Devensian thermal contraction networks and cracks at Somersham, Cambridgeshire, UK

Abstract

AbstractMiddle and Late Devensian thermal contraction networks and cracks are described from the southern Fenland, Cambridgeshire. They are associated with gravel aggradations of low terraces of the River Great Ouse. Two networks are described, one set younger than the other. Three types of contraction crack are described. Type 1 occurs within the gravel aggradation of a c. 3 m OD terrace; type 2 penetrates this gravel and is filled with a complex of sediments, including primary, secondary and trough fillings; type 3 penetrates younger lacustrine sediments and gravels associated with Lake Sparks, the lake formed by the damming of Fenland by Late Devensian ice at the Wash. The networks are related to type 2 and type 3 cracks. The relation of permafrost periods to the stratigraphy and landscape development is described, together with the chronology. Periods of sand‐loess and cover sand deposition are identified. A period of permafrost degradation may be associated with change of climate and/or the development and proximity of Lake Sparks. A major period of permafrost in the area is correlated with the Weichselian Late Pleniglacial major permafrost event in the Netherlands (24 ka to 17 ka BP) and a similar major event in Poland (22 ka to 19/20 ka BP). This appears to be, broadly, the period leading up to and associated with the advance of the Late Devensian (Weichselian) ice in Britain and on the continent.