Cambridge Underground 1981 pp 36-37


Jim Nebelsick

Caves often provide excellent exposures of fossils in various states of preservation ranging from beautifully planed off sections on smooth washed walls of wet stream passages to the preferentially non-eroded fossils that stick out sharply from cave walls thus ideally suited for ripping up both wetsuit and knee. Both types of preservation are helpful in determining the characteristics of the fossil and hence its identity. Fossils are useful for geologists in that they can be used for stratigraphic correlation and as palæoenvironmental indicators. Most caves in Britain are formed in the Carboniferous limestones of Visean age thus making them about 320 million years old.

The fossils that are commonly found in the Carboniferous caves of Britain are crinoids, corals and brachiopods. Although crinoids are common, corals and brachiopods are much more important as far as correlation is concerned. Crinoids are popularly known today as sea-lilies and were much more common in the past than they are today. Although originally the animal was quite elaborate, it is usually only found in a broken up state. Thus all we see are the ossicles which once made up the stalk of the crinoid. These occur either alone or connected to a few other ossicles as well.

Brachiopods are easily recognised fossils that show a great variety within the phylum. Today they are known as lamp-shells and are not very significant, however they, like the crinoids, were much more important in the past. They are characterised by two shells that are unequal in shape. Some of the most commonly seen in caves are the productids (Fig. 1) which consist of two saucer shaped shells with one shell neatly fitting inside the other. Another common shape is that of the rhynchonellids, which have a zigzag shaped opening that is cleverly shaped to allow the maximum opening space with a minimum diameter of the opening itself (Fig. 2). Corals are best distinguished by their cross section and are thus easily identified in cave walls. They are either solitary or colonial with the two illustrated being solitary ones (Figs. 3 & 4).

The stratigraphic use of fossils is only realised if the fossils are seen to change vertically through the rock strata. This happens in the Visean as the corals and brachiopods are used to subdivide the rock into zones and subzones. For example, the Great Scar Limestone occurs primarily in the S2 and D1 zones locally reaching down into the S1 and C2 zones where C=Syringothyris, S=Seminula and D=Dibunophyllum. The first is a brachiopod, the latter two are corals. Using these zones the rock can be correlated with other rocks of the same age throughout Britain. Some fossils are especially useful as marker beds, eg. the algal Girvanella band, which occurs at the entrance to Birks Fell Cave, occurs near the boundary of the Great Scar Limestone and the overlying Millstone Grit.

The other usefulapplication of fossils is as palæoenvironmental indicators. Using the Great Scar Limestone as an example, most of the fossils are found in a broken up state and deposited in massive beds along with many other bits of fossils. The normal habitat of these animals was a reef environment which is not developed to a great extent in the Great Scar Limestone. This fact along with the evidence from the broken up fossils suggesting transport, indicates a back-reef environment that periodically had reefal detritus washed onto it from the reef by storms or normal current or tidal activity. Thus fossils may in fact be found together even though they may not originally have lived in the same place.

Thus, next time that you are tramping down the stream passage of O.F.D., look out for the pure white fossils that show up in beautiful detail with the black limestone as a backdrop.

Further Reading:

British Museum (Natural History), 1975. British Palæozoic Fossils, 4th Edition.
Edwards, W. and Trotter, F.M. 1954. British Regional Geology. The Pennines and Adjacent Areas, 3rd Edition.
George, T.N. 1970. British Regional Geology. South Wales, 3rd Edition.

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