Leader: Dr Paul Davis, Natural History Museum, London.
A walk organised as part of the Lyme Bay Fossil Festival. Beginning at the foot of the cliffs to the east of the river and walking eastward approximately 2km.
Report and photos C. Popham.

The Jurassic period sees the first appearance of ammonites in the fossil record and the appearance of these creatures is used as the delimiter of the Triassic-Jurassic boundry. Examples of the earliest species are seen in the beds exposed in the cliffs of Pinhay Bay to the west of Lyme Regis. These beds form the lower lias sub group and are characterised as rhythmic beds and are particularly well exposed in Church Cliff.

Ammonites were free-swimming creatures, usually with a coiled shell, looking similar to and related to the modern Nautilus. They evolved many forms and the appearance and disappearance of individual species acts as a useful marker in the fossil record. Their hard shell meant they are abundant in the fossil record and because they were free swimming they were widely distributed in the prehistoric oceans consequently leaving a well distributed fossil record. When studying the fossil record we are able to identify the fossils of individual ammonite species in regions that are now geographically distinct. As a result it is possible to infer that rocks bearing the same ammonite fossil fauna were laid down at the same time in prehistory and thus correlate different rock types at different sites. This is the basis of stratigraphy.

The Charmouth mudstones are comprised of 4 sub groups: The 'Shales with Beef' (these are the oldest and are separated from the next group by a limestone horizon). Black Ven Marl Belemnite Marl Green ammonite beds. These groups were to be encountered in a progression as we walked eastward from Charmouth Heritage Centre and represent a gradual younging of the beds at a rate of approximately 1 million years for each mile walked.

Charmouth beds

The shales with beef are formed from sediments deposited at the bottom of an ocean approximately 200m deep but with a low oxygen level at the sea bed. This is ideal for fossil formation as there are few creatures on the bottom of the ocean to break up the dead creatures that sink there. The low oxygen level also implies little stirring of the water and so there will be little mechanical breakdown of the creatures. However, it also means that the only fossils likely are those of free swimming creatures.

The principal fossils are thus: ammonites and belemnites (free swimming squid like creatures), rarely swimming reptiles: ichthyosaurs and plesiosaurs and wood remains: cycads, ferns and conifers similar to modern redwoods.

The term beef refers to calcite bands that occur within the mudstones as it is said to resemble the fibres of fresh beef. As the mud was compressed by the deposition of further sediments there came a point where the clay particles begin to lithify: turn into solid rock, and during this process water was squeezed out forming layers in the sediments. Dissolved salts in the water were literally squeezed out of solution by the high pressure and began to crystallise. The most abundant salt was calcium carbonate and this formed the calcite crystals of which the beef is formed, aligned at 90 degrees to the lie of the beds.

The most predominant ammonite fossil, found washed out of a recent landslip at the beginning of the cliffs east of Charmouth was Promicroceras. This appeared as both pyritised fossils showing excellent preservation of the shell form but also as ammonite biscuits: ammonites fossilised by the beef forming process. Pyrite: iron sulphide, is precipited in low oxygen environments often in decaying organic matter. This is a very hard, fine grained mineral and may preserve very fine details of the fossil such as the suture lines in ammonite fossils.

Another abundant fossil ammonite was Microderoceras, characterised by a double row of spines on the shell, commonly found in birchi nodules. These are limestone nodules that precipitated rapidly from the sea water, possibly in a matter of hours. This rapid precipitation is indicated by the fact that entire undegraded fossil fish have been found in these nodules. Critically from the point of stratigraphy, Microderoceras only occurs below (ie earlier in history) than the limestone horizon that marks the transition from Charmouth mudstones to Black Ven Marls.

There then followed a discussion of the nature of ammonite shells. Many animal shells are made of a form of calcium carbonate called aragonite. This is unstable under the environmental conditions found at the surface of the earth and is only possible because the animals constantly renew the shell. Once the animal dies the aragonite reverts to calcite, the form of calcium carbonate that is stable on the surface of the earth. Is it possible that this change, in the high pressure environment found 200m down in an ocean, may have acted like a seed crystal dropped into supersaturated salty liquid. The change in the animal shell may have caused the birchi nodules to precipitate around the dead animal, thus explaining why so many well preserved fossils occur in these nodules, which are much sought by local collectors. As a footnote it was pointed out that there is a point at a depth of about 2000m in the ocean where the conditions are such that aragonite is the stable form of calcium carbonate.

The Charmouth mudstones and Black Ven Marls are both black shales. This is due to the high organic matter content of these beds. This was mostly derived from algae in the seawater, which on sinking to the bottom did not rot due to the low oxygen environment. In a circular argument it may be that this organic rain used up any available oxygen in the ocean depths and thus contributed to the low oxygen availability. Becoming compressed and pressurised, oils from the organic material are squeezed out giving the shales a distinctive smell but also contributing to the oil field in Poole harbour.

Moving further eastward the birchi nodule layer was observed in the cliffs as it approached beach level. This occurs because the strata in the cliff dip at an angle of about 3 degrees to the east. Several birchi nodules were collected from the beach. These nodules have a slightly green hue and may have faint layering whereupon they are termed Stonebarrow Flatstones.

The walk then encountered a prominent feature in the foreshore; a broken band of material about 1m wide running straight out to sea. Looking back at the cliff it was apparent that this pointed directly to a corresponding vertical discontinuity in the cliff where a small stream was making its way down. This is a fault where the ground to the east has been downthrown by about 2m due to earth movements. This was particularly apparent with respect to a prominent limestone horizon in the cliff. The nett effect however was that in the space of 1m lateral travel we had walked forward in history some tens of thousands of years and had made the transition into the Black Ven Marls.

Faulting

There was an immediate change in the fossil fauna. Promicroceras were still seen however we were unable to find any Microderoceras. In their place we found the ammonite Asteroceras a marker for the transition. Significantly however we also found the first fossil shellfish: Oxytoma. This immediately implied a significant change in the ocean environment, with sufficient oxygen at depth that benthic creatures were able to survive.

Fossils of the dinosaur Scelidosaurus have been found locally in and only in the Black Ven Marls. The implication is that these land animals were living on islands not too far away: Dartmoor and possibly the Mendips. Occasionally when these creatures died they were washed out to sea and when they sank became fossilised. There are a number of opinions as to why these dinosaurs only appear in this layer, including the possibility that only at this time was the high ground exposed as islands due to low sea levels or that only at this time was the environment favourable to them being in the vicinity.

Wood is another common fossil in this layer. Again this was washed out to sea from nearby land and may be in the form of whole tree trunks. Such large pieces stayed afloat for perhaps a number of years and during this time they became homes to colonies of crinoids: fan like creatures sometimes called sea lilies, related to sea urchins and starfish. Eventually the logs sank and the crinoids became fossilised beneath them.

Even within the Black Ven Marls there are marked changes in the abundance of ammonite fossils; at some points there are none to be found yet within a few meters they may become so abundant that there is one under every footstep. This indicates that there were changes in the environment, either the environment in the sea changed so that at times there were no ammonites present or the oxygenation level on the sea bed changed so that fossilisation became more or less likely.

Within the Black Ven Marls there are also a series of hard grey limestone beds. These are a further indication of a change in the environment. As outlined already, the dark mudstones indicate an abundance of organic material in the water column. In contrast, the limestone beds formed as the result of mineral precipitation from the water with very little organic material, possibly in a clear shallow sea. However, these beds also contain rare fossil brachiopods: benthic creatures, thus indicating sufficient oxygenation of the water to support life on the bottom. They also contain septarian nodules: limestone nodules which have been fractured by great pressure and the fractures infilled by calcite.

In this section of the Black Ven Marls there is an unconformity where 6 ammonite zones found elsewhere are missing. These may have been eroded away at this location or were not deposited. What is sure is that the limestone bed beneath these missing layers was exposed on the ocean bed and was colonised by boring worms as there are abundant trace fossils in what is known as coinstone. This marks the transition from the lower to upper Black Ven Marls, which are characterised by abundant pyritised ammonite fossils.

Coinstone

The transition to the Belemnite marls is marked by another unconformity: a limestone bed sitting upon a layer of rubble representing an erosion surface. The belemnite marls are paler in colour than the Black Ven Marls and contain almost limitless numbers of belemnite guards. These beds are exposed in the cliffs for several miles, stretching beyond Golden Cap as far as Seatown.

Belemnites were free swimming creatures much like a modern squid. The fossil guards are the remains of an internal body support which also aided in buoyancy control, equivalent to the hard white cuttlefish 'shell' frequently found on beaches.

The final stratigraphic units: the green ammonite beds, covered in this lias sequence could not therefore be examined closely on this walk. We were however able to see them capping the cliffs in several places, most notably as the crest of Golden Cap. These beds are made of golden sand, the name comes from the slightly greenish tint to the calcite which fills the chambers of the fossil ammonites in these beds.

Paul was thanked by all for a very informative walk and we were then free to make our way back to Charmouth at our pace examining the various features previously discussed.

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Words & Photos: Chris Popham

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