The distribution, variation and origins of pre-Devensian tills in eastern England

Abstract

In order to define quantitatively the lithological properties of the pre-Devensian tills in eastern England, calcium carbonate contents and mechanical compositions of 501 samples from 289 sites have been measured and heavy minerals counted in 102 of them. The results show that the tills may be divided into two groups: ( a ) a North Sea Drift group consisting of the Norwich Brickearth, the Cromer Tills, the Marly Drift of Cromer type and till members of the Contorted Drift, which is characterized by high sand and low opaque heavy mineral contents; and ( b ) a Lowestoft Till group including the Lowestoft Till of East Anglia, the Chalky Boulder Clay of the east Midlands, the Calcethorpe and Wragby Tills and the Lowestoft-type Marly Drift, which is characterized by low sand and high opaque values. The qualitative similarity of the mineral suites in the two groups, however, suggests a common origin in the North Sea basin. Automated contouring (SYMAP) has been used to represent the spatial distribution of till properties. These confirm that the Lowestoft Till group can be spatially separated from the North Sea Drift group, and divided into a Calcethorpe-Marly facies high in carbonates and lying astride the Wash, and a Lowestoft-Wragby facies with moderate but variable contents of calcium carbonate and occupying the rest of the region. Trend surface analysis has been applied to the Lowestoft Till group. At the first order level there are decreasing trends across the region, from northeast to southwest, in calcium carbonate, amphibole and epidote values and increasing trends in silt and clay. These are interpreted as showing a general movement from the North Sea of sandy and chalky material which became progressively modified by assimilation of Mesozoic clays. Higher order surfaces, particularly those of sand, garnet and amphibole values, point to the Wash as the focus of this glacial activity. It is proposed that the most vigorous stream of ice entered eastern England at this point, levelling the Cretaceous scarps and excavating the Jurassic clays of the Wash-Fens basin, and then fanned out into most of the region to deposit the clay-rich Lowestoft-Wragby facies. The Calcethorpe-Marly facies is considered to represent chalky North Sea material carried by marginal, and weaker, ice streams directly onto the Chalk of Lincolnshire and north Norfolk. The North Sea Drift group is believed to be the product of another ice body, penecontemporaneous with that depositing the Lowestoft group, which entered Norfolk from a more easterly part of the North Sea, incorporating sediments from this basin, but without crossing substantial outcrops of Jurassic or Lower Cretaceous formations or Tertiary clays. The Marly Drift includes a variant showing lithological affinities with both Lowestoft and Cromer Tills and which may be the product of complex interaction between the two ice sheets. All the tills studied seem most likely to be of Anglian age.