The dependence of diverse topographic forms on geological structure is nowhere better illustrated than in our Great Lakes. The valleys are excavated in shales, soft and impure limestones and sandstones, with their development retarded by the occurrence of overlying beds of hard rock. Such strata, consisting of the Niagara limestone, have given rise to the separation of the basins of five of the Great Lakes, that of Georgian Bay being distinct from the basin of Lake Huron. This feature is true, although the lakes trend in every direction, as is shown on the accompanying map.
Underlying the superficial drift the Niagara limestone forms the rockfloor of the country for a breadth of 5 or 6 miles or perhaps even 20 miles. This formation constitutes a barrier to Lake Erie, separates Georgian Bay from Lake Huron, and Green Bay from Lake Michigan, and finally forms . . .