The northern sector of the last British Ice Sheet: maximum extent and demise

• Published in: Earth-Science Reviews
• Main Authors: Bradwell, Tom, Stoker, Martyn S., Golledge, Nicholas R., Wilson, Christian K., Merritt, Jon W., Long, David, Everest, Jeremy D., Hestvik, Ole B., Stevenson, Alan G., Hubbard, Alun L., Finlayson, Andrew G., Mathers, Hannah E.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2008
• Subjects: Rona; Fennoscandian; Ice Sheet; North Atlantic
• Online Access: http://eprints.gla.ac.uk/226507/

Strongly divided opinion has led to competing, apparently contradictory, views on the timing, extent, flow configuration and decay mechanism of the last British Ice Sheet. We review the existing literature and reconcile some of these differences using remarkable new sea-bed imagery. This bathymetric data provides unprecedented empirical evidence of confluence and subsequent separation of the last British and Fennoscandian Ice Sheets. Critically, it also allows a viable pattern of ice-sheet disintegration to be proposed for the first time. Covering the continental shelf around the northern United Kingdom, extensive echosounder data reveals striking geomorphic evidence – in the form of tunnel valleys and moraines – relating to the former British and Fennoscandian Ice Sheets. The pattern of tunnel valleys in the northern North Sea Basin and the presence of large moraines on the West Shetland Shelf, coupled with stratigraphic evidence from the Witch Ground Basin, all suggest that at its maximum extent a grounded ice sheet flowed from SE to NW across the northern North Sea Basin, terminating at the continental-shelf edge. The zone of confluence between the British and much larger Fennoscandian Ice Sheets was probably across the northern Orkney Islands, with fast-flowing ice in the Fair Isle Channel focusing sediment delivery to the Rona and Foula Wedges. This period of maximum confluent glaciation (c. 30–25 ka BP) was followed by a remarkable period of large-scale ice-sheet re-organisation. We present evidence suggesting that as sea level rose, a large marine embayment opened in the northern North Sea Basin, as far south as the Witch Ground Basin, forcing the two ice sheets to decouple rapidly along a north–south axis east of Shetland. As a result, both ice sheets rapidly adjusted to new quasi-stable margin positions forming a second distinct set of moraines (c. 24–18 ka BP). The lobate overprinted morphology of these moraines on the mid-shelf west of Orkney and Shetland indicates that the re-organisation of the British Ice Sheet was extremely dynamic — probably dominated by a series of internally forced readvances. Critically, much of the ice in the low-lying North Sea Basin may have disintegrated catastrophically as decoupling progressed in response to rising sea levels. Final-stage deglaciation was marked by near-shore ice streaming and increasing topographic control on ice-flow direction. Punctuated retreat of the British Ice Sheet continued until c. 16 ka BP when, following the North Atlantic iceberg-discharge event (Heinrich-1), ice was situated at the present-day coastline in NW Scotland.