Geomorphic evidence for active and inactive phases of Late Devensian ice in north-central Ireland

Abstract

In the Clogher Valley, north-central Ireland, subglacial and proglacial landforms provide evidence for phases of active and inactive subglacial and ice-marginal processes during the Late Devensian (Weichselian) glaciation (∼22–13 kyr BP). Patterns of subglacial Rogen moraines and drumlins record east to west ice flow along the valley axis during maximal ice stages. Eskers, superimposed upon the Rogen moraines and drumlins, record later west to east ice flow and mark the first stages of ice retreat. Areas of large, ice-marginal, non-aligned ridges and mounds are separated by more extensive areas in which no deglacial landforms or sediments are present. The non-aligned ridges and mounds do not show a dominant directional signature, have intricate basal outlines, steep slopes and high relief (< 25 m), and are morphologically associated with meltwater channels and deep enclosed depressions (kettleholes). The ridges and mounds are interpreted as ice-marginal moraines formed by a combination of proglacial thrusting and gravity processes (high sediment mobility) during the melting out of buried ice blocks. An alternative model may involve sediment transport by glacially-driven, groundwater fluxes forming so-called ‘extrusion moraines’. Both models suggest that basal ice was polythermal in character and that alternating active (thrusting) and passive (stagnation) processes at the ice margin took place during overall westward ice retreat.