Local: Oshawa-Durham History - The Canadian Shield
The rocks that form the Canadian Shield were formed about four billion years ago during the Archeon Eon of the Precambrian Era. Erosion of this extremely rugged, mountainous landscape deposited enormous quantities of clays, silts, sands and gravels into the surrounding waters. Compressed by their sheer cumulative weight and the heat of the shifting Earth's crust, sedimentary and metamorphic rocks formed during the Proterozoic Eon of the Precambrian Era. 
More recent rocks that were formed above these ancient layers have since
been largely removed by the scouring action of glaciers that covered
northern North America in the several ice ages in the past
100,000 years.
The last ice age scraped the rocks in a NNE (north-north-east) to SSE
(south-south-east) direction. At the end of the last ice age, all the
waters in the Barrie area (and the great lakes) drained to the east,
toward the St Lawrence River. After the weight of the glaciers left this
area, the land slowly began to rise. The surface rose about 20 metres at
the eastern end of Lake Nipissing, and the French River began flowing to
the west into Georgian Bay. As the Niagara escarpment rose, the waters to the west flowed to Lake Huron,
and the waters to the east into either Georgian Bay and (further south) Lake Ontario. The soil on which trees and other vegetation
grow in this part of the continent are the result of gradual sediment
buildup since the last ice age.
More history of Oshawa-Durham
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