- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
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.