Rock platforms, pavements or ledges are a most fascinating feature of the beach at Kimmeridge Bay in Dorset. The cliffs are made up of alternating layers of various mudstones and bituminous shales making up the Kimmeridge Clay sequence. The ledges are seaward projections of the harder limestone rock layers that arise from the base of the cliffs. They are dolomitic limestone which is basically dark grey but when it contains a higher proportions of iron it is yellowy.
The ledges and platforms are covered with cracks and crevices forming interesting geometric shapes. Viewed from above, a larger pattern of shapes, termed megapolygons, is visible. I do not fully understand the processes that underlie the creation of these natural designs. I think the tremendous pressure of what was once a kilometre deep layer of sedimenatry rocks above this stratum, and crystallisation within the sediments of this particular layer, may have been contributary factors.
Whatever the cause of them, the patterns are intriguing; and I will show more photographs of them in later Posts. The smooth flat surfaces and the crevices of the ledges and pavements provide substrates on which seashore animals and plants can settle and shelter – thus contributing to the fantastic array of wildlife that occurs at Kimmeridge in this marine wildlife reserve.
For more information about the geology of Kimmeridge Bay see the most excellent academic guide written by Ian West of the National Oceanography Centre in Southampton University.
For more information about Kimmeridge Bay and its wildlife you can refer to the Purbeck Marine Wildlife Reserve Web Site.
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