Late pleistocene interglacial deposits at Tattershall Thorpe, Lincolnshire

Abstract

Interglacial deposits exposed in gravel pits near Tattershall Castle and at Tattershall Thorpe, Lincolnshire, are described. These consist predominantly of detritus muds and silts which fill channels cut into the underlying Wragby Till. The interglacial deposits at both sites are overlain by between 3 and 8 m of fluvial gravel and sand in which ice-wedge casts are preserved. Radiocarbon dates from silt lenses within these gravels confirm a Middle Devensian age for this aggradation. The interglacial deposits from both sites are rich in fossils (pollen, plant macrofossils, Mollusca, Ostracoda) which are described in detail. They also contain some foraminifera and vertebrate fossils in Tattershall Castle pit. The molluscan faunas were unusually rich, with a combined total of 86 taxa recorded including many land snails, two of which (Cochlicopa nitensandVitrea subrimata) have not previously been reported from the British Pleistocene. The fossil evidence from Tattershall Castle pit indicates that the interglacial sediments were deposited by a slow-flowing, well vegetated stream during substage lIb of the Ipswichian Interglacial. An early Ipswichian age is also suggested for the basal calcareous silts, which contain an open-country molluscan fauna, previously ascribed to the late Wolstonian. The correlation is strengthened by uranium-series disequilibrium dates suggesting an age between 75 and 115 ka B.P., a thermoluminescence date of 114±16 ka B.P. and by amino acid racemization data from molluscan shells. The occurrence of brackish-water species of Mollusca, Ostracoda and foraminifera indicates mildly brackish conditions occurred during deposition between —1.8 and —0.2 m O.D. Several plants and molluscs occur that are no longer present in Britain and these provide some evidence that summer temperatures may have been slightly warmer than those of today. Winter temperatures also appear to have been mild. The banding polymorphism of the snailsCepaeaspp., analysed here for the first time from a British interglacial site, might also give evidence of a warm climate. Evidence of interglacial conditions from Tattershall Thorpe comes from infills of two palaeochannels. The fossiliferous detritus muds in the channel studied in most detail (1979-1982) appear to result from deposition in nearly stagnant water under fully temperate conditions with regional mixed oak forest, during substage II of an interglacial. These deposits are at a higher elevation (5-6 m above O.D.) than those at Tattershall Castle and contain no evidence of brackish conditions. Silty clay from the second channel (studied mainly by other workers, in 1971) yielded pollen spectra referred to substage IV of the Ipswichian Interglacial, and a molluscan assemblage indicative of moving water in a river channel.