Late Devensian and Flandrian Environmental History of the Ancholme Valley, Lincolnshire: Molluscan and Ostracod Evidence

Abstract

Deposits of calcareous tufa occur in most of the dip slope valleys of the Middle Jurassic escarpment that forms the western margin of the Ancholme Valley, Lincolnshire. Although seldom more than 2 m thick these tufas formed, more or less continuously, during the major part of the early Flandrian. Close serial sampling and quantitative analyses of the contained nonmarine Mollusca and Ostracoda have provided important biogeographical information on the migration of certain taxa into this region during the Flandrian, as well as throwing considerable light on local environmental history. Biostratigraphical analysis of tufa have been undertaken at the following locations: (1) in the small valley between Blyborough Grange and Waddingham; (2) three sites within the valley occupied by the Springfield Beck near Castlethorpe. Another site, of Late-glacial age, near Sturton has also been investigated. The molluscan biostratigraphy of all the tufas exhibits an underlying similarity. Catholic species of wide ecological tolerance together with those typical of opencountry characterize the very early Flandrian. Occasional arctic-alpine species persist from the Late-glacial. Shade-demanding species appear progressively in an ordered sequence. The occurrence of the land snails Nesovitrea petronella and Discus ruderatus, both extinct in Britain today, are noteworthy. A temporary forest clearance episode is reflected by a replacement of woodland by grassland snails in the calcareous silt (slopewash) overlying the tufa at Castlethorpe (site 3). A radiocarbon date of 3410 ? 80 (BM-1795) indicates that this occurred towards the end of the early Bronze Age. The sequences are compared with those from equivalent sites in the British Isles and continental Europe. The molluscan zonation scheme proposed from comparable deposits in Kent is readily applicable to these Lincolnshire sites. Unlike the Mollusca, the ostracods shown no obvious successional patterns but merely reflect local changes in depositional environments.