The maps above, and the general cliff section, provide the introductory geological setting for the area and also location information. The general pattern is that on the hill tops there is weathered Cretaceous Upper Greensand (brownish green) with much chert and with some Chalk (of about 100million years old). These units lie unconformably on grey Liassic (Lower Jurassic) marine clays (of about 190 million years old) with ammonites, belemnites and, occasionally, ichthyosaurs and plesiosaurs. The occurrence of the Cretaceous sands and chalk above Jurassic clays is the cause of much landsliding in the area. The clay cliffs undergo rapid sea-erosion which reveals the fossils. The details of the geology are discussed below.

Lyme Regis is in a valley where the small river runs out to sea. Coaches and car for field trips can park at the large car park up the hill on the west side of the town. Cars can also park at beach level near the Cob. Note that it can be difficult for large coaches to pass through the centre of the town with its narrow steet and a sharp turning, if they are coming in on the road from Charmouth. In the town centre area there is an excellent museum and fossil shops. The field excursion described below is to the west of the small harbour - the Cobb. Usually there is access to the cliffs without major problems from the tide, but this may not be the case in stormy weather. Access to the coast eastward requires low tide. The Blue Lias is well exposed from Chippel Bay to Seven Rock Point and beyond.

Go to: Lyme Regis Town and Seafront webpage.

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STRATIGRAPHY:

The Lower Jurassic System of West Dorset.

The simplified outcrop map above shows the distribution of Jurassic strata at the surface on the British land areas. The offshore outcrops, such as in the North Sea and the English Channel are not shown here. Jurassic also underlies extensive areas in southeastern England, as in the Weald of Kent and Sussex. Lyme Regis is shown at the southwestern end of the Yorkshire to Dorset outcrop.

The table above gives the chronostratigraphy of the Jurassic System in terms of stages with dates mainly from Odin as given in the International Stratigraphic Chart of UNESCO, 2000. It is is recommended that checks be made with regard to correction and updating from time to time. It is likely to be modified but mostly only to a small extent. The Jurassic/Cretaceous boundary presents special problems and there is some uncertainty here.

A section in another diagram is a simplified and introductory scheme to the lithological sequence of the Lower and Middle Jurassic strata. A third diagram gives the traditional zonal scheme of the Lower Jurassic with lithological equivalents, based on House (1993) and Cope et al.(1980) .

The Blue Lias is the unit mostly discussed here. This consists of thin alternating shales, marls and argillaceous limestones at the bottom of this Jurassic succession. The Blue Lias overlies " Rhaetic " strata (Penarth Group) which is a lagoonal facies of Upper Triassic age (and in turn overlies Triassic red-bed, desert facies). The Blue Lias represents the first normal marine sediments resulting from the transgression of the sea over the deserts and lagoons of parts of the great supercontinent Pagaea. The marine fauna increases upwards in abundance and diversity as the sea opens and deepens and then ammonites and ichthyosaurs and plesiosaurs live in the fairly warm water of moderate depth above the muddy sea-floor.

The Blue Lias is the lowest unit discussed here. This consists of thin alternating shales, marls and argillaceous limestones at the bottom of this Jurassic succession. The Blue Lias is 32m thick, and includes all the Hettangian Stage - Psiloceras planorbis, Alsatities liasicus, Schlotheimia angulata plus the lower part of Sinemurian Stage - Arietites bucklandi, Arnioceras semicostatum . These limestones and shales overlie " Rhaetic " strata (Penarth Group) which is a lagoonal facies of Upper Triassic age (and in turn overlies Triassic red-bed, desert facies). The Blue Lias represents the first normal marine sediments resulting from the transgression of the sea over the deserts and lagoons of parts of the great supercontinent Pagaea. The marine fauna increases upwards in abundance and diversity as the sea opens and deepens and then ammonites and ichthyosaurs and plesiosaurs live in the fairly warm water of moderate depth above the muddy sea-floor.

Marine mud facies of the Shales-with-Beef, an oil source-rock, and the Black Ven Marls follow. The Shales with Beef are 25m thick and belong to the zone of Coenisites turneri . These are important fossiliferous units. The Belemnite Marls lie above.

STRATIGRAPHY:

Jurassic Palaeogeography

A large map is provided above showing the position of the shelf seas of Europe in the Jurassic. Because this is a very generalised and not for a specific time in the period it should not be used as a precise marker of palaeolatitudes. It does indicate the area of extensional tectonics and basin development at the northeastern end of the Atlantic at that time. Not only was the formation of rifted basins responsible for much accumulation of clay, limestone and sandstone sequences of the Jurassic. In addition, the breakup of the old supercontinent Pangaea led to an increase in the number of spreading centres in the oceans (Lemon, 1993). This resulted in a consequent displacement of seawater from the oceans producing a rise in sea-level. Both the rifting and this process caused the transgression of Jurassic marine deposits over Permo-Triassic, desert red-beds.

Also above is a simplified palaeogeographic map showing the generalised distribution of sea and land in the British area during Early Jurassic times. The southern part of the Atlantic Ocean was opening to the southwest, but the North Atlantic was not open at this time. Shallow shelf seas with some locally deeper basins occupied much of the British region. This map is to set the scene in broad terms and the details varied at different times within the Early Jurassic. The southwest peninsula was probably the landmass from which the tree remains in the Lias of the Lyme Regis and Charmouth areas have come. There is uncertainty about whether Wales was completely submerged or whether there was land in the area. Because of a general later tendency for Britain to tilt eastward towards the North Sea basin, the London-Brabant (or London-Belgium) Uplands are now below sea-level and buried under younger sediments. As this area of Palaeozoic, hard-rock hills has gone down with the tilt so the Palaeozoic area of the Welsh mountains has come up in the west. The London-Brabant uplands are mostly at about 500m depth in the London area but rise to within 50m of sea-level in North Buckinghamshire.

The Lower Jurassic (Liassic) sea was continuous between the Dorset coast, where the strata are now very well-exposed, through the Midlands to the Yorkshire coast, which is also notable for excellent exposures. There are smaller Lower Jurassic outcrops elsewhere, particularly in South Wales and in northwest Scotland (Skye etc.)

(provided for both the East and West Lyme Regis Webpages)

< SRC="http://www.soton.ac.uk/~imw/gif/1bluems.gif" ALT="Blue Lias - log">

This graphic log, largely based on the work of Lang (1914; 1924) , is provided to help recognition of the named (and numbered) beds of the Blue Lias and to enable specific fossil horizons to be located. It is not intended to give full and precise sedimentological details, merely to be some basic guidance for use in the field. The "shale" units are generalised, not distinguishing between blocky, conchoidal marl (calcareous mudstone) and bituminous shale. Nevertheless, because there are so many beds, it is necessary first to recognise some of the main argillaceous limestones, before commencing detailed work. The notes on beds and the fossil listing in the diagram should help in this respect.

The listing by Lang, used here, was not the original. Wright (1860) published a vertical section a listing of Blue Lias beds with details is in the old memoir of Woodward and Ussher (1911). Lang's work is an improvement on this, particularly with regard to ammonite zones. For the diagram given in the present webpage the strata have not been remeasured but there is some partial updating from House (1985; 1993). Further corrections and improvements will be made on the basis of fieldwork when time permits.

The limestone names originate from the quarrying of the cliffs for cement during the 19th Century and beginning of the 20th Century. The origin of the names of some of the quarried beds, such as Third Quick, Venty, Rattle, Top Copper and Iron Ledge can be guessed at. Others such as Glass Bottle are not so obvious (but "a cave has been discovered on the cliffs to the west of the town with some broken fragments of old French brandy bottles" - Anonymous, undated). Skulls refers to nodular, irregular, concretionary beds, to some extent resembling Chalk flint nodules in shape. Table Ledge presumably takes its name from the "Table Rock" that was original present on the shore at the eastern jetty of Lyme Regis (the synclinal structure takes the top of the Blue Lias down to near beach level at the Esplanade). Unfortunately, this shoreline feature (Roberts, 1834) was broken up long ago by coastal quarry workers (Woodward and Ussher, 1911).

The thicknesses given in Lang (1914) are approximate. The paper is on the Charmouth side of Lyme Regis so the thickness presumably apply to Church Cliffs. His 1924 paper gives more precise figures but they sometimes differ appreciably from the earlier estimates. Lateral changes are probably largely responsible and the details are unlikely to be the same at Monmouth Beach and Church Cliffs. In any case, some beds are impersistent. The zonal schemes of Lang have been modified. Used here are the zones as given in House (1993).

For some of the fossils listed in the log please go to the Lias fossils webpage. From time to time more fossils will be added to that page. There have been some changes of names since the publications of Lang. To some extent the list has been updated, but not necessarily completely so. Note that Coroniceras bucklandi is now Arietites bucklandi (in the very early literature it was Ammonites bucklandi); in some cases a particular oyster Ostrea may be now Liostrea; the bivalve Gryphaea incurva is now Gryphaea arcuata ; records of Lima may refer to Plagiostoma; some species of what was listed as Rhynchonella may now be termed Calcirhynchia

The Lias of Chippel Bay, west of Lyme Regis

Study will now be made of the cliffs in Chippel Bay (map reference SY 331913) between the Cobb and Seven Rock Point. If time permits the field trip can be extended to see the base of the Lias and the top of the Triassic Rhaetic (Penarth Group) in Pinhay Bay (map reference SY 318908)

The first good cliff sections encountered are past the yachts and the chalets (where there were once brick kilns) and at the west side of Poker's Pool. Here we see the Shales with Beef overlying the Blue Lias at the foot of the cliffs. Because the general dip direction is eastward we only see the top of the Blue Lias here and in a while we will go further west to see lower parts. The Shales with Beef form the clayey unit that slopes back in the upper part of the cliff, whilst the Blue Lias consists of alternating argillaceous limestones and shales and forms a vertical cliff, although small just here. Landsliding has caused much slumped debris to accumulate at the foot of the cliff.

The Shale-with-Beef are important because they have a high organic content and are significant oil source rocks in the Wessex Basin. The " beef " is an old quarryman's term form of fibrous calcite formed by diagenesis under burial. Such diagenetic fibrous calcite (" cone-in-cone " and "beef ") veins are common features in shales and marls throughout the stratigraphic column; their morphology has been studied for more 130 years (Sorby, 1860). Controversy remains over the time and mechanism of emplacement and the chemical environment in which precipitation took place (Marshall, 1982). Vein growth is commonly shown to be displacive; impressions of a fossil can be found on the upper and lower surfaces of the same vein and shale inclusions may be ruptured into crude conical form (Marshall, 1982 and references therein). Fibrous calcite of this type is normally associated with organic matter and aragonite. Although some suggestions have been made about an early origin, many veins postdate concretions whose curged internal laminae reveal syncompactional growth and some veins have been shown to postdate hydrocarbon emplacement (see Marshall, 1982 for Sorby and futher references).

A particularly interesting aspect of the beef here is its relationship to septarian nodules. 30 cm beneath the Birchi Tabular bed occurs the Birchi Nodular. These nodules, which were described by Bellamy (1980) as compound carbonate bodies, have have a very interesting diagenetic history. They usually have a core of calcite, surrounded by ferroan dolomite and then an outer layer of beef. Sometimes the calcite core is fractured by crystal growth of the later dolomite, in accordance with Todd's (1913) nodule expansion theory. The main point to note is that the beef is of burial diagenetic origin and later than all septarian nodule development, even including the dolomite.

INTRODUCTION:

The Blue Lias Cliffs

(For more photographs and information on the Blue Lias, please go to: Blue Lias part - Lyme Regis to Charmouth Webpage.)

Here are the cliffs in Chippel Bay. The lower part is steep and consists of alternating dark grey shales and light grey, argillaceous limestones (cementstones). This is the Blue Lias. The Shales-with-Beef form an inclined crumbly cliff above. The dog is inspecting the Mongrel Bed, which almost marks the base of the Arietites bucklandi zone of the Sinemurian Stage of the Lower Jurassic System. A closer view of this part of the succession, with an arietitid ammonite is shown in the bottom photograph.

Here is a small fault of rotational type in the Blue Lias argillaceous limestones and shales. This is Sinemurian part. The photograph gives a realistic impression of the appearance and state of the cliffs here on a damp and grey day with some light rain. Notice that the argillaceous limestone beds vary considerably in thickness. When the tide is high the geologist will necessarily be fairly close to these cliffs and there is some risk from falling rock.

Students are seen making a close study of the Mongrel Bed and underlying shales They are studying the macrofossil content and the trace-fossils. They note whether trace-fossils extend down from the limestone into the shales and whether the converse also occurs.

There are various examples of ammonites and nautiloids on the ledges and loose blocks of this area. The example above might seem to the casual observer as advolute but closer examination indicates post-mortem damage. This, however, is of an unusual type.

Higher up the cliff slopes back and consists of shale. Here are the Shales-with- Beef (fibrous calcite - cone-in-cone), the most organic-rich part of the sequence. Beef is a common indicator of organic-rich shales in other sequences (such as the Purbeck).

The bituminous shales form part of the source rocks for the oil of the huge Wytch Farm oilfield. Total organic carbon in the Liassic bituminous shale beds (these are usually thinly-laminated - paper shales) is usually about 6 weight per cent (Ebukanson and Kinghorn, 1990). In argillaceous limestone beds, such as those of the Blue Lias, it is lower, round about 0.5 weight per cent . The organic matter is in the state of kerogen, a microscopic brown waxy substance dispersed through the shale. The kerogen is of sapropel origin (Type II - liptinic), a mixed type consisting of marine algal plankton with some zooplankton (microscopic marine animals). It can, and has, produced both oil and gas if buried for a long period at a few kilometres down where the temperature is round about 100 degrees centigrade. Here, at Lyme Regis, it has not been buried deeply enough and it is not thermally mature. Deeper basins, however, occur to the south and south-east under the sea, and there it is mature.

This photograph was taken at Seven Rock Point further west, in the yellowish light of a fairly low, western sun. The view is from the west towards the east. Here, up-dip compared with Chippel Bay, the top of the Blue Lias rises higher in the cliffs. The base of the cliff is stratigraphically higher. It is now in the angulata Zone of the Hettangian, near the level of the Brick Ledges. Above the vertical Blue Lias section the Shales-with-Beef and Black Ven Marls form an extensive upper, sloped and slumping part of the cliffs, most of it not visible from here and sloping back for some distance. The Blue Lias has no major slip planes in it and does not move; its low-dipping (to the east) stone bands give the resistance to form a vertical wall and the bituminous clays render it mostly impervious. Above though, on Ware Cliffs, there are landslides in the relatively unsupported, overlying clays. Lubricated by water (notice the marks of this on the cliff), the mud from the upper landslides moves seaward over the top, bringing with it some bushes and trees. This is more pronouced during particular wet winters, and the cliffs may be relatively stable in summer.

More beach material has been accumulated here. The beach in the foreground contains much (Cretaceous) Upper Greensand chert and some flint pebbles. These subangular pebbles are light in colour and, consisting of silica, are harder than steel (they should not be hammered because of danger of flakes flying off). These clasts lie between boulders of sandstone (Cowstones) from the Upper Greensand and argillaceous limestone blocks from the Lias. The Upper Greensand boulders tend to be rounded and gritty with bioturbation, some fossils and small dark glauconite grains, visible if examined closely. The Cretaceous material has come from debris fallen from the unconformable Cretaceous strata (Upper Greensand and Chalk) on the high ground landward of here. At low tide the Blue Lias stone bands can also be seen as brown worn reefs on the shore. In the distance is Stonebarrow Hill and Golden Cap, both with brown unconformable Cretaceous on their summits.

Here, east of Seven Rock Point more trees have fallen over the cliff. This photograph was taken in April, 2000 when a slip-plane above the Blue Lias was supplying clay and trees from Ware Cliffs. The process happens sporadically between here and the head of Pinhay Bay. Grotesque tree roots can sometimes be found in Pinhay Bay amongst the logs that have come over the cliff. Occasionally the beaches are dangerously under attack from falling material and not safe to traverse (someone has once had to jump into the sea at Seven Rock Point, many years ago to avoid a rock like a cannon-ball descending and bouncing on the ledges).

Note the similarity of movement of material on horizontal slip-planes here with such movement in the Eocene Barton Clay at Highcliffe on the Hampshire/Dorset border.

The Blue Lias rises in the cliffs in a westward direction so that entering Pinhay Bay there is a high cliff with an obvious difference between the thicker- bedded parts of the Blue Lias, already seen, and the thinner-bedded lower part. Most of this lower sequence, representing the lower half of the Hettangian Stage, has not been quarried for cement and, as a result, most of these thinner, argillaceous limestones do not have names. Further along this stretch of cliff, further into Pinhay Bay is the base of the Psiloceras planorbis Zone which is, of course, the base of the Jurassic System. The actual horizon is not easily found because it occurs within the cyclical limestones of the Blue Lias and is not conspicuous. The lowest part of the Blue Lias is thus Triassic in age and beneath it is the White Lias of the Penarth Group (or "Rhaetic). This is seen near the corner of the bay in the distance at the foot of the tree-covered slopes.

If you study this part of the coast do please take care with this cliff; it is high and much debris falls from above. Notice the pile of fallen debris in the foreground. As far as possible keep back from the foot of the cliff.

STRATIGRAPHY:

Blue Lias:

Diagenetic Development of Stone Bands

There are repeating sequences of calcareous mudstones, bituminous shales and argillaceous limestone in the Blue Lias cliff sections. A truely cyclical interpretation of these requires that the carbonate bands be at least partly primary in origin. There is, indeed, trace-fossil evidence for this. A problem, though, is how much is the present form of the carbonate of concretionary (diagenetic) origin. Both primary sedimentary differences and secondary diagenesis are, in fact, involved here (see papers of Hallam and of Sellwood and other authors).

Here is some obvious evidence for diagenetic processes involved in the formation of stone bands. The student is looking at an early concretionary carbonate nodule (is it the result of input of carbonate from the sea floor or of unmixing?) in cyclical calcareous shale/ bituminous shale sequences. The carbonate-shale sequences at the top of this image involve both primary differences and secondary concretionary carbonate. Sequences like this elsewhere may not always be correctly interpreted. Here, however, as previously mentioned, there is good evidence for a primary difference from trace-fossils (burrows have carried down carbonate), and good evidence for diagenesis from the lack of lateral continuity seen in the case here.

As shown in this photograph there is, the additional evidence for early lithification shown by the absence of compaction in fossils in Blue Lias limestones. Within the stone bands the ammonites are very well-preserved and uncrushed. In the shales they are compacted. For more examples of uncompacted ammonites from stone bands, go to the ammonite section of the Lias Fossils Webpage.)

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BLUE LIAS

Cyclicity in the Blue Lias

This photograph shows the lower to middle part of Blue Lias in Pinhay Bay. Close to medium-spaced cycles are seen.

For comparison here are cyclical sequences in the upper part of the Blue Lias at Church Cliffs. These are wider-spaced cycles with more clay between them.

The main features of the Blue Lias cycles are shown schematically. Note particularly the various details of the limestone lithologies, including the bivalves, the burrow etc.; such features are shown in various photographs in this web page and the Lyme Regis to Charmouth webpage. A direct comparison between diagram and photograph of a cycle is provided in the webpage on the Blue Lias at Church Cliffs .

The origin of this apparent cyclicity is still problematical (see for example - Hallam, 1986). Are these 38 thousand year Obliquity of the Ecliptic or 22 thousand year Precession of the Equinox cycles? See House (1985; 1986; 1993) . There has been signicant recent new work and discussion by Weedon et al. who support the 38 ka obliquity cycle as the main one for the Blue Lias etc. (Rhaetian - Sinemurian). They have tried improved methods, with magnetic susceptibility measurements, and using time-series analysis. You will note from the paper that, although, obliquity cycles are favoured, there has been some argument, there are various uncertainties involved and that this seems to represent the best that can be done at present. The cycles may be better understood in the future with improved absolute dating of the Jurassic.

Notice that if the alternations between limestone and clay truely represent Milankovitch cycles, then the sedimentation rate for the lower part of the Blue Lias (the lower part of the cliff) was significantly less than for the upper part. The sequence represents a marine transgression following the deposition of the lagoonal limestones and shales of the Penarth Group ("Rhaetic" - uppermost Trias). Perhaps there was a lower sedimentation rate in the shallower water. Did the water deepen until the organic-rich Shales-with-Beef were deposited? Did the cycles expand, as a consequence of increased sedimentation rate, as deepening occurred? If this happened then why was so much organic matter accumulated at a time when sedimentation rate seemed to be higher?

Go to - Fossils of the Lias

Pyrite (iron sulphide - FeS2 ) is common in the Lower Lias. In the left-hand image above is a fallen blocks containing the uncompacted shell of an arietitid (i.e. large and strongly ribbed) ammonite which has been replaced by pyrite. The shell would have originally been of aragonite, the orthorhombic form of calcium carbonate, and the pyritisation has most likely occurred after burial. The pyrite has not replaced the shell in a neat pseudomorphic manner but is rather irregular and nodular.

In the right-hand photograph pyrite is seen as as veins in the Top Copper Bed, just above the Mongrel. Presumably the bed received its name from the abundance of pyrite (but note that this is iron pyrites not copper pyrites or chalcopyrite). The reducing (anoxic) conditions in the sea-floor sediments, and sometimes in the lower water column, were produced by the large supply of organic matter. Sulphate-reducing bacteria reduced the sulphate ions of seawater to sulphide, producing the unpleasant gas - hydrogen sulphide in solution. This would have reacted with any iron available to produce the brassy-looking ferrous sulphide - pyrite (not necessarily directly but, perhaps, via hydrotroilite). Pyrite is easily formed by decomposing organic matter in the presence of iron.

" Mr Pepys records the accidental discovery of the production of Iron Pyrites from the decomposition of the bodies of some mice, which had fallen into a pitcher containing several quarts of a solution of iron that had remained unnoticed for a year." (Damon, 1884).

Pyrite is normally abundant in black shales with high organic content. It is frequently present in oil source rocks. An unusual feature is that the pyrite is aligned along some pattern of joints (cracks) but not all the present, open ones. In theory pyrite should have been produced early after sedimentation and later, when the strata entered the methane fermentation zone, its precipitation should have ceased. However, the limestone were lithified and subject to brittle fracture and yet hydrogen sulphide was still present.

Lyme Regis was once celebrated for its hydraulic lime and cement. This was quick setting in marine conditions (Woodward and Ussher, 1911) and was particularly useful for harbour works. The Lyme Cement Works operated in the Ware Cliffs as shown above, between Devonshire Head and Monmouth Beach, and was established by Mrs Eleanor Coade of Belmont House, Lyme Regis (Legg, 1999). She died about 1820. De la Beche remarked that when Smeaton built the Eddystone Lighthouse he obtained Lias lime from Aberthaw in Glamorgan, but if the geological knowledge had been available at that time he could have got it from Dorset. Relics of a 19th Century cement industry at Lyme Regis are old railway lines on the shore in Chippel Bay, and which are still visible at low tide. The argillaceous limestones of the Blue Lias were used and so also were cementstones in the clays and shales above. The stone was originally collected from the fallen blocks on the beach in the bay, and when this had been used up, the cliffs were blasted to bring down more material. This industry resulted in the naming of individual stone beds, to the benefit of geologists. The quarrying seems to have been done regardless of some concern in the area about this type of human coast erosion (particularly complaints about destruction of ledges near the central part of Lyme Regis). There was a cement factory at Monmouth Beach, west of Lyme Regis from 1850 to 1914 (Thomas, 1993). the kilns here probably gave off the polluting, sulphurous fumes that are associated elsewhere with limekilns burning pyritous limestone! Much material has also been shipped from the Lyme Regis coast in the raw state, at one time about 120 tons a week (Woodward and Ussher, 1911). These authors mentioned that the cementstones, with a higher clay content and more aluminium and soluble silica, made better cement that set somewhat harder and had a higher density.

See also the Charmouth Field Guide, regarding the cement factory at Charmouth.

LOCALITY:

Pinhay Bay (west of Lyme Regis)

ACKNOWLEDGEMENTS

I am very grateful to the Geologists' Association and particularly to the Editor of the Proceedings of the Geologists' Association, Professor Richard Howarth, for permission to use the old photograph of Pinhay Bay, above. This is much appreciated. The students and staff both of Southampton University and of the Civil Engineering Department of London South Bank University have generously participated in fieldwork and photography on these cliffs. I am particularly grateful to Dr Paul Sandford, Professor Mike Gunn and other members of staff for their help and cooperation.

Other Webpages on the Lias and Lyme Regis Area