The last glaciation of Shetland : local ice cap or invasive ice sheet?

Abstract

The question of whether the Shetland Islands were covered by an ice cap or by an ice sheet during the last glacial cycle (40–10 ka) remains unresolved. This paper addresses this problem using existing and new data on glacial erratic carry, striae, glacial lineaments and roche moutonnée asymmetry. Its focus is on eastern Shetland, where ice-cap and ice-sheet glaciation would lead to opposed ice-flow directions, towards and away from the North Sea. Evidence cited in support of ice-sheet glaciation of Shetland is questioned. The primary survey of striae correctly identified striae orientation but the direction of ice flow from striae on eastern Shetland was misinterpreted: it was not from, but towards the North Sea. Glacial lineaments interpolated to cross the spine of Shetland instead are discontinuous and diverge away from an axial ice-shed zone that lacks lineaments. Glacitectonic structures cited recently as evidence for westward flow of an ice sheet across eastern Shetland have been partly misinterpreted and other ice-flow indicators in the vicinity of key sites indicate former eastward ice flow towards the North Sea. Westward carry of erratics over short distances in N and S Shetland may be partly accounted for by shifts in ice sheds during ice-cap deglaciation. Collectively, the evidence for movement of the Fennoscandian ice sheet across Shetland is weak. Any ice-sheet incursion over Shetland occurred before the last glacial cycle. The cleansed ice-flow directional data for Shetland show a simple pattern of divergent ice flow from an axial ice-shed zone beneath an ice cap. The deglaciation sequence for the ice cap is evident from sea-bed moraine systems. The Shetland ice cap at the Last Glacial Maximum was substantial, attain ing a thickness of 1 km and a diameter of >160 km. The ice cap was of sufficient size to restrain the Fennoscandian ice sheet on the western edge of the Norwegian Channel and to divert the British ice sheet over Orkney. Glacial landscapes on Shetland indicate that ice-cap glaciation has been the dominant mode of glaciation during the Pleistocene.