HOLOCENE ALLUVIAL SEDIMENTATION AND VALLEY FLOOR DEVELOPMENT - THE RIVER SWALE, CATTERICK, NORTH YORKSHIRE, UK

Holocene river channel and floodplain response to environmental change is examined using field and laboratory data collected from the valley of the River Swale at Catterick. Multiple methods including the study of archaeological evidence, EDM survey, geochemical analysis, geomorp hological mapping, ground penetrating radar (GPR), logging of cut-bank exposures and trenches, and C-14 dating have been employed to date al luvial units. Archaeological excavations of the 12th century St Giles Hospital on the bank of the Swale at Catterick reveal sedimentary sequ ences dating from the late Neolithic - early Bronze Age overlain by fi ne-grained alluvium, colluvium and an Iron Age palaeosol. A series of terraces and multiple palaeochannels inset below the St Giles hospital , show the channel has been laterally and vertically active in the pos t Roman period. GPR images across these lower gravel terraces reveal a series of broad channels (up to 100 m wide) into which a series of sm aller channels (c.50-70 m wide) have been cut. Charcoal from fine sand y deposits overlying one of these channels has been C-14 dated to the 15th century. In addition, geochemical analysis of fine-grained sedime nt demonstrates variations in heavy metal concentrations, in particula r Pb, which can be related to metalliferous mining upstream in Swaleda le. Much of the lower Yorkshire Ouse basin has aggraded primarily as a consequence of Holocene sea-level rise and an increased fine sediment supply. In contrast, the River Swale at Catterick remains confined be tween Late Pleistocene bluffs, has coarser bed-load, and a steeper cha nnel gradient than downstream reaches. Consequently, the river has con tinued to rework and incise its valley floor, depositing alluvial sequ ences which are predominantly of late Holocene age.