A preliminary analysis of the particle sizes at the Gravel Hill site shows why it is a good site for cobbing - both now and in the Iron Age. The glaciofluvial debris (probably from MIS 13, just over 400 kyr ago) includes a good mixture of coarse clasts, sand and fine clays.
Of the material finer than 70 microns (.07 of a mm), a relatively high proportion is very fine, including the clays <2 micons, .002 of a mm.
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