End moraine construction by incremental till deposition below the Laurentide Ice Sheet: Southern Ontario, Canada

Abstract

Eyles, N., Eyles, C., Menzies, J. & Boyce, J. 2010: End moraine construction by incremental till deposition below the Laurentide Ice Sheet: Southern Ontario, Canada. Boreas, 10.1111/j.1502-3885.2010.00171.x. ISSN 0300-9483. Just after 13 300 14C a BP in central Canada, the retreating Ontario lobe of the Laurentide Ice Sheet briefly re-advanced westwards through the Lake Ontario basin to build a large end moraine. The Trafalgar Moraine (27 km long, 4 km wide) is composed of a distinctly red-coloured silt-rich till (Wildfield Till, up to 16.5 m thick) formed by the reworking of proglacial lake deposits and soft shale bedrock. The moraine has a pronounced ramp-like longitudinal form passing upglacier into fluted till resting on exposed shale. Analysis of water well stratigraphic data, drilled sediment cores, downhole gamma-ray logs and exposures in deep test pits shows that within the moraine the Wildfield Till is built of superposed beds up to 7 m in thickness. These are inferred to result from the repeated incremental deposition of fine-grained debris being moved towards the ice margin as a deforming bed such as identified at modern glaciers. A total till volume of 0.81 km3 was produced in a very brief time-span along a transport path probably no greater than 10 km in length. Subglacial mixing of pre-existing sediment and soft shale was clearly a very effective process for generating and moving large volumes of till to the ice margin. Similar till-dominated end moraines occur widely around the margins of the Great Lake basins, where the markedly lobate margin of the retreating Laurentide Ice Sheet re-advanced repeatedly into proglacial lakes and over fine-grained sediment. This suggests the wider applicability of the till transport and incremental depositional model presented here.