On the southern tip of the Isle of Portland, in Dorset, England, raised beach deposits provide evidence of higher sea levels at some periods during the Quaternary. There are two separate raised beaches, the more publicised Portland West Beach just to the west of Portland Bill Lighthouse – and another 200 metre long section of Portland East Beach to the east of the lighthouse and behind the Lobster Pot Café.
The Portland East Beach raised beach deposits are shown in these photographs. They sit on top of layers of limestone and are made up of pebbles, larger beach stones, and boulders of Portland and Purbeck limestone, flint and chert – intermixed with seashells. The shells in the images here are all marine gastropod shells of the common periwinkle. All the beach material is bound together in a cement-like matrix of high calcium content, the whole being frequently stained rusty by iron from percolating water. It is interesting to note how many of these ancient beach stones were already riddled with small holes and burrows made by sea creatures, such as Polydora-type worms and boring bivalve molluscs like piddocks, before being isolated and consolidated into the solid layer of beach deposits when the sea levels went down. If you look closely, you can actually see the piddock shells retained within the bore holes of some of the stones..
The whole raised beach layer is up to 0.45 metres thick but is mostly patchy in its distribution. Amino-acid analysis of the mollusc shells suggests that they date from the Ipswichian period of 125,000 years before present, which was the last interglacial. The sea level at that time would have been about 6.95 – 10.75 metres above present sea level. The Portland East Beach is a more recent deposit than the Portland West Beach that has been dated by the same method to 210,000 BP, at which time the shoreline would have been about 14.5 metres above present levels. The higher sea levels are thought to have resulted from the melting of the ice caps in these warmer interludes in the Ice Age, and it is also possible that some local tectonic uplift of the land mass may have played a part.
REFERENCES
Barton, C. M.; Woods, M. A.; Bristow, C. R.; Newell, A. J.; Westhead, R. K.; Evans, D. J.; Kirby, G. A.; Warrington, G. (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, Compiled by M. A. Woods, British Geological Survey, Keyworth, Nottingham.
Davis, K. H. and Keen, D. H. (1985) The age of Pleistocene marine deposits at Portland, Dorset, Proceedings of the Geologists’ Association, 96, pp 217 – 225.
Goudie, A. and Brunsden, D. (1997) Classic Landforms of the East Dorset Coast, Series Editors Rodney Castleden and Christopher Green, The Geographical Association, ISBN 1 899085 28 9.
Keen, D. H. (1995) Raised beaches and sea levels in the English Channel in the Middle and late Pleistocene: problems of interpretation and implications for the isolation of the British Isles, in Island Britain: a Quaternary perspective, Preece, R. C. (editor). Geological Society of London Special Publication, No. 96, pp63 – 74.
Keen, D. H. (1985) Late Pleistocene deposits and mollusca from Portland, Dorset, Geological Magazine, 122, pp 181 – 186.
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