Periglacial discontinuities in Eocene clays near Denham, Buckinghamshire

Abstract

Abstract In the investigation for part of the M25 motorway near Denham, Buckinghamshire, several types of sheared and unsheared discontinuities were found within the Eocene London Clay and Reading Beds clays which are considered to have formed under Pleistocene periglacial conditions. These consisted of two types of low angle, near-surface solifluction shears with associated discontinuous, random accommodation shears. These overlay and truncated high angle shears believed to have formed by collapse on thawing of the top of the permafrost. Deeper, low angle shears of two types, one continuous, subhorizontal and planar, the other discontinuous, random and undulose, are tentatively attributed to shearing at the base of a permafrost layer at a thawing front. Subvertical, unsheared discontinuities are considered to be contraction cracks.