Geomorphological mapping using detailed morphometric analyses of newly available high-resolution (5 m) digital elevation datasets across more than 9000 km2 of southern Ontario shows that the Greater Toronto Area and its immediate environs, Canada’s largest urban area, is built across the former till beds of two fast-flowing ice streams (the newly named Simcoe Ice Stream (SIS) and the Halton Ice Stream (HIS)) within the last Laurentide Ice Sheet (LIS). These formed during regional deglaciation sometime around 13 000 years before present and are directly analogous to fast-flowing (>3 km year−1) ice streams in today’s Antarctic and Greenland ice sheets. Their subglacial footprint in southern Ontario consists of flow sets of highly elongated and closely spaced flow-parallel till ridges up to 4 km in length and 100 m in width (megascale glacial lineations) that converge on the Oak Ridges Moraine (ORM) from the north (SIS) and south (HIS). ORM is southern Ontario’s largest glacial landform extending from the Niagara Escarpment to Kingston over some 160 km and was deposited as a series of en echelon glaciolacustrine fan deltas in a complexly evolving glaciolacustrine depocenter trapped between the converging ice streams; meltwater channels cut into the bed of the Simcoe Ice Stream suggest that glaciofluvial sediment was primarily supplied from the north. The sedimentology of the ORM is comparable to “morainal banks” at the margins of ice streams terminating in water; some 70 km3 of sediment accumulated very rapidly (>300 years?) reflecting the ability of ice streams to move very large fluxes of sediment, combined with trapping of sediment between SIS and HIS. To date, more than 117 paleo ice streams have been recognized in the LIS and this study confirms the presence of additional corridors of fast-flowing ice in its eastern Great Lakes sector. The ice stream paradigm provides a uniformitarian foundation for a comprehensive re-evaluation of much existing glacial geologic mapping and stratigraphic work in southern Ontario that hitherto stressed the role of catastrophic regional subglacial megaflooding in the formation of the ORM and many other landforms. This paper advocates complete remapping of the glacial landscapes of southern Ontario using an “ice stream landsystem” approach to determine the full number of ice streams in the eastern Great Lake sector of the last ice sheet.

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