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

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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

INTRODUCTION:

Location and Geological Maps

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".

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.

INTRODUCTION:

Safety; the Hazards of Kimmeridge Cliffs

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.

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.

INTRODUCTION:

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

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.

STRATIGRAPHY:

Cliff Section

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.

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.

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 .

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.

LOCATION:

Rope Lake Head Area

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.

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.

STRATIGRAPHY AND LOCATION:

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

The Rope Lake Head Dolomite Bed is a thin stone band, about 0.5 metres thick, easily found because it is not far above the Blackstone, and because it can be seen on the shore at Rope Lake Head, and just to the west of the 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).

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.

STRATIGRAPHY AND LOCATION:

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

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.

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 .

STRATIGRAPHY AND LOCATION

The Short Joint Coal
(K43/3, low hudlestoni Zone, Upper Kimmeridge Clay)

In terms of oil shale, perhaps the next-most important bed to the Blackstone is the Short Joint Coal. This is bed 43/3 of the hudlestoni Zone. It is higher than the Blackstone and is 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 bed is less than 30 cm thick and was not economical to mine for oil shale. The organic content is probably quite high.

The Short Joint Coal is oil shale on above and belwo a bed of an unusual type of limestone. It was described as coccolith-rich limestone within oil shale by Cox and Gallois (1981). However, 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 P 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 568ppm 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 bed is unusual in some respects, and containing both oil shale and a rhomb limestone. It is not an oil shale throughout in spite of the name.

STRATIGRAPHY AND LOCATION;

Rope Lake Head:
Ichthyosaur Remains

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.

STRATIGRAPHY AND LOCATION

Rope Lake Head:
The Shingle Beach

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.

STRATIGRAPHY AND LOCATION:

Rope Lake Hole and the Basalt Stone Band (KC 44/2)

(Very conspicuous dolomite with dicey weathering in the Pectinatites hudlestoni Zone of the Upper Kimmeridge Clay; not basalt of course.)

The Basalt Stone is a grey, argillaceous dolomite or dolostone with very high magnetic susceptibility and very low gamma ray values (because of low clay content its K, U and Th values are low). It bed 2 in unit KC 44 of the Pectinatites hudlestoni Zone of the Upper Kimmeridge Clay. It is best seen to the east of Rope Lake Head, but this is not always an easy place to reach because of problems with the tide (you can get cut off at Clavell's Hard, so careful planning is needed). The Basalt Stone is persistent in the Kimmeridge area, unlike some thin dolomite 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.

The name "Basalt Stone" has arisen because it has a superficial similarity in appearance to a basalt sill ( Arkell, 1933). It is of a grey (and locally slightly brownish) colour; it has a massive rather than laminated structure, and has a characteristic cuboidal or "dicey" fracture pattern. It is about 1.25 m. thick. The 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.

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 dicey weathering and they contrast markedly with the blocks of the finely lamined White Stone Band, which also descend in the same area.

WHITE STONE BAND (Coccolith Laminite Limestone)

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

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).

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 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.

WHITE STONE BAND continued:

Lamination and Details

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.

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.

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

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.

MIDDLE WHITE STONE BAND

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.

FRESHWATER STEPS

Approaching from the West (from Rope Lake Head)

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.

FRESHWATER STEPS

Approaching from the East (from Egmont Bight)

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.

FRESHWATER STEPS AND FRESHWATER STEPS STONE BAND -

General

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!).

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) .

FRESHWATER STEPS STONE BAND - General Features

FRESHWATER STEPS STONE BAND

The Uppermost Coccolith Laminite

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.

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).

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.

FRESHWATER STEPS STONE BAND

Petrography by Acetate Peels

PALAEONTOLOGY

Ammonites from above the Freshwater Steps Stone Band

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 (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.

PALAEONTOLOGY

Ammonites from the Rope Lake Head area.

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 (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.

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.

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?

BIBLIOGRAPHY AND REFERENCES - Kimmeridge

Please see separate Bibliography and References

ACKNOWLEDGEMENTS

Dr. Ramues Gallois is particularly thanked for photographs of the shoreline and cliffs at Freshwater Steps, a very 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).
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 am 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.

Copyright © 2011 Ian West, Catherine West, Tonya West and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

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

Webpage - written and produced by:

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

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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 is an unfunded, private activity, and does not necessarily represent the views of Southampton University. Any field activities shown are not necessarily those of any specific organisation and mostly represent private field work.