Small-Scale spatial variability of some sedimentary properties of lodgement till and slumped till

Abstract

Macrofabric, particle size and stone content measurements have been taken from a random scatter of sample points across two till sections in south Hertfordshire. Most results from Hertford indicate a spatial pattern which changes systematically from the base to the top of the section in a manner that is ascribed to lodgement till deposition by a dynamic, but diminishing, glacial energy system progressively adjusting to the alignment of the underlying relief. In contrast, all the results from Hatfield indicate a random spatial pattern which is interpreted as the result of multidirectional slumping caused by the melting of buried ice. Comparison of the results from the two tills indicates that elongate stones in lodgement till are more likely to exhibit a highly significant preferred orientation, a limited directional variation, and a relatively low angle of dip, than are similar stones in slumped till. Consideration of the representativeness of single sample and sample mean macrofabric values indicates that lodgement till may give potentially reliable generalisations about the direction of ice movement, but slumped till will give results that are both statistically and geologically unreliable. It is emphasised that, in all cases, the sedimentary processes responsible for till deposition should be determined before any predictions can be made about the directions of ice movement from fabric data.