The tills of southwestern Ontario derived their igneous and metamorphic rock and mineral content from areas of the Canadian Shield north of Lake Ontario and north and northeast of Lake Huron. The magnetic susceptibility (MS) of samples of till taken from the area north of Lake Ontario is significantly higher than that for samples taken from the area north and northeast of Lake Huron.The MS of samples of surface till deposited from the Ontario and Erie ice lobes reflects the higher values of MS from that area of the Shield north of Lake Ontario and these samples are significantly higher in magnetic material than samples taken from the Huron lobe on the east side of Lake Huron. Thus the MS of till may be used to differentiate tills in southwestern Ontario on the basis of source area.Measurements of MS from two tills of differing age in the Toronto area show that the two have significantly different MS values and consequently the mean MS values may be used to trace these tills in the subsurface and in areas remote from the sampled sections.The results indicate that the MS of tills provides a rapid and accurate technique for the glacial geologist to identify the source area of tills and to differentiate tills found south of the Precambrian–Paleozoic boundary.

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