Lots of serious fossil hunters go to Seatown in Dorset to find fossil ammonites that have fallen to the beach from the cliffs. The cliffs for the most part are composed of Green Ammonite Member which is part of the Charmouth Mudstone Formation laid down in the Jurassic Period. The ammonites that are most commonly found in this type of rock are Aegoceras, Androgynoceras, Liparoceras, and Oistoceras. I haven’t found any decent fossils of the type I could pick up and take home, but there are plenty of fossils and ammonite impressions to be seen lying in pieces of rock on the shingle beach where people with hammers have broken them open. These pictures show some of the specimens that I found on my last visit. I am not sure which species they represent but maybe some local geologist may be able to look at these images and tell me what they are.
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.
The most common fossils at Seatown on the Dorset coast are belemnites. These are bullet-shaped internal hard parts of a type of extinct cephalopod (think cuttlefish, squid and octopus). For a great deal of the length of the beach, the rock strata are hidden by debris falling down from layers above. There are lots of minor mudslides and landslips. However, as you get nearer to the western extremity of the beach, approaching Golden Cap, a continuous kerb-like, harder, and more calcareous stone layer makes an appearance. This is the Belemnite Stone that has been raised to view by a small anticlinal flexure. Below it are many layers of Belemnite Marl that can be seen in cross-section in the vertical face at the base of the cliff; and also extending out horizontally beneath the gravelly beach and exposed at low tide. They alternate light and dark layers. Fossils are abundant with belemnites predominating but ammonites are also common. The huge numbers of belemnites are thought to have resulted from mass die-offs following mating frenzies.
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.