Abstract
<p>Comprehensive mapping and the Briticechrono geochronology provides a reconstruction of the last advance and retreat of the only land-terminating ice lobe of the western British Irish Ice Sheet. The Irish Sea Glacier was fed by ice from Lake District, Irish Sea and Wales, and extended to maximum limits in the English Midlands. During ice retreat after 27 kyrs, a series of reverse bedrock slopes rendered proglacial lakes endemic in the land-system. Not resembling the more extensive definitions of the classical ‘Glacial Lake Lapworth’, these ice contact lakes were smaller time transgressive moraine- and bedrock-dammed basins that evolved with ice marginal retreat. Combining, for the first time on glacial sediments, 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, and constrained for the Irish Sea Glacier a post 30ka ice maximum advance, 26.5±1.8ka maximum extent, and 25.3±1.6 to 20.6±2.2ka retreat vacating the region. With retreat of the Irish Sea Glacier an opportunistic Welsh re-advance 19.7±2.5ka took advantage of the vacated space and rode over Irish Sea Glacier moraines. Our geomorphological chronosequence shows a glacial system forced by climate, but mediated by piracy of ice sources shared with the larger and marine terminating Irish Sea Ice Stream to the west. The Irish Sea Glacier underwent changes flow regime and fronting environments driven by stagnation and decline as the primary impetus to advance was diverted. Ultimately, the glacier of the English Midlands display complex uncoupling and realignment during deglaciation and ice margin retreat towards upland hinterlands ~17.8 kyrs (Lake District and Pennines) and asynchronous behaviour as individual adjacent ice lobes became increasingly important in driving the landform record.</p>
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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