GEOLOGY OF THE TORONTO REGION
The hills of Archaean rocks to the north of the Palaeozoic beds were powerfully scoured on the northeastern (stoss) side, showing roches moutonnees forms, but the lee side is often rugged and more or less covered with boulder clay or loose erratic blocks.
Some have supposed that the basins of the lakes were largely hollowed by the continental ice sheets, but it is improbable that any important amount of excavation was accomplished in this way. The damming of valleys by morainic materials was far more important and no doubt gave rise to the innumerable rocky lakes of the Archaean region to the north.
The heaping up of a great interlobate morainic mass between the valleys of Lake Ontario and Lake Huron blocked the channel of the ancient Laurentian River and deflected the waters round the southwest-ern peninsula of Ontario into Lake Erie and the Niagara River, thus changing the whole arrangement of land and water.
This land of irregular morainic ridges may be followed from a point near Trenton to Aurora, north of Toronto, and then to the top of the escarpment toward the west ; while less conspicuous loops surround the southwestern end of the Ontario basin.
Between the sheets of boulder clay there are beds of stratified sand and clay evidently formed by water, and hence interglacial. The earliest of these inter-