The mineralogy and weathering of Weichselian tills in eastern England

Abstract

The mineralogy and particle size distribution of a large number of till samples from eastern England were studied. Only two members of the Devensian till succession of the Holderness area of Yorkshire are recognised, not three as earlier supposed. These two tills, the Purple and Drab, car be distinguished texturally, mineralogically, and by their colour. However, both weather by oxidation to a reddish brown till, and are then virtually indistinguishable in the field; this weathered material was previously thought to represent a distinct unit, the Hessle Till. The lower Devensian till (the Drab) extends on to the eastern foot slopes of the Yorkshire Wolds, and is correlated with the Marsh Tills of Lincolnshire and the Hunstanton Till of north-west Norfolk. The upper Purple Till occupies only an arcuate area of south-eastern Holderness. A grey older till at Welton-le-Wold (Lincolnshire) is tentatively correlated with the pre-Devensian Basement Till of Holderness. Soil development on these tills was investigated, mainly in a profile on the Purple Till at Tunstall in eastern Holderness. Particle size, mineralogical and chemical analyses were used to establish the uniformity of the original parent material in this profile, and to quantify weathering and other changes that occurred during its development in post-Devensian times. The calculations were based on the method outlined by Barshad (1964), using the quarts and felspar content as an index of weathering. The profile is oxidised to a depth of about and leached of calcium carbonate to 0.7m. Gleying is prominent at depths of 0.2 - 2m. Pock fragments have been disaggregated and fine clay formed from coarser fractions in the upper part of the profile. Much of this fine clay has been translocated to lower horizons (O.3-1.4m depth), but there is little remaining evidence for translocated clay in the micromorphology of these horizons. The main mineralogical charge in fine soil fractions was the weathering of mica to fine clay sized vermiculite.