198 THE GLACIAL LAKE AGASSIZ. 



by which Lake Agassiz outflowed, where now are Lakes Traverse, Big- 

 Stone, and Lac qui Parle ; the St. CIroix River and Lake St. Croix, formerly 

 the course of drainage from the west part of Lake Superior when that 

 lake was held 500 feet higher than now by the barrier of the receding 

 ice-sheet ; and the Illinois River, the outlet of the glacial Lake Michigan, 

 flowing through Lake Peoria. 



Eroded cliffs. — This type of shore-lines, denominated sea cliffs by Gil- 

 bert, is developed where a glacial lake has formed a terrace, usually in the 

 unmodified glacial drift or till (see fig. 7, p. 26). Waves at these places 

 have been efficient to erode, by undercutting at the base of the terrace; 

 and shore currents have borne away the eroded material, excepting usually 

 a considerable number of large bowlders. Only a small portion of the 

 shores of Lake Agassiz examined by me consists of these wave-cut slopes of 

 till; and they nowhere form conspicuous topographic features, their range 

 in height being from 5 or 10 to 30 feet. Much higher cliflPs of till of simi- 

 lar origin exist on some parts of the shores of the present great lakes of 

 the St. Lawrence and Nelson rivers, where erosion has been in progress 

 ever since the time of the glacial recession. Scarboro Heights, on Lake 

 Ontario, near Toronto, extending 9 miles, with a height of 170 to 290 feet, 

 consisting of till and interglacial beds, aire cliff's thus produced by post- 

 glacial lake erosion. The duration of the glacial lakes 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 

 Agassiz to form the margin of a plateau that stretches Avith a moderately 

 undulating 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 



