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.
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.
Just east of the outlet of the River Char on Charmouth beach, in the area close to Raffey’s Ledge, the upper shore is strewn with many large irregular stones. Amongst these, the most noticeable are those with white patterns and lines, which on closer inspection turn out to be crystalline calcite-filled cracks in the matrix of the rock. I have been looking at these strange stones over the years and wondering what they were (see the earlier post Pebbles with white lines on Charmouth beach). Now I think I have the answer. They are the worn remnants of the inner cores of Birchi Nodules. Birchi Nodules appear high in the cliff above this section of shore and have a complex structure resulting from a series of processes in the sediments that took place millions of years ago before the sediments compacted into rock. The large ovoid or discoid Birchi Nodules can be seen scattered along a line below the more continuous stratified rocky Birchi Tabular Bed at the top of the cliff. The rest of the cliff below is mostly composed of darker thinly-bedded shales.
These remnants of the inner cores of Birchi Nodules are also a kind of septarian nodule. The stones illustrated here from Charmouth are partial remains that have been worn smooth by rolling around on the beach for a long time. Further east along the coast at Ringstead I have seen complete septarian nodules that have freshly fallen from the cliff face of a different type of rock formation (Septarian Nodules at Ringstead).
[I found out about Birchi Nodules from the most excellent on-line resource for the geology of the Dorset Coast written by Ian West. This is a veritable cornucopia of information but requires that you continuously scroll down the page to locate the items in which you are interested. It is well worth the effort if you really want to find out the information.]
When I visited Seatown today, the sea was spectacular. Although the weather had not been particularly windy or stormy, the waves were huge and crashing with great force against the pebble bank on the shore. Each new wave increased the white foam till it looked like whipped cream, or the penultimate stage of egg whites beaten for meringues. Here are some pictures of the seafoam being generated by the tremendous energy of the sea. Click on any image to enlarge.
In my previous post about rip-rap rocks on Seatown Beach in Dorset I mentioned that the boulders had been positioned to protect the soft cliffs from erosion. This post shows the first of a series of photographs illustrating these soft eroding cliffs on the western side of the beach.The cliffs are made of Lower and Middle Lias rocks from the Jurassic Period.
The lower part of cliff section nearest to the rip-rap, and just west of the fault valley with the River Char, is the Green Ammonite Member of the Charmouth Mudstone Formation; this is principally a medium or dark grey mudstone which can be silty or sandy in the top part. The upper area of the cliff belongs to the Dyrham Formation; this is more variable in composition and includes pale to dark grey and greenish grey micaceous mudstone with interbeds of siltstone or very fine sandstone that weathers to brown or yellow (Woods 2011). Clay, sandstone, limestone layers and concretions are much more common in the Dyrham Formation. It is difficult to see exactly what is going on with the stratification here because so much is obscured by landslips and mud slides. Small streams cut down from the cliff top, softening the mudstones and sandstones, causing areas of collapse, and carrying the muddy fluid and liquid clay down to the pebble shore. The pattern of erosion and weathering is repeated all along the beach towards Golden Cap.
As usual, I am still working out my understanding of these perplexing cliffs, trying to find key fixed points or features from which I can work outwards to unravel the stratigraphy. At first it all looks a bit of a mess but I am sure I can interpret it in the end. At least there was a great array of natural textures and patterns to capture, and many processes of accumulation and disintegration to study.
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.
The cliffs are eroding in many places along the Dorset coast, particularly where the rocks are soft. This results in land slips and mud slides. It has always been going on but in recent years the erosional processes seem to have accelerated along with changing weather conditions. At Seatown on the coast near Chideock in Dorset, large boulders have been imported to protect the shore from the sea adjacent to the Anchor Inn that sits at the mouth of the River Char. I cannot name the rock types represented in the rip-rap for certain since they are not local to the area and have been chosen specifically because they are harder and more resistant than the cliffs on this beach. I am not even sure that they were quarried in Britain. However, some of them remind me a lot of Carboniferous limestone with fossils, calcite and haematite inclusions. Anyway, they are really interesting and well worth a closer look. The patterns, colours, and textures are amazing. I would like to visit again when the rocks are wet and the more subtle colour variations would be highlighted.
Click HERE to buy prints of these and other rock images featured on Jessica’s Nature Blog.
For information about the geology of this location please look at:
John C. W. Cope Geology of the Dorset Coast, Geologists’ Association Guide No. 22, Geologists’ Association, 2012, pp 159-167, ISBN978 0900717 61 1.
West, Ian. Kimmeridge Bay, Dorset. Geology of the Wessex Coast of Southern England. An on-line information resource.
M. A Woods (compiler) Geology of south Dorset and south-east Devon and its World Heritage Coast, British Geology Survey, NERC, 2011, pp 61 – 67, ISBN 978 085272654 9.