The size, shape and composition of the mobile sediments on the beach at Chapmans Pool in Dorset varies according to the location. There is generally a lot of fine gravel and very small pebbles overall – as you can see in the picture below. This deposit covers a horizontal rock platform over which the varying depths of the shingle move. Scattered around are large boulders that have fallen from high up on the cliffs.
In the region of the boathouses to the southeast of the bay, the shore comprises angular boulders of different sizes – see the earlier post on Strandlines and flotsam at Chapmans Pool (2) .
The more southerly part of the this area of beach, on the outer or seaward side of the boathouses, is made of sharp cobble-sized stones as seen below (the walking pole is to give scale). These rocks must have originated in the Portland Group of sandstones and limestones that outcrop in the bare scarp face on the top of Emmetts Hill.
The following photograph shows the size of the shingle occurring in most parts of the bay. It is predominantly made up of flints and cherts.
In one place close to the head of the bay in the north, near where the stream (ironically known as the “lake”) flows onto the beach, the pebbles are a different character. Here they are flattened smooth grey pebbles of Kimmeridge Clay derived from the fine layered strata of the lower cliff. There are also here some larger flat slabs of the same that have recently fallen; and some largish stones from the rotunda nodule layer within the Kimmeridge Clay. The flat slabs and large stones sometimes contain fossils.
Beyond that point to the northwest, the shingle becomes even finer – as shown below. At least this was the situation when I visited in June. I get the impression that the smaller shingle shifts around a lot depending on the strength of the waves. It can be heaped up or spread out. It can reveal rock platform or conceal it. It can be obscured by new rock falls. The shingle components can be better sorted and graded by the elements on one occasion rather than another.
Very fine shingle (with scale) in the northwest of Chapmans Pool.
Very fine shingle (without scale) in the northwest of Chapmans Pool.
A closer look at the very fine grey and yellow shingle in the northwest of Chapmans Pool.
And below, an even closer look at the very fine shingle in the northwest of Chapmans Pool showing how ill-sorted the pebbles are with a wide range of sizes and varying degress of angularity and smoothness.
Revision of a post first published 6 July 2009
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