An air photograph taken in July 1958 near Abbot's Salford, on the Avon river, south-central England, shows a polygonal pattern that is considered to represent the result of a natural process modifying subsoil permeability. The former presence of permafrost is inferred, with a mean annual isotherm 13 1/2 degrees C lower than at present. The condition is assumed to have obtained at the time of the Irish Sea glaciation maximum.

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