil shale by Cox and Gallois (1981). However this is not the case. A HREF="http://www.southampton.ac.uk/~imw/kimref.htm#Bellamy"> Bellamy (1980) showed that the carbonate bed is a rhomb-limestone of calcite rhombs. It is laminated (although not finely so like the coccolith limestones) and very pyritic and with much kerogen. There has been much replacement by pyrite of calcite rhombs. Ostracods, foraminifera and mollusc fragments are very scarce and when seen are heavily pyritised. It is interesting that fish scales and phosphate is common in this bed and other lithologies rich in calcite rhombs (Bellamy, 1980). (Dunn, 1974) reported phosphorous at 1250 ppm. which is fairly high. He found organic matter (i.e. kerogen) for the limestone to be 11%. The underlying bed, which is oil shale, has 38.3 % organic matter, a moderate figure for a Kimmeridge oil shale.

Bellamy (1980) found that the rhomb calcite of the limestone within the Short Joint Coal has the composition:
Ca 96.60 Mg 2.07 Fe 1.30 Mn 0,03 CO
3 . Thus it is slightly magnesian.

Sr is 568 ppm in a tested sample, and thus is fairly low and unremarkable. Stable isotope data is:
delta C13 = -4.8%; delta O18 = -2.4% wrt PDB.

Thus, this pair of beds is unusual as a comination of a thin oil shale and a thin rhomb limestone. Dolostone beds are common in the main part of the Kimmeridge Clay in the Kimmeridge coast type section, but limestones (spindle crystal calcitic limestones) occur in the Kimmeridge Clay at Ringstead and Yorkshire. Limestones in the Kimmeridge Clay are of special interest at the present, because limestones of the hudlestoni Zone are much thickly developed in the Weald of Sussex. They are of potential economic importance at Balcombe, Fernhurst and Wisborough Green. They lie above the Blackstone and, although not conventional carbonate reservoirs, they might be used as effective reservoirs if acidized or hydraulically fractured. (Further details will be discussed elsewhere in another webpage).

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STRATIGRAPHY AND LOCATION;

Rope Lake Head:
Ichthyosaur Remains

Ichthyosaur bones

A series of vertebrae and associated bones in matrix were found in the Upper Kimmeridge Clay of Freshwater Bay by Dr Dru Drury, and were excavated by him and Captain G. Fenwick-Owen, by permission of Lady Hamilton Russell (Delair, 1960).

Not far away at Rope Lake Head, in 1955, I was with Mr G. Symes of Bournemouth when he discovered ichthyosaur vertebrae, of Ophthalmosaurus a short distance above the beach. We found other bones and excavated as much as we could. A sketch of these which I made at the time is reproduced here. The bones were at the promontory itself and were found in what was then listed by Arkell (1947) as grandis zone, 0.76m (2 feet, 6 inches) below the base of the wheatleyensis subzone. In modern terminology this is within the hudlestoni zone. The site was a few metres east of the most extreme southern point of the cliff and just above a ledge forming a small platform near the base of the cliff. It was from time to time covered by an apron of shale scree and it was not possible to excavate safely far into the cliff. The skull was missing but a number of other bones including part of the verterbral column, numerous ribs and some limb bones were preserved. The central part of the mass of bones was pyritised, and where this had happened the vertebrae were badly distorted. In other words they had been pyritised under burial and during compaction. Some of the bones are at Bournemouth School and some at Southampton University. Delair (1986) has a published a short account of these bones in a survey of little-known ichthyosaurs from Dorset.

The early diagenesis of the Ichthyosaur skeleton was of interest. The centre of the body had been pyritised. The pyritised vertebrae were deformed by compaction, whereas those in phosphatic condition were undeformed. This suggests that some pyritisation was taking placing rather later than would be expected, during major burial compaction (the sulphide reduction zone is usually relatively shallow and early). Burial compaction should not have been appreciable in the early sulphide reduction zone as in the later and lower methanogenesis diagenetic zone.

The locality was examined again more recently, including on the 2nd July 2010, but no bones were seen there. At that date the site was fairly free from talus.

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STRATIGRAPHY AND LOCATION

Rope Lake Head:
The Shingle Beach

A shingle beach of pebbles of Portland Chert between Rope Lake Head and Rope Lake Hole, east of Kimmeridge, Dorset, 2010

An unusual small shingle beach occurs between Rope Lake Head and Rope Lake Hole (i.e. east of Rope Lake Head). This is mainly composed of small brownish chert pebble that have been derived for the hillwash on the cliff tops here. This hillwash or head has moved down from the outcrop of Portland Cherty Series up the hill, towards Swyre Head, to the north. The photograph above shows brownish debris of hillwash origin smearing the cliffs. The beach is very localised, and is in a relatively undisturbed locality. Few people can reach this point because of tide problems, particularly at Clavell's Hard. It can only be approached a good low tide, and it is only possible to stay there for an hour or so. The cliffs at the back show the succession of the White Stone Band, the Middle White Stone Band and the Freshwater Steps Stone Band, amongst other strata of the Kimmeridge Clay, Pectinatites pectinatus Zone.

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BASALT STONE BAND (DOLOSTONE)

Introduction:

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This bed is KC 44/2. It is a conspicuous grey, ferroan dolostone with cuboidal ("dicy") fracture in about the middle of the Pectinatites hudlestoni Zone of the Upper Kimmeridge Clay. It is about 1.25 m in thickness. It can be seen very easily in the cliffs at Rope Lake Head. Further east at Rope Lake Hole it descends to the shore. It forms a conspicuous grey shore-ledge to the east of Rope Lake Hole. This shows the jointing which is a feature of this bed. The name comes from the basalt-like appearance in the cliffs (of course, it has no connection with basalt or any igneous rock and is just an argillaceous dolomite rock). The name was in use by Arkell (1947) and in earlier publications.

At Rope Lake Head and for a short distance eastward, blocks of the Basalt Stone fall to the beach. They are very distinctive because of the cuboidal or dicey weathering and they contrast markedly with the blocks of the finely lamined White Stone Band, which also descend in the same area.

The Basalt Stone is a grey, argillaceous dolomite or dolostone with very high magnetic susceptibility and very low gamma ray values. In spite of its grey appearances the content of clay is actually low. For the same reason the contents of K, U and Th are also low. It is bed 2 in unit KC 44 of the Pectinatites hudlestoni Zone of the Upper Kimmeridge Clay (in some old literature you may find it placed in the Wheatleyensis Zone). The bed is low in kerogen content and is of interest in containing unreplaced aragonite shell material (discussed further below). Like other dolostone beds lower in the Kimmeridge Clay it shows some "fish-tail" symmetry in the magnetic susceptibility record, but not so markedly as in lower dolostones. The reason for the approximate symmetry is because the Fe content is a little higher at top and bottom of the bed. This type of symmetry suggests some parallel influx of Fe-dolomite at top and bottom of the bed during its (fermentation zone) diagenesis. It does not show major expansion features like those of the Flats and Yellow Ledge dolostones, and does not contain internal thrust planes. It is effectively transitional in type between the hard brown Yellow Ledge type of dolostones and the weaker, grey "cementstones" like Cattle Ledge and Grey Ledge.

The Basalt Stone is persistent in the Kimmeridge area, unlike some thin dolostone beds above the White Stone Band ( Bellamy, 1980). Thus the same grey stone band can be seen in Brandy Bay, west of Kimmeridge, and it is only slightly thinner at that locality ( Cox and Gallois, 1981). It does not seem to be present at other localities, such as Ringstead.

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Jointing pattern seen in the dolomite bed known as the Basalt Stone, Rope Lake Hole, east of Kimmeridge, Dorset, aerial photo of the Channel Coastal Observatory, 2003

Looking down from Rope Lake Head into Rope Lake Hole and the Basalt Stone dolomite ledge, east of Kimmeridge, Dorset, July 2008

Basalt Stone Ledge, east of Rope Lake Head, seen from above, October 2005, Kimmeridge, Dorset

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The Basalt Stone, an argillaceous dolostone, seen from above on the 9th October 2005, east of Rope Lake Head, Kimmeridge,  Dorset, a larger view

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The Basalt Stone, a grey dolostone or dolomite, east of Rope Lake Head,  Kimmeridge, Dorset, 2010

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The Basalt Stone Ledge, east of Kimmeridge Bay, Dorset, 2010, with the tide rising rapidly

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Tide rising into the joints of the Basalt Stone Ledge, Kimmeridge, Dorset, 2010

The Basalt Stone, a dolomite bed in the hudlestoni Zone of the Upper Kimmeridge Clay, just east of Rope Lake Head, near Kimmeridge, Dorset, 2006

Fault displacement of the Basalt Stone dolomite bed, the White Stone Band, the Middle White Stone Band and the Freshwater Steps Stone Band, east of Rope Lake Head, near Kimmeridge, Dorset, 2006

Fallen and partly sea-eroded blocks of the Basalt Stone, a grey dolomite from the Upper Kimmeridge Clay, at Rope Lake Head, Kimmeridge, Dorset, 2006

Many blocks of dolomite with cuboidal fracture from the Basalt Stone bed in the cliff at Rope Lake Head, near Kimmeridge, Dorset, 2006

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A rock fall of Basalt Stone debris over the Little Stone Band, at Rope Lake Head, east of  Kimmeridge Bay, Dorset, with Ian West, 31st March 2014

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A rock fall of argillaceous dolomite blocks from the Basalt Stone Bed, in the Hudlestoni Zone of the Upper Kimmeridge Clay, Rope Lake Head, east of  Kimmeridge Bay, Dorset, 31st March 2014

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BASALT STONE BAND (DOLOSTONE)

Composition and Diagenesis

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Diagenetic history of the Kimmeridge Blackstone or oil shale in comparison to that of the Kimmeridge Basalt Stone, a dolomite microsparite, strata of the Upper Kimmeridge Clay, east of Kimmeridge Bay, Dorset

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The Basalt Stone is not a micrite, but a dolomite microsparite [a carbonate rock with components between 10 and 63 microns]. It has constituent dolomite crystals have a mode between 20 and 30 microns. It does not appear to be as kerogenous as some Kimmeridge dolomites and this is not surprising since it is developed in a calcareous mudstone facies rather than a facies rich in bituminous shales. Presumably the relative lack of compressible kerogen is, at least in part, the reason for its massive rather than laminated character. An interesting effect of the relative lack of organic matter is that early pyritisation has not occurred. White aragonitic ammonites and aragonitic bivalves are still preserved as such and have not been pyritised in the bacterial sulphate-reduction diagenetic zone (i.e. shallow reducing zone near the sea-floor). The carbonate content is ferroan (ferroan dolomite), and thus it is clear that if sulphide ions (i.e. hydrogen sulphide) were available early then pyrite would have been formed in significant quantity.

It may be important to note that this bed is a non-sulphurous rock, of fermentation zone diagenesis, and very different from the very sulphurous Kimmeridge Blackstone or oil shale (which has undergone diagenesis in the sulphate-reduction zone).

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BASALT STONE BAND

Fauna

A coarse-ribbed ammonite, still preserved as aragonite, in the Basalt Stone Band, a ferroan dolostone, Hudlestoni Zone, Upper Kimmeridge Clay, Rope Lake Head, east of  Kimmeridge Bay, Dorset, 31st March 2014

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The Basalt Stone contains abundant ammonites but they only easily seen when blocks have fallen and broken up. Most are of Pectinatites type with close-spaced ribbing. However, an unusual example with rather coarse, biplicate ribbing is shown above. This was found by Jonathon Pim of Perenco UK, during an oil company field trip, and I am grateful to him for pointing it out so that I could photograph it.

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WHITE STONE BAND - GENERAL
(Coccolith Laminite Limestone, the lowest of three main beds)

(For more on the White Stone Bands go to:
Brandy Bay, Kimmeridge)

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A broad view in good weather of the Three White Stone Bands, Pectinatus Zone, Upper Kimmeridge Clay, Kimmeridge Bay, Dorset, photograph by Ramues Gallois, 28th October 2011

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The three White Stone Bands, coccolith laminites, in the cliffs between Rope Lake Head and Freshwater Steps, east of Kimmeridge Bay, Dorset, 3rd April 2014, photo by Ian West

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The Three White Stone Bands, coccolith laminites of the Upper Kimmeridge Clay, seen from the Basalt Stone Ledge, east of Rope Lake Head, Kimmeridge, Dorset, 2nd July 2010

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The three White Stone Bands in the Upper Kimmeridge Clay, east of Rope Lake Hole, Kimmeridge, Dorset, 2010, introduction

The three White Stone Bands, or coccolith limestones in the Upper Kimmeridge Clay at Brandy Bay, west of Kimmeridge Bay, Dorset, October 2011

The White Stone Band is a conspicuous, laminated coccolith limestone of the Upper Kimmeridge Clay. It is one of three coccolith limestones in this sequence and it is the best-developed. A labelled photograph above shows the three limestones in the sequence which is that of the top Pectinatites hudlestoni and basal Pectinatites pectinatus Zones. The White Stones Bands are associated with short cyclical sequences of bituminous shales and calcareous mudstones. They are clearly linked, in terms of facies, with oil shales, and the lowest of the three contains a thin oil shale within it (near the base).

Also shown above, for comparison, are the three White Stone Bands at Brandy Bay. The sequence there is similar but generally slightly thinner.

The three stones bands east of Rope Lake Head are discussed separately below, starting with the lowest, thickest and main one, The White Stone Band.

With regard to composition, Bellamy (1980) analysed a sample of the White Stone Band and found it contained 97% calcite and no dolomite. For comparison the Middle White Stone Band was found to contain 84% calcite and 5% dolomite, and the Freshwater Steps Stone Band (the highest) contained 74% calcite and 6% dolomite. Note though the beds vary from top to bottom so that the analyses cannot regarded as average for each bed. Apart from inorganic minerals, there is a variation clay content and in kerogen content; a thin oil shale occurs within the White Stone Band. Nevertheless, this bed, the lowest of the three beds is by far the best example; it is the thickest and has the highest carbonate content.

The White Stone Band is easily studied both in the cliffs and in blocks on the shore. The coccolith content is shown in a section further down. The intial photographs and diagrams here are for location of the bed in the field; more details are given in following sections. The bed is imperfectly laminated and is not throughout a true laminite. There is a burrowed horizon in the central part. Thin oil shales occur near the top and bottom. Saurian vertebrae and bones have been found in this bed (Arkell, 1947).

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The White Stone Band in weathered condition near the cliff top, Rope Lake Head, Kimmeridge, Dorset, 31st February 2014

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The White Stone Band and associated strata of the Upper Kimmeridge Clay in the cliff at Rope, Lake Head, east of Kimmeridge, Dorset, July 2010

The cliff top east of Rope Lake Head showing coccolith limestone in shale of the pectinatus Zone, Upper Kimmeridge Clay, east of Kimmeridge, Dorset, 2005

Log of the succession of part of the Upper Kimmeridge Clay including the white coccolith limestones east of Kimmeridge Bay, Dorset, after Cox and Gallois

The cliff at Rope Lake Head, east of Kimmeridge Bay, Dorset, with hudlestoni Zone, Upper Kimmeridge Clay, March 2006

The White Stone Band in the cliff top on the east side of Rope Lake Head, east of  Kimmeridge Bay, Dorset

Rope Lake Head is the first place where the White Stone Band is encountered when walking eastward along the shore. It is present in the top of the cliff here, but fallen blocks are present on the shore. From here the bed descends eastward to an in situ exposure on the shore as a very jointed Kimmeridge ledge.

The White Stone Band and other coccolith limestones seen from above Rope Lake Head, looking eastward, Kimmeridge coast, Dorset, 2005

Rope Lake Hole east of Kimmeridge Bay, Dorset, seen from the cliff top at Rope Lake Head. Note the White Stone Band, a coccolith limestone

The three white coccolith limestones, the White Stone Band, the Middle White Band, and the Freshwater Steps White Band at Brandy Bay, Kimmeridge, Dorset, March 2012

Rope Lake Hole east of Kimmeridge Bay, Dorset, seen from the cliff top at Rope Lake Head. Note the White Stone Band, a coccolith limestone - labelled version

Aerial photograph of eastern part of Rope Lake Hole, showing faulting and jointing in the White Band, a coccolith limestone, from a Channel Coastal Observatory image

The eastern side of Rope Lake Hole and the Spur below Swyre Head, seen from the foot of the cliff at Rope Lake Head, east of Kimmeridge, Dorset, UK

A labelled view of the cliff east of Rope Lake Head, near  Kimmeridge, Dorset, 2006, showing the Basalt Stone and the White Stone Bands

The bay between Rope Lake Head and Freshwater Steps, Kimmeridge, Dorset, with the Basalt Stone and White Stone Band in the cliffs and as sea ledges, 2010

The White Stone Band in the cliff can be seen to contain two, thin, black oil shale bands, east of Rope Lake Head,  Kimmeridge, Dorset, 2010

The White Stone Band descends to the shore between Rope Lake Head and Freshwater Steps, Kimmeridge, Dorset

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The White Stone Band near the base of the cliff, east of Rope Lake Head, east of Kimmeridge Bay, Dorset, 2010

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The White Stone Band, a coccolith laminite, at sea level between Rope Lake Head and Freshwater Steps, East of Kimmeridge, Dorset, 17th September 2012

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Standing above the White Stone Band, coccolith laminite ledge, east of Rope Lake Head,  Kimmeridge, Dorset, 17th September 2012

The best exposure of the White Stone Band is shown above. It descends to the shore between Rope Lake Head and Freshwater Steps (nearer to Rope Lake Head). As it reaches the beach it is faulted and descends again a short distance further east. It then forms a ledge in the sea in the area known as White Lias Rocks. The upper surface of the bed as seen in the ledge has conspicously rectilinear jointing. Is this a pre-tectonic compaction feature? Has it any similarity to the vertical jointing in the Blackstone, where thin sheets of calcite occupy the joints? It is quite different from jointing in the diagenetic dolomites, lower in the Kimmeridge Clay.

The photographs above show the White Stone Band descending to the ledge on the shore in Rope Lake Hole and in the cliffs to the east. The well-defined jointing of this bed is clearly visible in the aerial photograph. The Middle White Stone Band and the Freshwater Steps Stone Band are above and these, too, are laminated coccolith limestones.

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THE WHITE STONE BAND, COCCOLITH LIMESTONE - LAMINATION AND LENSING:

Close-up Details and Disruption Effects

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Please note: Various photographs have been taken at Rope Lake Head and eastward at different dates and different years. The coast here east of Kimmeridge Bay, has been visited many times, of course, when the tide has been suitably low. The close-up photographs all tend to show that there is some very small-scale disruption (on 1 mm or less, vertically). Whether this is the result of compaction, of slumping of seismic shocks is not known actually at present. Different photographs may shown different speculative interpretations in their captions, so please treat these with caution. The real origins of the small-scale bed structure have not been solved, as yet!

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The White Stone Band, coccolith laminite, Kimmeridge Clay, Upper Jurassic, seen in situ, at the cliff foot between Rope Lake Head and Freshwater Steps, East of Kimmeridge Bay, Dorset, 17th September 2012

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Very small-scale, soft sediment deformation in the White Stone Band, an imperfect coccolith laminite of the Upper Kimmeridge Clay, near Rope Lake Head, east of  Kimmeridge Bay, Dorset, 31st March 2014

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Disruption of finer laminae, but survival of thicker bands, the White Stone Band, coccolith laminite, fallen blocks at Rope Lake Head, east of Kimmeridge Bay, Dorset, 31st March 2014

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The White Stone Band is a coccolith laminite, but a very imperfect one. The coarser laminae are usually fairly continous. Thinner laminae on a millimetre scale are often disrupted to various extents. The reason for this is not known. It might be the effects of minor earthquakes, or it might be the consequences of some compaction process. Water escape structures are not obvious. This small-scale, soft sediment deformation is not satisfactorily explained. Other examples are shown below.

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The White Stone Band, coccolith limestone, at beach level east of Rope Lake Head, Kimmeridge, Dorset, and showing some subdivisions

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A fallen block of the White Stone Band, a coccolith laminite, Rope Lake Head, east of  Kimmeridge Bay, Dorset

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The White Stone Band, a coccolith laminite with dark oil shale laminae, Rope Lake Head, east of  Kimmeridge Bay, Dorset

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Cross-section through a specimen of the White Stone Band, a bituminous coccosphere laminite of the Upper Kimmeridge Clay, east of  Kimmeridge Bay, Dorset; note the disruption which might be due to flowage during early compaction

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Laminites and disrupted beds within the White Stone Band, a coccolith limestone, east of Rope Lake Head, Kimmeridge, Dorset

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Minute lenses of white coccolith carbonate in the White Stone Band, ex situ shore block, near Rope Lake Head, west of  Kimmeridge, Dorset

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Oil shale in the White Stone Band, a coccolith micrite, details shown enlarged, Pectinatus Zone, Upper  Kimmeridge Clay, east of Kimmeridge Bay, Dorset, 17th September 2012

Various fallen and wave-washed blocks from the White Stone Band, a coccolith limestone, Upper Kimmeridge Clay, east of Rope Lake Head, Kimmeridge, Dorset, 2010

Burrows in the White Stone Band, Brandy Bay, Kimmeridge, Dorset, March 2012

The large blocks shown above have been to some extent eroded and cleaned by the sea. The White Stone Band consistantly contains some burrows in the central part, presumably of crustacean origin. It is interesting that impressions of small decapod crustaceans, resembling Palaeopolycheles in morphology, occur in both the Freshwater Steps Stone Band and in the Middle White Stone Band (Ensom, 1986). Both of these are very similar, laminated coccolith limestones of the Upper Kimmeridge Clay. It seems quite likely that the burrows in the White Stone Band have been made by the decapod Palaeopolycheles. For more on the occurrence of Polychelidae crustaceans in Upper Jurassic black shale facies see Aguirre-Urreta et al. (1990).

An oil-shale horizon is conspicuous in the lower part of the same block of the White Stone Band (the block is believed to be the right-way-up as shown). The White Stone Band is also seen at Brandy Bay and is remarkably similar there. Some photographs from this location are added below and can be compared to photographs of the bed at the main exposure east of Rope Lake Head.

Blocks of the White Stone Band, a coccolith limestone of the Upper Kimmeridge Clay, on the shore at Brandy Bay, west of Kimmeridge Bay, Dorset

Details of the imperfect lamination in the coccolith laminite, the White Stone Band of Brandy Bay, west of Kimmeridge Bay, Dorset

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Fallen blocks of the White Stone Band, seen on the shore east of Rope Lake Head, and near the White Stone Band ledge at sea level, Kimmeridge, Dorset

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Immiscible lenses of coccolith carbonate and some micro-thrusting in the organic-rich White Stone Band, an Upper Kimmeridge Clay argillaceous micrite, Rope Lake Head, Kimmeridge, Dorset

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A sea-washed section through the White Band, a coccolith laminite with some burrows, east of Rope Lake Head, Kimmeridge, Dorset, 2010

[Incidently, notice in the photographs above the burrowed horizon is present both east of Rope Lake Head (the main exposure) and at Brandy Bay. There are remarkable similarities even though these localities are more than 4 kilometres apart and that the Kimmeridge Clay sequence is thinned at Brandy Bay.]

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At first sight the White Stone Band just appears as a fairly well laminated, coccolith bed, associated with oil shale. Only closer examination shows small-scale sedimentary structures, including what seem to be minute ripple marks (?), and some very early disruption of the fine sediment. There is some lenticular lamination with very small scale features resembling ripples. Here and there some miniature scale thrusting of white, coccolith, carbonate laminae occurs. This produces overlap of fragments of laminae.

The burrows in the middle of the bed cut cleanly through the lenticular and laminar structures. The (apparent) miniature-scale ripple marks and other disturbances are of primary sedimentary origin and not due to some later redistribution during burial. The burrows are clean-edged without indications of any deformation. This is not remarkable but it is emphasised here to avoid any suggestion of deformation after burial. The bed is seen now, just as it was, but perhaps with some compaction. Its fundamental structure has not been changed by diagenesis (as has happened in the case of dolostone beds) or deep-burial processes.

This might have petroleum reservoir or fracking implications. The dolostones have changed greatly after about the initial one kilometre or so of burial (see discussion of this elsewhere). This rock is probably very little altered. The "Kimmeridge Micrite" of Balcombe has about 1% porosity, presumably mostly primary, but it also has some ferroan dolomite, so it has been diagenetically affected, like so many Kimmeridge carbonates. [This topic is not properly discussed here, but more information may be added later.]

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WHITE STONE BAND continued:
Lower Oil Shale Band and Internal Unconformity

A small-scale internal unconformity in the lower part of the White Stone Band, east of Rope Lake Head, Kimmeridge, Dorset

The White Stone Band, an imperfect coccolith laminite has an oil shale horizon near the top and another one near the base. The lower oil shale and associationed bituminous shale form a thin black band. Beneath this the lowermost coccolith laminite does not seem to be perfectly parallel. A very small-scale unconformity is present, as shown in the photograph above. This, however, has not been studied in the field and need confirmation. If this is truely the case, then the White Stone Band is probably a composite of several beds: (1) a basal coccolith laminite, deposited and slightly deformed and eroded, (2) the lower oil shale with associated bituminous shale, (3) coccolith laminite, fairly thick, without burrowing, (4) the burrowed central band, (5) a higher unit of fairly thick, coccolith laminite (6) the upper oil shale, (7) the upper coccolith laminite.

It does seem to be a composite of several lithologies, and may, in fact, only appear as one band when seen at a distance, without close inspection. This may be true for other coccolith laminite beds; the Middle White Stone band contains cephalopod fossils in its central band according to Steve Etches (personal communication, 2014).

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WHITE STONE BAND continued:
Coccoliths and the Sediments of Algal Blooms

Coccospheres make up a large part of the White Stone Band, a laminite of the Upper Kimmeridge Clay, east of  Kimmeridge Bay, Dorset

The White Stone Band is quite well-known as a coccolith limestone. Under the scanning electron microscope it has the similarity to Cretaceous chalk by consisting of calcareous nannoplankton. A difference from most of the Chalk is that this Kimmeridge Clay bed is mainly composed of complete coccospheres (the complete calcitic armour of the planktonic algal cells) rather than the separated coccoliths (the individual oval structures that make up the coccosphere). Strata consisting of chalk-like coccolith sediment but associated with black oil shales have been referred to as "Black Chalk" Pearson et al., (2004). This bed, of course, is not black but brownish in a fresh surface and when weathered or sea-washed it is almost as white as chalk.

A major difference from the more homogeneous and bioturbated Chalk is that the Kimmeridge White Band shows fine lamination, at least in part. There are mainly coccosphere-rich layers, separated by thin organic-rich laminae. In addition there are some thicker oil shales layers associated. The striking lamination, which causes the rock to be classified as a laminite, requires special explanation.

The white bands in the Kimmeridge Clay were first identified as coccolith-rich horizons by Downie (1957). An early and key work on the distribution of these, and on the coccolith content (however see Young and Bown (1991) re coccoliths), is that of Gallois and Medd (1979). The White Stone Band of Dorset, 0.9 metres thick, is the thickest of these coccolith deposits and is composed of more than 100 laminae. Elsewhere the bands are generally less than 0.1m. thick and may occur as a single lamina only a few mm in thickness. Some of the other examples are not only thin but are disturbed by burrowing. The White Stone Band has only a limited amount of burrowing and this is in the central part. Thus this bed in the Dorset cliffs is the best example to see in England.

An investigation into this bed has recently been made by Pearson et al., (2004). They considered that the mixed organic matter input that is present was the result of storm transport. There had been seasonal input with a coccolith bloom followed by a more diverse assemblage including dinoflagellates and photosynthetic chlorobiacean bacteria. They theorised that at times the photic zone extended into the euxinic water column. Then organic matter which was exported to the sea bed underwent minimal cycling through oxidation and subsequently became preserved through sulphurization with greatly increased sequestration of carbon. Thus organic-rich, oil shale-type laminae were formed. The authors thought that this process was significantly increased by late season storm-driven mixing of euxinic water into the photic zone. Increased frequency of storm systems would therefore dilute the coccolith input to give an oil shale.

The content of coccoliths in the White Stone Band has been investigated in detail by Young and Bown (1991). The dominant coccolith is the species Watznaueria fossacincta (Black, 1971). There are rare specimens of species from a few other genera. Both coccoliths and coccospheres of Watznaueria fossacincta are present. Particularly interesting is that this unusual rock types has preserved an ontogenetic sequence, that is fossil forms from different stages in the life cycle. There are not only fully grown coccoliths but also specimens at earlier growth stages, including proto-coccolith rings. The early growth phases have previously been described as separate species; these have now been recognised as early stages and thus the former names become synonyms.

There is an interesting palaeoenvironmental implication of this work. Incomplete coccolith morphotypes are unusually abundant in the White Stone Band of the Kimmeridge Clay (Young and Bown, 1991). Work on living species of coccolithophores have shown that incomplete coccoliths are most abundant in samples during the rapid phase of growth of cultures grown in high nutrient media. The ecological analogue is that of bloom conditions. These condition have been independently suggested for the Pectinatites zones of the Kimmeridge Clay by Gallois (1976), in order to explain the coincidence of high productivity, monospecific assemblages and anoxic conditions. The White Stone Band is a classic example of coccolith sedimentation from a phase of plankton blooms in the late Jurassic sea.

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MIDDLE WHITE STONE BAND

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Access to the Middle White Stone band and the Freshwater Steps Stone Band can be achieved at full moon or new moon, when the low tide is very good, but this is not easy and not necessarily advised! The shore exposure of the bed is some distance east, beyond Rope Lake Head. This locality is out-of-site of Clavell's Hard, the tidal trap, and is a significant walk on a rock and slippery shore. It should not attempted except in good tidal conditions and when the sea is calm. There is no route up the cliffs at Freshwater Steps or at Clavell's Hard. Slow walkers may be taking a greater risk than fast walkers because of a possible necessity of hurrying back to Clavell's Hard before the tide rises too far. Geologists, inevitably like to study details and may easily become pre-occupied and not realise that the tide is rising. If the time, tide, or weather conditions are not good, then it may be wise to be less ambitious and to visit more easily accessible, and safer, Kimmeridge Clay cliff sections, as at Kimmeridge Bay or between Yellow Ledge and Clavell's Hard. Large parties or groups of mature geologists may prefer to visit the cliffs closer to Kimmeridge Bay. The Middle White Stone Band and the Freshwater Steps Stone Band exposures are really a place for specialists, on timed and planned field trips and they require appropriate precautions.

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Sequence of coccolith limestones, the White Stone Band, the Middle White Stone Band and the Freshwater Steps Stone Band, east of Rope Lake Hole,  Kimmeridge, Dorset, 2010

Middle White Stone Band, coccolith limestone at Brandy Bay, Kimmeridge, Dorset

The Middle White Stone Band as a ledge on the  shore near Freshwater Steps, East of Kimmeridge Bay, Dorset, 17th September 2012

The Middle White Stone Band, a coccolith laminite in a ledge, west of Freshwater Steps,  Kimmeridge, Dorset, seen fairly close, 17th September 2012

Laminite details, the Middle White Stone Band, Upper Kimmeridge Clay, west of Freshwater Steps, Kimmeridge, Dorset, 17th September 2012

Above the White Stone Band the Middle White Stone Band is another coccolith laminite. It can be seen at some distance from Rope Lake Head when walking along the shore eastward and approaching Freshwater Steps. It is also well-exposed in the cliff at Brandy Bay west of Kimmeridge Bay. The Middle White Stone Band lies above bituminous shales (Coe et al., 1999). On the log and contrasting with the ferroan dolomite, the coccolith limestones generally have very low magnetic susceptibility and this the case with regard to the Middle White Stone Band.

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FRESHWATER STEPS

Approaching from the West (from Rope Lake Head)

A general aerial view of Chapman's Pool, Houns-tout Cliff and Egmont Bight, Dorset, courtesy of the Channel Coastal Observatory

A view of Freshwater Steps in the distance beyond a cliff showing the Freshwater Steps Stone Band, east of Kimmeridge, Dorset, 2006

Cliffs east of Kimmeridge and towards Freshwater Steps, showing the Freshwater Steps Stone Band of the Upper Kimmeridge Clay, 2010

Freshwater Steps, at the cliff top, can be approached easily by the cliff path eastward from Kimmeridge Bay. The cliff top locality can also be approached by walking down from Houns-Tout cliff on the east side. The cliff top, however, no longer gives any access to the beach, and there is no escape up from the beach here.

The shore and cliffs at Freshwater Steps can be approached from the beach via Clavell's Hard and Rope Lake Head, although it is a fairly long walk on a rough beach. It is easier from Rope Lake Head onwards because there is narrow beach of chert and limestone debris from the Portland Stone. However, this route by the beach requires careful attention to tide conditions or there is danger of being cut off at Clavell's Hard on the return walk.

If the tide is very low, it is possible to approach from the opposite direction, from Egmont Bight, and reach the shore at Freshwater Steps by walking round the small promontory. Again, it is necessary to be cautious about tide conditions and watch out for any rise in water level over the access ledges. You could be trapped if you were unwise regarding the tide. See the photographs below.

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FRESHWATER STEPS

Approaching from the East (from Egmont Bight)

Looking from the cliff edge of Houns-tout across Egmont Bight to Freshwater Steps, east of Kimmeridge Bay, Dorset, November 2011

It is easy to visit Freshwater Steps shore from the east, if, but only if, a time of very good low tide is chosen. One route is to park a car at Kingston, walk to the summit of Houns-tout. Descend halfway down the coast path on the east side; then turn left on the middle terrace (there is narrow path) and continue eastward until the collapsed, low angle slopes into Egmont Bight are reached. There is a very poorly-defined path that descends an old landslide to the beach. From the eastern end of Egmont Bight, walk to Freshwater Step promontory. Now take much care regarding the tide! Under no circumstances go round the promontory if there is risk of being cut off by a rising tide. Note that it is also possible to reach Egmont Bight by walking round from Chapman's Pool.

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FRESHWATER STEPS AND FRESHWATER STEPS STONE BAND -

General

The Freshwater Steps Stone Band, a coccolith limestone, forms two ledges because of a small extensional fault, east of Kimmeridge Bay, Dorset, October 2011, low tide

Freshwater Steps promontory, from the chert pebble beach on the west side, location east of Kimmeridge Bay, Dorset, photographed by Ian West, 26th November 2011

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A broad general view of the Freshwater Steps Stone Band, a coccolith laminite, in the Kimmeridge Clay cliffs near Freshwater Steps, east of Kimmeridge Bay, Dorset, Ramues Gallois, 2011

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Freshwater Steps shoreline and cliffs, east of Kimmeridge Bay, Dorset, photographed by Ramues Gallois, 28th October 2011

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Freshwater Steps shoreline and cliffs, east of Kimmeridge Bay, Dorset, photographed by Ramues Gallois, 28th October 2011 - labelled version

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Freshwater Steps promontory, east of Kimmeridge Bay, Dorset, seen from the sea ledges at low tide, 26th November 2011

Freshwater Steps in the 1930s when the stream was flowing over the promontory, Encombe, east of  Kimmeridge, Dorset

Freshwater Steps on the 13th March 1965, when the stream was still flowing over the end of the promontory, east of Kimmeridge, Dorset

A wave breaks on the Freshwater Steps Stone Band, Freshwater Steps, Encombe Estate, east of Kimmeridge, Dorset, 2005

Formerly at Freshwater Steps there were stone steps to the beach from the South Gwyle Valley. This was a route to the sea from Encombe House. In 1954 the estate was private up to the cliff edge and there was no public coast path. I used to walk from Chapman's Pool over Houns-tout Cliff and descend by the steps so as to gain access to the Kimmeridge Clay cliffs from the east. Most of the stone steps were still there but the lowest part has been eroded and there was a steep, but not difficult, clay slope to descend at the bottom. The stream flowed over the end of the promontory. This may have been its natural stream bed, and it is not clear as to whether it was retained in its channel by large stone blocks at the side. The waterfall was effectively an ornamental feature of the estate. Now the stone blocks have broken away and the stream flows, less spectacularly down the west side of the promontory. The stone steps have been eroded away, although there was a ladder for some years, now there is no access to the beach here (and it it dangerous to approach the edge!).

The Freshwater Steps Promontory, east of Kimmeridge Bay, Dorset, seen from Houns-tout, 2007

A view of the Freshwater Steps promontory in October 2005, when the stream had broken through to the east, near Kimmeridge

The Freshwater Steps Promontory, east of Kimmeridge Bay, Dorset, in 2007

Steps to nowhere - the remains of Freshwater Step at the top of a vertical cliff of Kimmeridge Clay, east of Kimmeridge Bay, Dorset, as seen in 2005

The stream at Freshwater Steps was formerly channelled to the end of the promontory, Dorset

Details of the Freshwater Steps promontory, east of Kimmeridge Bay, Dorset, photograph of 1999 or 2000

The Freshwater Steps promontory at the western end of Egmont Bight, near Chapman's Pool, Dorset, September 2007

At Freshwater Steps there were once stone steps down to the beach from the path at the top which comes from Encombe House. Unfortunately these have been eroded away by the sea (I often used to descend here in the 1950s but have never been able to do so in recent years). Now the cliff is vertical and very dangerous, and normally there is no access from above to the beach. At one time there was a rather hazardous, vertical ladder but even that does not seem to be present now (2011).

The thin coccolith limestones of the White Stone Band, the Middle White Stone Band and the Freshwater Steps Stone Band are well-exposed in the cliffs between Rope Lake Head and Freshwater Steps. The promontory of Freshwater Steps is formed by the Freshwater Steps Stone Band forming a ledge at the foot of the cliffs with a bituminous shale, higher, at the level of the waterfall lip. Pterosaur bones have been found in the shales just above this bed (Taylor and Benton, 1986) .

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FRESHWATER STEPS STONE BAND - General Features

Freshwater Steps Stone Band, a coccolith laminite, in the cliff top, east of  Kimmeridge Bay, Dorset

Coccolith Laminite -Freshwater Steps Stone Band, Kimmeridge

The Freshwater Steps Stone Band, a coccolith laminite, at Freshwater Steps, east of Kimmeridge, Dorset, older photograph

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FRESHWATER STEPS STONE BAND

The Uppermost Coccolith Laminite

The Freshwater Steps Stone Band, a coccolith laminite in relation to supra and subjacent strata, Freshwater Steps, east of  Kimmeridge Bay, Dorset, 26th November 2011

Freshwater Steps Stone Band, a coccolith laminite, in the lower part of the cliff west of Freshwater Steps, east of  Kimmeridge Bay, Dorset, 26th November 2011

At Freshwater Steps, east of Kimmeridge, Dorset, there is an excellent exposure of a coccolith laminite, probably the best in the Jurassic of Britain, photo 2011

The Freshwater Steps Stone Band is the highest of three coccolith laminites east of Kimmeridge. It has been figured and discussed by Gallois and Medd (1979) - see this paper for details. It is quite a good coccolith laminite but thinner than the main White Stone Band.

Like the Middle White Stone Band, the Freshwater Steps Stone Band is associated with bituminous shales or oil shales which contain enhanced potassium for the Kimmeridge Clay (but notethere is some problem on the chart regarding potassium and uranium, with different labels at the top to those within the columns). Coe et al.(1999) found values of up to 8ppm near these two coccolith laminites. These seem to be the highest figures for the Kimmeridge Clay of the Dorset Coast.

Coccolith laminite with well-developed lamination, the Freshwater Steps Stone Band, Upper Kimmeridge Clay, Freshwater Steps, east of Kimmeridge, Dorset, 2011

Coccolith laminite details, presumably with annual laminae, Freshwater Steps Stone Band, Upper Kimmeridge Clay, Freshwater Steps, east of Kimmeridge, Dorset, 2011

Another view of the coccolith laminite, the Freshwater Steps Stone Band, at Freshwater Steps, east of  Kimmeridge, Dorset, showing fine carbonate-kerogenous shale lamination, 2011

As noted above the Kimmeridge Clay of Dorset has three main coccolith limestones, each generally less than half a metre thick. They show some fine-scale lamination of alternating white coccolith bands and darker bands of kerogenous shale. Some of the lamination is on a scale of about a third of a half of a millimetre.

The White Stone Band is the most conspicuous, thickest and most well-known of these. It is also the most accessible. It has been studied in detail by Pearson et al. (2004), particularly with regard to palynofacies. It is 80cm thick, compared to the Freshwater Steps Stone Band which is 40cm thick.

The Freshwater Steps Stone Band, a less conspicuous unit in the cliffs, probably has the best sequence of coccolith - shale laminae. Photographs of the closely-spaced lamination in this bed are shown above. This might be one of the best laminites in the Jurassic System of Britain. The bed has been studied particularly with regard to coccoliths (Gallois and Medd, 1979; Young and Bown, 1991).

Coccolith-shale laminite, with podded cabonate and other disruption, Freshwater Steps Stone Band, pectinatus Zone, Upper Kimmeridge Clay, Freshwater Steps, east of Kimmeridge Bay, Dorset, 26th November 2011

An interesting aspect of the coccolith laminites, the White Stone Bands of the pectinatus Zone, is the disruption of carbonate laminae. The clay laminae are fairly continuous. Unexpectedly the clay has not flowed much, but the coccolith carbonate has done so. Bands of this calcite have in many cases been broken. They are particularly prone to form pods, small lumps of carbonate. The carbonate has, at some time in the past, probably during compaction behave like an immiscible fluid moving, to some extent, into pods or globules. This two-phase mechanical behaviour is common in the White Stone Bands or coccolith limestones, and is particularly noticeable where the carbonate bands are relatively thick.

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FRESHWATER STEPS STONE BAND

Petrography by Acetate Peels

Coccospheres seen in an acetate peel from the Freshwater Steps Stone Band at high magnification, after Abineri, 1989

A probable Classopollis pollen grain amongst coccoliths in the Freshwater Steps Stone Band, Freshwater Steps, east of Kimmeridge, Dorset, after Abineri, 1989

Pollen, probably of Classopollis, in the Freshwater Steps Stone Band, Freshwater Steps, East of Kimmeridge Bay, Dorset, after Abineri, 1989

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PALAEONTOLOGY

Ammonites from above the Freshwater Steps Stone Band

Pectinatites - ammonite

Ammonite genus Pectinatites from above the Freshwater Steps Stone Band. This is the holotype (the original named specimen) of Pectinatites (Pectinatites) dorsetensis Cope, 1978. For full details see Cope's paper. This holotype is from the top of Cope's bed 6, 18.2m above the Freshwater Steps Stone Band. The stratigraphical ranges of Pectinatites dorsetensis is: Upper Kimmeridgian, pectinatus Zone, paravirgatites Subzone, ranging from 11.0 to 18.2m above the Freshwater Steps Stone Band (Beds 7-6d of Cope, 1978).

Pectinatites cf. nasutus  - ammonite

Pectinatites cf. nasutus (Buckman), according to Arkell (1947). The specimen is from about 100m west of Freshwater Steps, 2 - 3m above the Freshwater Steps Stone Band. Image modified after Arkell (1947), which should be seen for details and discussion.

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PALAEONTOLOGY

Ammonites from the Rope Lake Head area.

Subplanites - ammonite

An example of a crushed ammonite from the Upper Kimmeridge Clay. This is from Rope Lake Head, 9 metres east of the fault, 4.6 metres above the Rope Lake Head Dolomite Bed. It is after Arkell (1947) and is labelled as Subplanites accordingly, although that identification may or may not be correct. At that time it was listed as an ammonite of the Subplanites grandis Zone. It would now be within the Pectinatites hudlestoni Zone (Cox and Gallois, 1981). For more information on and for identification of Kimmeridge Clay ammonites see Cope (1967; 1978).

Subplanites cf grandis  - ammonite

Subplanites cf. grandis (Buckman), according to Arkell (1947). The specimen is from between Clavell's Hard and Rope Lake Head, 2 - 3m below the Blackstone or Kimmeridge Coal. Image modified after Arkell (1947), which should be seen for details and discussion.

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PALAEONTOLOGY:

Vertebrates of these Cliffs

The Upper Kimmeridge Clay of the coast between Swyre Head and Chapman's Pool and including the eastern part of the area discussed here , has yielded specimens of plesiosaurs, ichthyosaurs, crocodilians, a pterosaur, and a chelonian Taylor and Benton (1986) . The area is currently the most important reptile locality in the Upper Kimmeridgian of Britain. Crocodile bones have been found from the ledges below Swyre Head to Freshwater Steps.

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OTHER LOCALITIES:

East and West

Go east to:

Chapman's Pool, Houns-tout Cliff and Egmont Bight?

Go west to:

Clavell's Hard with oil shale?

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BIBLIOGRAPHY AND REFERENCES - Kimmeridge

Please see separate Bibliography and References

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ACKNOWLEDGEMENTS

Dr. Ramues Gallois is particularly thanked for photographs of the shoreline and cliffs at Freshwater Steps, a relatively inaccessible place, and elsewhere on the Kimmeridge coast. He is the well-known specialist on the Kimmeridge cliffs, and there is much reliance in this webpage on his detailed work (Gallois papers, including Cox and Gallois, 1981). I thank the Kimmeridge Clay specialist and collector, Steve Etches, of Kimmeridge, well-know for his large and important fossil collection, for helpful information and identification of ammonites.
In terms of sedimentology, Dr John Bellamy who wrote a thesis and a paper on the dolomite beds has provided much valuable information. This excellent thesis is particularly good with regard to the Rope Lake Head area. I am much obliged to Dr Geof. Townson for pointing out the Rhizocorallium trace fossils in the Rope Lake Head Dolomite Bed. The late Keith Abineri kindly contributed photomicrographs of details of the Freshwater Steps Stone Band. I am very grateful to the Channel Coastal Observatory for permission to use their impressive aerial photographs of the Kimmeridge coast. Denise Noel has kindly discussed the White Stone Band in the field and afterwards and I much appreciate this. Pari White and Michael Bauer have accompanied me on field work and helpfully provided photographs. I much appreciate the opportunity to reproduce a low-resolution version of one the coastal photographs taken from the sea by Richard Edmonds. I particularly thank Doreen Smith for a old photographs of Freshwater Steps. See her website at: Exmouth to Milford on Sea 1800-2000 (including the East Devon and Dorset World Heritage Site). I thank the staff of Perenco UK who accompanied me on a field trip to the Kimmeridge Blackstone, particularly Jonathon Pim, who found some good ammonite fossils in the Basalt Stone. I much appreciate the advice and help of my daughter, Tonya Loades of Bartley West, Chartered Surveyors.

I am, of course, very much obliged to the Dean and Staff of the Faculty of Natural and Environmental Sciences of Southampton University for kindly supporting this website. iSolutions of Southampton University are thanked for hosting the website.

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Copyright © 2014 Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

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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.

.

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.