LATE PLEISTOCENE DEPOSITS AT BLOCK-FEN, CAMBRIDGESHIRE, ENGLAND

Organic sediments in a gravel quarry at Block Fen, Cambridgeshire, for m a sheet dividing lower from upper gravels. Analyses of pollen, macro scopic plant remains and molluscs from these organic sediments are pre sented. They indicate the presence of temperate freshwater and slightl y brackish fine Floodplain sediments, which, on the basis of the palae obotany, are correlated with the temperate Ipswichian Stage. The fresh water sediments, ascribed to Ipswichian substage IIb, occur at ca. -3 m OD. Marine-influenced tidal sediments, ascribed to Ipswichian substa ge III, occur at ca. -6 m OD. No evidence was found for the presence o f more than one temperate stage in the sequence. The lower gravels are then correlated with the cold Wolstonian Stage and the upper gravels with the cold Devensian Stage. In contrast to the woodland environment s indicated by the palaeobotany of the Ipswichian organic sediments, p ost-Ipswichian pollen diagrams and macroscopic plant remains in the up per suite of sands and gravels indicate open tree-less vegetation typi cal of the cold Devensian Stage. They also contain a typical cold-stag e mollusc fauna. The sediments containing these floras and faunas are associated with thermal contraction cracks, indicating the presence of permafrost. The final sand and gravel aggradation in the Devensian fo rms the Block Fen Terrace, near 0 m OD. The evidence indicates that it is younger than the lacustrine sediments resulting from the blocking of the Fenland at the Wash by Late Devensian ice al ca. 18.5 ka BP. Th e sequence at Block Fen is related to nearby Ipswichian and Devensian sediments at Chatteris, March, Wimblington and Mepal, and to deposits at Wretton on the east margin of Fenland. The correlation permits an o utline reconstruction of the history of the valley carrying the River Great Ouse between the Isle of fly and the Chatteris and March 'island s' from the time of a gravel aggradation before the Ipswichian to the Flandrian. The reconstruction shows the time and level of the Ipswichi an marine incursion into the Middle Level of Fenland and the extent of aggradation and erosion in the Devensian.