Sedimentary evidence against a local ice-cap on the Shetland Isles at the Last Glacial Maximum

The Shetland Isles occupy a critical position equidistant to major ice-dispersal centres of the British-Irish and Fennoscandian ice sheets during the last glaciation, and have long been the subject of controversy regarding the timing, geometry and style of glaciation on the Shetland Isles. Previous reconstructions of the last glaciation of Shetland, favour development of a local ice-cap, possibly subsequent to early inundation by Fennoscandian ice. However, this model requires complex ice shed migrations and dynamic changes to explain the evidence. Recent studies identify an extensive landform record of NW-flowing ice extending across the region from the North Sea Basin, incompatible with evidence from striae and erratic boulder transport studies. This paper presents glacigenic lithofacies descriptions and structural (clast-fabric, shear-planes) analysis of seventeen sections from around the Shetland archipelago. The sedimentary record of glaciation supports a model of inundation by ice from the North Sea Basin, and includes no sediments representing multiple phases or dynamic local ice-cap glaciation. A preliminary framework which integrates all previous datasets with the results of this study proposes: (1) an early phase of local glaciation that was subsequently overwhelmed by extension of a major ice stream flowing from the North Sea Basin at the LGM; and (2) absence of an extensive local ice-cap formed over Shetland during deglaciation. This study concludes by identifying key areas that require further investigation, most notably more detailed analysis of ice-flow indicators and sediments in the eastern islands of Whalsay, Fetlar and Bressay, and further work to establish the temporal relationships between glacially eroded bedrock and associated subglacial till emplacement.