Main Late Devensian glaciation of the Isle of Man

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From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. British regional geology: Northern England. Fifth edition. Keyworth, Nottingham: British Geological Survey.


Introduction

Sea cliffs at Shellag Point, Isle of Man, cut in glacigenic sediments of the Bride Moraine that have been subjected to glacitectonic deformation. (P649468a).
Schematic transects across the north of the Isle of Man showing lithostratigraphical relationships. P916099.

The Isle of Man exhibits a very complex sequence of glacigenic sediments because of its location in the centre of the Irish Sea basin, where it has been affected by successive phases of ice flow from Scotland. The entire island was likely to have been glaciated at the LGM, now constrained to the Late Devensian by a radiocarbon date of about 36 ka BP obtained recently from organic sediments beneath the glaciogenic sequence at Strandhall, in the south of the island; crucially, no glacial deposits underlie the organic layer within a topographic basin otherwise conducive for till deposition. Judging from this new evidence, and contrary to previous interpretation, any Early Devensian glaciations affecting the Irish Sea basin must have been of limited extent.

The lowlands in the north of the island are underlain by up to 250 m of Pleistocene sediment assigned to the Irish Sea Coast Glacigenic Subgroup. The upper part of the sequence is exposed in a 25 km stretch of coastal cliff sections that reveal an exceptional range of glacigenic deposits and large-scale glacitectonic structures (P649468). The products of three glacial episodes are contained within three offlapping and southward-tapering glacigenic formations that are separated by significant unconformities and abut the Bride Moraine, a major push moraine complex (P916099). The Shellag Formation, which formed during the LGM, is composed mainly of deformed, shelly, clay-rich, clast-poor deformation till with subordinate beds of sand and gravel. The stratigraphically higher Orrisdale and Jurby formations are composed of complex interbedded units of deformation till, glaciofluvial outwash, sediment gravity flows, and glaciolacustrine and deltaic deposits that possibly record up to seven readvances of the ice sheet margin. Based on radiocarbon dates on organic sediment within kettlehole basins in the Orrisdale and Jurby formations, the readvances are thought to have occurred between 22.4 and 17.3 ka BP.

Bibliography

Bowen, D Q (editor). 1999. A revised correlation of the Quaternary deposits in the British Isles. Geological Society of London Special Report, No. 23.

Chiverrell, R C, Plater, A J, and Thomas, G S P. 2004. The Quaternary of the Isle of Man and North West England: Field Guide. (London: Quaternary Research Association.)

Ehlers, J, Gibbard, P L, and Rose, J (editors). 1991. Glacial deposits in Great Britain and Ireland. (Rotterdam: Balkema.)

Lambeck, K, and Purcell, A P. 2001. Sea-level change in the Irish Sea since the Last Glacial Maximum: constraints from isostatic modelling. Journal of Quaternary Science, Vol. 16, 497–506.

McMillan, A A, Hamblin, R J O, and Merritt, J W. 2004. An overview of the lithostratigraphical framework for Quaternary and Neogene deposits of Great Britain (Onshore). British Geological Survey Research Report, RR/04/04.