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The rocks of the Archaean are usually well exposed on the flanks of hills, while the valleys are more or less drift-covered; but the Palaeozoic rocks seldom rise as hills and their beds have only a slight dip, so that they are commonly buried under boulder clay or old lake deposits. Their outcrops are to be looked for mainly along lake shores or river valleys and there are hundreds of square miles of southern Ontario where no exposures of solid rock have been found.

The Ordovician (Lower Silurian or Cambro-Silurian) forms the bed rock in most of the Toronto region, occurring at many points on the shore of Lake Ontario and less often at a distance from it. The subdivisions usually recognized are as follows:

Queenston. Lorraine.

Ordovician Utica.

Collingwood. Trenton. Black River.

Feathering out toward the north upon the uneven surface of the Archaean peneplain south of Bala, Washago and other points between Georgian Bay and the Thousand Islands one finds solid beds of Black River limestone. They are well exposed in quarries



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