The 80 metre high cliffs on the east shore at Seatown in Dorset along the Jurassic Coast are subject to land slips and rock falls. Large lumps of shattered blue-grey clay are common on the beach. They come from cascades of Eype Clay Member material that forms the lower part of the cliff exposures.
The beach at Seatown in Dorset comprises a series of steep pebble banks. You can see how far up the shore the last high tide has been by the line of natural debris extending along the shore parallel to the water’s edge. On this occasion the strandline was almost entirely made up of dried red seaweed which contrasted well with the pebbles. Quite a number of white cuttlefish bones rested on the weed. I am fascinated by their beautiful structure. For some reason, the shape of the more concave surface (as in image 5) always makes me think of angels.
The shingle shore at Whitstable in Kent is protected by massive wooden groynes or breakwaters. At the time of my visit, the tide was high and the flint and other mostly worm-holed pebbles were steeply banked. The flat top of the beach was stabilised by vegetation with pink and white valerian and yellow ragwort the most colourful flowers. Pale bands of white empty oyster shells (mostly the rock oyster Crassostrea gigas) were high, dry, and dull on the shingle between the groynes; while lower down splashed by waves or heaped up against the wooden sea defence structures was a great variety of other empty shells including winkles, cockles, mussels, limpets, slipper limpets, whelks, netted whelks, Manila clams, and sting winkles. These were jumbled up with wet and dry seaweed, horn wrack, small pieces of driftwood, and flotsam. There was a marked contrast in the appearance of the shells and stones between the water’s edge where the wet shells were brighter and more colourful and the upper shore where everything was dry.
Broughton Bay is a wide sandy expanse on the north shore of the Gower Peninsula in South Wales, facing the Loughor Estuary or Burry Inlet. A small promontory called Twlc Point at the western end of the beach has an interesting geology with an exposure of Hunts Bay Oolite from the Carboniferous Period. I have written about these strata in earlier posts such as:
On this particular visit I was content to appreciate the way that pebbles of many types and colours on the upper shore were clustered around outcrops and boulders of the limestone which were often pink-tinged and sometimes fossiliferous.
Images of patterns on the beach caused by wave action winnowing out lighter black sediments from the heavier yellow sand grains have been shown in earlier posts. Here are three microphotographs of the black sediments from a small sample that I scooped up on the shore at the time. Sorry there is no scale because I haven’t fathomed out how to include one in the images yet.
Hopefully these shots are clear enough to show that the numerous black particles are small fragments of waterlogged wood; shiny hard pieces of coal (coal dust); a few large pale seeds not yet identified; minute fragmented seashells such as mussels; and many white slender organic objects which I think are small fish rib bones but could be or include heart urchin spines. It was these bones that gave the dried sample in the petri dish an almost fibrous appearance. I am going to see if I can find someone from the world of archaeology who would be familiar with the analysis of this type of material – to see if I can get some more specific information about these black sediments from the beach at Rhossili in Gower.
Flint nodules embedded in the low cliff between two minor faults in Studland Chalk Formation exposures at South Beach on Studland Bay, Dorset, England. At the top of the cliff face is a layer of ironstone and iron-stained flints that has caused the rusty stain on the chalk below. Elsewhere the rocks are covered with a fine coating of green algae.
A couple of useful references for the geology of the area in which these photographs of the chalk and flints were taken:
Barton, CM, Woods, MA, Bristow, CR, Newell, AJ, Weathead, RK, Evans, DJ, Kirby, GA, Warrington, G, Riding, JB, Freshney, EC, Highley, DE, Lott, GK, Forster, A, and Gibson, A. 2011. Geology of south Dorset and south-east Devon and its World Heritage Coast. Special Memoir of the British Geological Survey. Sheets 328, 341/342, 342/343, and parts of 326/340, 327, 329 and 339 (England and Wales), 9–100.
Cope, JCW, 2012 Geology of the Dorset Coast, Geologists’ Association Guide No. 22, Guide Series Editor SB Marriott, The Geologists’ Association, 191-194. A serious guide for the more dedicated amateur and professional.
Ensom, P and Turnbull, M 2011 Geology of the Jurassic Coast, The Isle of Purbeck, Weymouth to Studland, published for the Jurassic Coast Trust by Coastal Publishing, ISBN 978-1-907701-00-9, pages 96-117. A beautifully illustrated beginner’s guide to the geology of the area – one of a series of excellent publications by the Jurassic Coast Trust.
Swanage Solid and Drift Geology (map), British Geological Survey (Natural Environment Research Council) 1:50,000 Series, England and wales Sheets 342 (East) and part of 343
Walking back from Spaniard Rocks now, I took a route closer to the dunes where the character of the shore is quite different from the wet sand and strandlines between high and low tide levels. Here there are pebbles. Rhossili’s pebbles intrigue me. I love scrambling over the banks of stones at the very top of the beach. The colours are lovely pastel shades with pinks and blues and overall reminding me of sugared almonds. A total delight. Many rock types are represented. Some have interesting patterns.
I like the way that the numbers of beach stones seem to increase or decrease depending on how they are pushed around the shore between one visit and the next, and how the sand changes its level and distribution throughout the year and the transition from season to season. This time the wooden ribs and keel of the shipwrecked ketch Anne were only just visible above the sand and pebbles. I like the way that pebbles are arranged partly buried in the damp sand that quickly dries to a different hue and texture. The pebbles underlie the tall sand dunes of the Llangennith Burrows. The dunes have been scooped out by stormy seas and footsteps in many places to demonstrate that even wind-blown sand is stratified; and marram grass roots exposed to air show how deep they penetrate the soft fine sediments to bind them together and stabilise the dunes.
The next stage of the walk from Hill End to Spaniard rocks saw an even greater reduction in the numbers of washed up starfish, and correspondingly greater concentrations of seashells in the strandlines, mostly empty shells of the bivalve Pharus legumen. The fine black detritus arranged itself in elaborate patterns mostly to do with the drainage of water back down the beach.
The sample of the fine dark particles that I took at the time, turns out to have a very interesting composition now I have had a chance to examine it under magnification back home. There is a fair proportion of small dark decaying wood fragments but most of the black material is composed of minute shiny hard particles of coal (what you might call coal dust). The coal is not difficult to account for since coal mining and its export from nearby docks was a major industry in the past. There are many ways the coal could have been accidentally deposited in the sea. Together with the coal dust there are various seeds that I am not able to identify and, most surprisingly of all, what seem to be myriads of delicate fish bones. In fact, so many small threadlike rib bones that the dried sample seemed to have a fibrous texture. Amazing.