The soft smooth almost liquid muds that flow down the cliffs at Seatown after rain, pool and sink into the shingle on the beach. It doesn’t take long to see amazing networks of tracks and trails on the mud surface. These are made by a myriad of small invertebrate seashore creatures like worms, snails, and sandhoppers as they walk across, burrow, and tunnel into it, foraging for food and seeking shelter from exposure. The number of distinct track marks is amazing and I have no idea which mark was made by which animal (that is a whole new project requiring the collection of some mud samples for identification of the occupants of this habitat). Large bird footprints from crows and gulls show that these areas are also good places for them to feed on the creatures in the mud.
Images can be seen in greater detail by clicking on any photograph to view in the gallery, and then clicking “View full size” below the picture.
The alternating dark and light rock layers of the Belemnite Marls (belonging to the Lower Lias division of the Jurassic Period) at Seatown in Dorset, England, are riddled with small trace fossil burrows. These are mostly tunnels that were dug into the soft seabed sediments by marine organisms such as marine worms and crabs before the sediments became lithified or converted to hard stone. The patterns of these trace or ichno fossils in the cliffs show a wide range of sizes in the burrows with cross-section and longitudinal section views. Some of the tunnels are branched, some are u-shaped, and many are irregular. The shape and size of the burrows, and the particular location of the stratum in which they appear, provide clues to the identity of the creatures responsible. The burrows include Rhizocorallium, Thalassinoides, and Chondrites (Woods 2011). Most of the burrows shown in the photographs here are easy to see because of their contrasting colour – they have been excavated in layers of the darker sediment and have at a later stage been in-filled with the lighter coloured sediments from the layer above. The opposite can also happen, with burrows in lighter sediment being infilled with darker material from above, as seen in a couple of the pictures. Not all the trace fossils are burrows. Some traces appear to be a breaking up of the semi-solidified surface deposits with inter-mixing of sediment from the deposit above.
Woods, M. A. (compiler) (2011) Geology of south Dorset and south-east Devon and its World Heritage Coast: Special Memoir for 1:50,000 geological sheets 328 Dorchester, 341/342 West Fleet and Weymouth, and 342/343 Swanage, and parts of sheets 326/340 Sidmouth, 327 Bridport, 329 Bournemouth and 339 Newton Abbot. British Geological Survey, Keyworth, Nottingham. ISBN 978-085272654-9, pp 28-33.
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.
You don’t exactly have to keep your nose to the ground to see them but you do have to be a keen observer to notice all the different tracks and trails left on the soft wet sediments of the beach at low tide. Larger marks left by people and vehicles are the first ones you see. Bird footprints are every where. The birds are feeding on all sorts of invertebrate seashore creatures like worms, small crustacea and molluscs – all of which leave holes, burrows and furrows as they move in and out of the sand and across the surface. Some of the pictures shown here simply aim to give the general context for the area of Whiteford Sands that I was walking across. If you look closely the other images, you will see not only the ripples in the sand but also the intricate network of traces left by the virtually invisible organisms that inhabit this ecosystem. The larger furrows in photos 1, 12 and 13 are made by the common winkle (Littorina littorea Linnaeus). I cannot name each animal that is responsible for each of the other types of trace. However, I am sure that there will be some specialists out there who could, especially those researchers concerned with the interpretation of trace fossils (the ichnologists).
Fossils can include not only the actual preserved remnants of organisms from times long past (like shells and bones), and the replacements of such organisms (such as infilled moulds and casts), but also the evidence of their existence – for example their modified habitats, trails, burrows, tubes, marks and structures left by their feeding activities. Such evidence of long dead creatures is referred to as trace fossilisation; and the study of fossil traces is termed ichnology.
Part of the process of identifying the exact nature of trace fossils and reaching an understanding of their significance, involves making observations of the behaviour of related or similar present day organisms. This includes a study of their activities and the impact of them on their immediate environment in life. It also involves making records of the way in which the organisms change – what happens to them and their habitat after death (taphonomy).
Many of the trace fossils which I have discovered while walking along seashores and looking at cliffs, outcrops and boulders on beaches were made by various invertebrate seashore creatures in soft intertidal sediments that have later been buried, compressed, and hardened into rock. Recent marine worm tubes of various kinds are a fairly common occurrence on beaches and also as fossils in the sedimentary rocks of, for example, the Dorset coast in England. A short while ago I posted some photographs of fossil worm tubes that I found at Winspit.
For these reasons I was particularly interested to see the millions of sand grain tubes of marine polychaete worms on the low tide beach at Whiteford Sands on the north coast of the Gower Peninsula in South Wales. Many could be seen in situ, no doubt with worms in residence, projecting from the surface of the wet sand like some kind of stunted crop with seashells scattered among them. On the shallow bank created by a beach stream wending its way seawards, it was fascinating to see how far down into the sediments the tubes extended, a thick layer of unknown depth. It reminded me of a fantastic illustration in a book I recently acquired by Wilhelm Schäfer called Ecology and Palaeoecology of Marine Environments, originally published in Edinburgh by Oliver & Boyd in 1972. Copyright considerations mean the book is not old enough for me to reproduce the drawing here for you without written permission but Figure 190 on page 326 shows a cross-section through bedded sands and muds and shell deposits with many-branched and frequently extended dwelling tubes of Lanice conchilega cutting across the layers – just like the deposits on Whiteford Sands. Even though the book is old it is still widely available secondhand and is a wonderful repository of information.
Boulders on the back of the quarried ledge at Winspit contain fossils and trace fossils. A recent discovery of mine when I last visited were numerous very small worm tubes, frequently amassed in discrete areas or layers, around and below black chert nodules in the Portland Stone Cherty series rocks. It is difficult to be certain of the identification of these worm tubes (maybe someone can look at these pictures and tell me) but it is known that serpulid worms called Glomerula gordialis are found in this particuar geological and geographical location, and I am assuming for the time being that these are the same species.