NATURAL HISTORY, TORONTO REGION
grey clay, due to the grinding up of grey shales ; but it becomes red or brown where the underlying rock is of that colour, e.g., over the Queenston shale, showing how local the source of the clay has been.
There are at least five sheets of boulder clay of different ages exposed in the vicinity of Toronto, but in most other parts of the lake region only one or two are to be found. The oldest tills are harder and more resistant to weathering than the later ones. The latest sheet is considered to have been formed by the Wisconsin ice sheet of American geologists. The oldest may be Pre-Kansan. It has not been possible to separate these till sheets with any certainty up to the present, except at Scarborough Heights, where almost the whole series is displayed.
The ice which covered the region came from the Labrador centre, but did not pass, as might have been expected, southwards or southwestwards across the country, since the valleys of the present Great Lakes deflected it into more westward directions. The ice followed these wide and deep depressions as great lobes, which were confluent when the glaciation was at its maximum, but which separated again when the ice sheets began to wane.
The ice sheets, after passing through the Ontario valley, spread out towards its western end and then climbed the escarpment, leaving polished and striated surfaces on the Niagara limestone beyond the cliffs. Striae and even deep gouges and grooves were left on the Devonian limestones around Lake Erie.