. The Canadian field-naturalist. Nearly all the lakes north of the Ottawa valley came into existence as the great continental ice cap drew back toward the north, leav- ing many of the pre-glacial valleys, which it had overspread at the beginning of the Ice Age, blocked in various places with moraines. This conversion of graded stream valleys into lakes by the irregular dumping of ice-transported debris produced thousands of the Ontario and Quebec lakes, but not all of them. After the birth of these glacier-begotten lakes, a depres- sion of the Ottawa and St. Law- rence valleys brought the Otta
. The Canadian field-naturalist. Nearly all the lakes north of the Ottawa valley came into existence as the great continental ice cap drew back toward the north, leav- ing many of the pre-glacial valleys, which it had overspread at the beginning of the Ice Age, blocked in various places with moraines. This conversion of graded stream valleys into lakes by the irregular dumping of ice-transported debris produced thousands of the Ontario and Quebec lakes, but not all of them. After the birth of these glacier-begotten lakes, a depres- sion of the Ottawa and St. Law- rence valleys brought the Ottawa valley a few hundred feet below sea level, and the marine clays and sand were laid down, produc- ing the generally level terraces which are conspicuous features nearly everywhere along the Ottawa and St. Law- rence valleys. When the region was finally up- lifted again, a few depressions, resulting perhaps from tidal rips or eddies, interrupted the otherwise level plains of submarine origin and these formed a second and considerably younger series of lakes than those which in the higher lands had persisted from the time of the withdrawal of the ice. McKay Lake belongs to the younger sea-born lakes, and one of the recently placed markers directs attention to the difference in age between the glacier-born and the sea-born lakes of Ontario. It is believed that the information which such roadside markers may give the passerby will add much to the interest of the vacation trip of the auto tourist. The geologist would certainly wel- come such information, displayed as geological finger-boards directing him to significant features. Fig. 3. Roadside marker at McKay Lake, Ottawa. in areas unfamiliar to him. The writer, if he were driving from Canada to the southern states would like to find on the highway followed a roadside marker indicating just where the con- tinental ice cap stopped (and started on its retreat to Greenland) after covering more than half of North America. Many of o
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