The glacial geomorphology of the tweed valley and surrounding area, eastern British Isles.

Abstract

This project focuses on reconstructing the glacial dynamics of the Tweed Valley and surrounding area in the Scottish Borders and northeast Northumberland from the glacial geomorphological record. Previous investigations in the region are heavily descriptive and there has been little focus on reconstructing the regional ice configuration, dynamics and style of retreat of this sector of the British Ice Sheet. Geomorphological mapping from digital elevation models conducted in this study has identified three main landform assemblages. Landform Assemblage A is comprised of highly attenuated longitudinal subglacial bedforms, situated in an arc to the north of the Cheviot Massif. Landform Assemblage B consists of glaciofluvial and glaciolacustrine complexes found at the northeast edge of the Cheviot Massif and on the North Northumberland Coastal Plain. Landform Assemblage C is comprised of meltwater channels cut into the lower flanks of the Cheviot and Lammermuir Hills. The geomorphological data was supplemented by sedimentological surveys at selected sites. From the geomorphological and sedimentological characteristics of these landforms, and also their spatial relationship with each other, inferences have been made on regional glacial dynamics. The organisation of meltwater channels, streamlined bedforms and eskers have been used to reconstruct regional ice flow trajectories. It has been shown that a polythermal ice sheet existed, formed of a warm-based, fast-flowing Tweed Ice Stream that was coalescent with the cold- based Cheviot Ice Cap. It is proposed that the Tweed Ice Stream was diverted southwards along the North Sea coast by the North Sea Lobe during the Late Devensian. During deglaciation, which is inferred to have been relatively rapid, the Bradford Interlobate Complex formed in the suture zone between these two ice masses. Following ice streaming, the Cornihill-Wooler Glaciofluvial Complex formed and meltwater channels were cut as the Tweed Ice Stream and Cheviot Ice Cap separated along the ice stream lateral shear margin. This study has revealed that this sector of the British Ice Sheet was more dynamic than originally thought and has highlighted the importance of the geomorphological record for ice sheet reconstructions.