Athough the pebbles of the shingle beach at Ringstead in Dorset first look much of a muchness, a closer inspection reveals that they have different characteristics and geological origins. I collected together some small groups of different types of pebble when I was last down on the beach and these are shown in the following photographs.
The majority of the pebbles are medium to large (almost cobble-sized), irregular, knobbly, flints like the white-coated ones shown immediately above and below. The natural dark grey or black colour of the glassy matrix shows through where the coating is chipped. The matt, powdery, white coating can get stained a rusty colour if the pebbles get buried deep in the sediments for a while. The dusty coating forms from the constant abrasion of one stone against another as the pebbles are moved around the beach by the waves.
Some types of pebbles are rusty-orange, browns or reds with a grainy, sand-like texture. They have a porous surface and white inclusions. Others are dark grey, even purple-grey or dark red. I wish I could tell you exactly what kind of rock they are made from. Unfortunately, the rock strata in the cliff along the stretch of coastline from White Nothe to Osmington Mills via Ringstead Bay is extremely varied and complex. I have been unable to determine with any confidence the real nature of the different pebble types.
For anyone desperately wanting to find out what rocks these pebbles are, may I suggest reference to Ian West’s web site on the geology of this area, which gives a full account of the stratigraphy and fossils.
Revision of a post first published 26 October 2009
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