Mudslides are common on the cliffs between Lyme Regis and Eype in Dorset, England. On the western half of the beach at Seatown, the lower cliffs are composed of soft dark grey mudstones of the Green Ammonite Member, part of the Charmouth Mudstone Formation, dating from the early Jurassic period. The mudstones become more silty and sandy in the upper part of the exposure, and the top of the member is marked by the base of a micaceous sandstone known as the Three Tiers. Above this is the Dyrham Formation comprising mostly soft pale, dark or greenish grey sedimentary rocks some of which weather to brown or yellow colours (Woods 2011 pages 30 -34).
After heavy rain, water streams down the cliff face carving narrow valleys through the soft mudstone and sandstone strata, liquidising it, and causing flows of pale viscous mud that ooze over the bright orange pea gravel and pebbles on the shore. I stopped to photograph the interesting contrast of colour and texture as the two materials mixed, and it started to rain. The scattered drops pitted the mud surface and I moved in for a closer shot of the patterns. I then noticed that the mud showed shallow flow marks and very small tracks and trails. It was not until I returned home and examined the pictures that I discovered the small creatures that were actually making the trails. They were small amphipod Crustaceans, probably sandhoppers, that were bogged down and trying to break free. Presumably, they had been sheltering from exposure in the damp gravel while the tide was out and suddenly found themselves engulfed by the mud.
I found all this particularly interesting from a palaeontological point of view, or perhaps I should say an ichnological one. Trace fossils are often found in rocks and there is a constant quest to interpret marks that have become preserved in the stone, whether made by elemental conditions or by creatures. Observing events and phenomena in the present time helps in the understanding of the phenomena of the past. It is possible to imagine that the rain pits and sandhopper trails in this mudslide could themselves be preserved as future trace fossils. All that would be needed is some hot sun and drying wind, quickly followed by burial in a fresh deposit of liquid clay or mud or gravel.
Intriguingly, just a few hundred metres further along this west shore at Seatown, the rocks are absolutely riddled with millions of trace fossil burrows made by marine invertebrates like worms and crabs – but more about that later.
