Abstract
ABSTRACTHere we reconstruct the last advance to maximum limits and retreat of the Irish Sea Glacier (ISG), the only land‐terminating ice lobe of the western British Irish Ice Sheet. A series of reverse bedrock slopes rendered proglacial lakes endemic, forming time‐transgressive moraine‐ and bedrock‐dammed basins that evolved with ice marginal retreat. Combining, for the first time on glacial sediments, optically stimulated luminescence (OSL) bleaching profiles for cobbles with single grain and small aliquot OSL measurements on sands, has produced a coherent chronology from these heterogeneously bleached samples. This chronology constrains what is globally an early build‐up of ice during late Marine Isotope Stage 3 and Greenland Stadial (GS) 5, with ice margins reaching south Lancashire by 30 ± 1.2 ka, followed by a 120‐km advance at 28.3 ± 1.4 ka reaching its 26.5 ± 1.1 ka maximum extent during GS‐3. Early retreat during GS‐3 reflects piracy of ice sources shared with the Irish‐Sea Ice Stream (ISIS), starving the ISG. With ISG retreat, an opportunistic readvance of Welsh ice during GS‐2 rode over the ISG moraines occupying the space vacated, with ice margins oscillating within a substantial glacial over‐deepening. Our geomorphological chronosequence shows a glacial system forced by climate but mediated by piracy of ice sources shared with the ISIS, changing flow regimes and fronting environments.
Highlights
The eastern sector of ice masses located in the Irish Sea Basin (ISB) is unusual in the former British–Irish Ice Sheet (BIIS) in that it comprises an ice mass generated on land, passing offshore, and flowing back onto land to maximum limits in the English Midlands (Fig. 1) (Thomas, 1989, 2005; Chiverrell and Thomas, 2010)
At the Last Glacial Maximum’ (LGM), ice sourced dominantly in southern Scotland and Cumbria converged as a coherent ice mass in the eastern Irish Sea and expanded reaching maximum limits in the English Midlands (Fig. 1B) (Chiverrell and Thomas, 2010; Clark et al, 2012)
The few moraines form a broad arcuate lobe at the maximum extent (Fig. 1C), but are dissected by deeply entrenched valleys floored by glacial outwash deposits
Summary
The eastern sector of ice masses located in the Irish Sea Basin (ISB) is unusual in the former British–Irish Ice Sheet (BIIS) in that it comprises an ice mass generated on land, passing offshore, and flowing back onto land to maximum limits in the English Midlands (Fig. 1) (Thomas, 1989, 2005; Chiverrell and Thomas, 2010) This Irish Sea Glacier (ISG) is the only land‐ based terminus of the western outlets of the former BIIS, and unambiguous evidence (sensu Stokes and Clark, 1999; Stokes, 2018) for ice streaming is lacking. These differences in the pace of retreat have been attributed to the absence of ice streaming and the terrestrial nature of the eastern terminating ice lobe (Chiverrell et al, 2016)
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