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 besnn to wane. 
The ice sheets, after passing through the Onisito 
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. 
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