A.M. McCabe

New data for the Last Glacial Maximum in Great Britain and Ireland

Abstract Understanding the history of the British and Irish Ice Sheet (BIIS) at the Last Glacial Maximum (LGM) has been advanced by new approaches, in particular, by cosmogenic nuclide surface-exposure dating, aminostratigraphy of ‘shelly’ glacial deposits, AMS radiocarbon dating, and the evidence from continental margin marine cores, all of which supersede the previously weak geochronologic control. It was formerly believed that Great Britain and Ireland was largely ice free between the last interglacial (oxygen isotope sub-stage 5e) and the Late Devensian, when the LGM occurred. As such the BIIS was effectively out of phase with Laurentide and Scandinavian ice sheets, as well as inferences of ice volume from oxygen isotope stratigraphy. The BIIS during the Late Devensian maximum was also perceived as having been relatively stationary. New evidence shows that the LGM was an important event during the evolution of an earlier BIIS when the extent of ice was greater. Repeated iceberg rafting events over the past 50xa0ka are shown by marine cores, while the derivative inference of numerous corresponding glacial advances is supported by several clusters of 36 Cl ages on glaciated surfaces and glacial boulders, that are indicative of deglacial events between 40 and 12xa0ka. These appear to be associated with Heinrich events, the earliest being inferred as Heinrich 4 at about 40xa0ka. During this advance, the BIIS and Scandinavian Ice Sheet (SIS) were in contact and all of Ireland was glaciated. The ice sheet appears to have fluctuated several times between 40 and 25xa0ka, although evidence for this is poorly preserved. But the 36 Cl and 14 C evidence is clear that the BIIS reached its LGM maximum size about 22xa0ka, soon after Heinrich Event 2, when the BIIS and SIS were not in contact. One cluster of 36 Cl and 14 C ages, at 21.4±1.3xa0ka, records an initial pulse of deglaciation that was followed by extensive deglaciation about 17.4±0.4xa0ka just before Heinrich Event 1, when the ice sheet readvanced. Contrary to previous views, the BIIS probably existed throughout much of Devensian time as a mobile and sensitive ice sheet, during which the LGM advance was but one important event. In places, glacial deposits of the earlier Devensian glaciation have previously been incorrectly identified as products of the later LGM glaciation.

Quaternary deposits and glacial stratigraphy in Ireland

Quaternary stratigraphic studies in Ireland have been based largely on lithostratigraphy. The division of complex sedimentary sequences into the traditional framework of Munsterian and Midlandian events has created a series of stratigraphic scenarios which cannot be fully substantiated either by biostratigraphy or by facies basin analysis. Many of the current problems of glacial stratigraphy in Ireland therefore arise directly from the field procedures used and the frameworks used to subdivide sedimentary sequences. These problems are centred on the selection of type-sites, the subdivision of sedimentary sequences into unnatural divisions, the concepts of ‘direct’ and ‘indirect’ stratigraphy, classical models of climatically determined ice advance/retreat cycles and the genetic inferences of the term till. The conceptual basis of various stratigraphic schemes has changed little since the work of Farrington (1934–1966). It is argued that a more realistic approach to description and assessment of glacigenic sediments should be based on basinal facies analysis and must include a documentation of the composition and geometry of three dimensional facies variations.Palaeoenvironmental interpretations of complex glacigenic sequences, especially in coastal locations, should take account of the influences of marine ice sheets on the sedimentary record. In the past most sequences were interpreted in terms of glacioterrestrial models. A large body of sedimentary evidence now indicates that shallow, glaciomarine environments occurred along the margins of the last ice sheet during its maximum and subsequent retreat phases. The locations and patterns of associated icemarginal systems indicate that the magnitude of, and patterns of crustal deformation were greater and more complex than formerly realised. Thus the primary control on sea-level variation at successive ice sheet margins was largely a function of glacioisostatic disequilibrium.Although complex glacial and glacially-related sequences are known it is argued here that most of the multiple glacigenic sequences in Ireland can be accommodated within the Midlandian Cold Stage. The basal units from certain sequences and erratic fans have been attributed to events earlier than the Midlandian. However, it is difficult to compare these traces of earlier events from different sectors of the island due to poor dating correlation and the fact that ‘direct’ and ‘indirect’ stratigraphy is not adequately defined or separated.The age of the Gortian-style interglacial sequences, the implications of their floristic composition and their relationships to drift stratigraphy remains a major problem in stratigraphic constructions of the Irish Quaternary. The stratigraphic significance of many till units must remain largely unknown until these matters are resolved. At present it is not clear if the Gortian sequences represent more than one interglacial.A series of exposures in Ulster provide evidence for major environmental changes during the Midlandian Cold Stage. These include: an early stadial, an early forest biozone (>48.1 ka BP), an early herb biozone (>41.5 ka BP), a middle ‘interstadial’ phase of cold but non-glacial aspect (30.5 ka BP), a late stadial (max. 24-22 ka BP) and a late glacial climatic oscillation. The Late Midlandian stadial comprises a major glaciation that almost covered the island and, a later, more restricted phase when drumlins formed in the northern and western parts of the island (max. 17 ka BP).

Sedimentation in an ice-contact subaqueous setting: The mid-Pleistocene ‘North Sea Drifts’ of Norfolk, U.K.

In East Anglia, the mid-Pleistocene ‘North Sea Drifts’ extend over some 1500 km2 of the coastal zone of North Norfolk. They are the oldest Pleistocene glacial sequences exposed in Britain, deposited during the Anglian Glaciation (ca. 450 ka BP?). Laminated and stratified pebbly mud (diamict) facies of the North Sea Drifts are widely regarded as the product of subglacial deposition below a terrestrial ice margin. In this paper these facies are identified as the product of subaqueous deposition in front of the margin of an ice sheet terminating in a large water body. Laminated, stratified, and massive diamict facies were deposited by the ‘rain-out’ of fine-grained sediment from suspension and coarse debris from floating ice, combined with downslope sediment gravity flow. Intraformational folds are the product of episodic downslope slumping and identify a regional paleoslope to the east, away from a major ice marginal moraine ridge (The Cromer Ridge). Exposures at Cromer show bulldozed ice-contact subaqueous sediments and ice-thrust chalk rafts. A fall in water-level emplaced shelly shoreface sands and beach gravels which are loaded into the underlying diamicts as large pan-shaped ‘sag basins’ with diapiric structures at their margins. Comparable sedimentary sequences are exposed elsewhere around the southern North Sea Basin; a glaciomarine setting in response to glacio-isostatic depression around the Anglian ice sheet is suggested.

The stratigraphic and sedimentological significance of Late Devensian Ice Sheet surging in Holderness, Yorkshire, U.K.

Amino acid analyses of marine mollusc valves show that the Basement Till of Holderness, Yorkshire, is of Late Devensian age (ca. 20,000 BP) and not pre-Ipswichian (>125,000 BP) as is traditionally supposed. Together with the overlying Skipsea and Withernsea Tills the Basement Till is argued to be a ‘deformation till’ resulting from the repeated onshore surging of Late Devensian ice over a muddy sea floor, and the subglacial transport, attenuation and mixing of marine sediment. Silts on the surface of the Basement Till, yielding 14C dates of 18,500 and 18,240 BP, have been regarded as minimum ages for Late Devensian glaciation in eastern England (e.g. the Dimlington Stadial of Rose, 1985, 1989). The amino acid data presented here indicates that the maximum of the Late Devensian glaciation in eastern England occurred earlier. Sedimentological interpretation of the Late Devensian tills of Holderness as glacially-reworked marine sediments supports previous glaciological models of the British Ice Sheet involving surging of ice lobes along the coast of eastern England. The Basement, Skipsea and Withernsea tills are separated by shallow marine sediments and may be representative of ‘deformable bed’ till facies deposited below Pleistocene ice sheets moving over soft, fine-grained sediments.

A geological view of drumlins in Ireland

Facies modelling of the internal structure of drumlins has resulted in recognition of five major facies associations within the Irish drumlin belt. The facies associations are linked with a sequence of depositional events during the last deglacial phase (ca. 20-16 ka BP). Drumlin sediments were formed by basal till deposition, subglacial fluvial-lacustrine sedimentation, sheet flow, debris flow and subglacial cavity deposition. Subglacial deposition was initiated by the infilling and blocking of subglacial networks which caused a reduction in hydraulic transmissibility at the base of the ice sheet prior to drumlinisation. Drumlins then formed as subglacial erosional forms during surge-type events when subglacial hydraulic transportational processes were active and sediment fluxes to the ice margins were high. It is argued on geologic evidence that basal boundary conditions of this type coupled with high rates of ice wastage by calving at the ice margin contributed to rapid disintegration of the last ice sheet in western Britain.