246 W. UPHAM GLACIAL LAKES IN CANADA. 



indeed a very slight elevation in comparison with the cliffs of till of similar 

 origin on some parts of the shores of Lake Michigan and others of the Lau- 

 rentian lakes, where erosion has been in progress from the time of the glacial 

 recession to the present day. Scarboro' heights on Lake Ontario, near To- 

 ronto, extending nine miles with a height of 170 to 290 feet, consisting of 

 till and iuterglacial beds, are cliffs thus produced by postglacial lake erosion. 

 The duration of the glacial lake appears to have been much shorter than the 

 postglacial epoch. 



It is important, however, to note here that cliffs of preglacial erosion, 

 which remained as prominent escarpments through the vicissitudes of the 

 ice age, became in some places the shores of glacial lakes. Of this class are 

 the bold highlands of Pembina, Riding, and Duck mountains, which rise 

 steeply 100 to 1,000 feet from the highest western shore line of Lake Agas- 

 siz, to form the margin of a plateau that stretches with a moderately undu- 

 lating surface westward. Even where this lake washed the bases of the cliffs, 

 it doubtless eroded them only to a slight extent. The horizontal Cretaceous 

 beds of this great escarpment originally extended eastward a considerable 

 distance, as believed by Hind and Dawson, probably so far as to cover the 

 areas now occupied by Lake Winnipeg and the Lake of the Woods ; and we 

 must attribute the erosion of their eastern portion, leaving this steep line of 

 highlands, to river action during the Tertiary era, not in any important 

 degree to glaciation, and least of all to shore-cutting by the glacial lake. 



Beaches. — The course of the shore of a large glacial lake is usually 

 marked by a deposit of beach gravel and sand, forming a continuous, 

 smoothly rounded ridge, such as is found along the shores of the ocean or of 

 our great lakes wherever the land sinks in a gently descending slope beneath 

 the water-level. The beach ridges of Lake Agassiz, and of the glacial rep- 

 resentatives of the Laurentian lakes, commonly rise three to ten feet above 

 the adjoining land on the side that was away from the glacial lake, and ten 

 to twenty feet above the adjoining land on the side where the lake lay. In 

 breadth, these ridges vary from ten to twenty-five or thirty rods. The beach 

 deposit takes thus the form of a broad wave-like swell, with a smooth grace- 

 fully rounded surface. Like the shore accumulations of present lakes and 

 of the sea coast, these glacial lake beaches vary considerably in size, having 

 in any distance of five miles some portions five or ten feet higher than others, 

 due to the unequal power of waves and currents at these parts of the shore. 

 Moderate slopes bordering the greater glacial lakes w^ere favorable for the 

 formation of beach ridges, and such ground frequently displays many beaches 

 at successive levels, which marked pauses in the gradual elevation of the 

 land when it was relieved of its ice-burden, and in the subsidence of the lake 

 as its outlet became eroded deeper or as the glacial retreat uncovered new 

 and lower avenues of discharge. 



