The last glaciation in Caithness, Scotland: revised till stratigraphy and ice-flow paths indicate multiple ice flow phases

摘要

This paper reviews existing information on the last glaciation of Caithness and presents new evidence for additional till units and for long distance ice-flow paths based on till palynomorphs, indicator erratics and striae. Early, radial expansion of Northern Highland ice probably occurred at 31 – 29 ka. After ice withdrawal from the north coast, Moray Firth ice returned before a second withdrawal. Thereafter Moray Firth ice advanced to limits close to the Atlantic shelf edge between 21 and 18 ka. Deglaciation of hill summits was completed by 18.4 – 17.8 ka but the low ground south of Wick only became ice free after 16.5 ka.\ud\udRecognition of these multiple ice-flow events is consistent with the dynamic behaviour of the last ice sheet exhibited in mathematical simulations. The event sequence differs, however, from nineteenth century and recent two-stage flow set reconstructions as each main flow set is shown to represent multiple events following similar paths but under different ice sheet configurations. Various configurations allow ice to flow northwards across Caithness and so remove any requirement for the Fennoscandian Ice Sheet to directly block and divert the outflow of ice from the Moray Firth during the last glaciation.\ud\udDuring the Late Pleistocene, the British and Irish Ice Sheet (BIIS) discharged ice via the Moray Firth from a source area that covered c. 1700 km2 in the Northern and Grampian Highlands of Scotland (Geikie 1877) (Fig. 1). Part of the flow of Moray Firth ice moved across Caithness (Peach & Horne 1880) to deposit ‘shelly boulder clay’, a mud-rich till unit including erratics of Pleistocene marine shells and Mesozoic rocks derived from glacial erosion of the bed of the inner Moray Firth (Croll 1870). Ice flow towards the north over Caithness (Fig. 2a) has long been viewed as a result of deflection of the outflow of the Moray Firth glacier by the presence of the Fennoscandian Ice Sheet (FIS) in the North Sea (Croll 1870).