We report direct dating, by thermoluminescence (TL) methods, of key Quaternary stratigraphic horizons in the Toronto area. TL ages indicate that (1) most of the exposed sediments of the Don Formation at the sampled site were probably deposited during the closing phase of the Sangamon Interglacial (∼80 ka; oxygen isotope stage 5a), not the climatic optimum substage 5e (∼120∼130 ka),as generally supposed; (2) overlying deltaic strata of the Scarborough Formation, which accumulated in an ice-dammed ancestral Lake Ontario, were deposited between 60 and 50 ka (early Wisconsin, oxygen isotope stage 4); (3) the Sunnybrook diamict, overlying the Scarborough Formation, was deposited in the middle Wisconsin at ∼40-45 ka (oxygen isotope stage 3), likely by glaciolacustrine processes, rather than before 65 ka, as previously supposed; (4) sediments of the Upper Thorncliffe Formation lying stratigraphically just beneath the last-glacial-maximum Hallton Till were deposited at ∼23 ka, consistent with reliable 14C dating. Thus, the earliest Wisconsin incursion of the (proto) Laurentide Ice Sheet into the Lake Ontario basin, as recorded by the Scarborough Formation, probably did not take place until at least 60 ka, and did not extend south and southwest of the Toronto area until after ∼25 ka. Ice-free conditions probably persisted over large areas of midcontinent North America from the end of the Illinoian glaciation at ∼130 ka to the late Wisconsin glacial maximum at ∼20 ka.

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