EROSION TERMINATING THE GLACIAL EPOCH. 245 



as shown thus by wave-cut cliffs, wave-built beaches, and deltas brought by 

 inflowing rivers, extend far along both sides of the present hydrographic 

 basin, often rising slightly and regularly northward, instead of sinking in 

 that direction, as they would do if there had been a depression of the land 

 at the north. When traced carefully with levelling, they are found, some- 

 times after an extent of hundreds of miles, as on the glacial Lake Agassiz 

 and about the great lakes tributary to the St. Lawrence, to terminate 

 abruptly where the basin attains its greatest width. Hence it is manifest 

 that the barrier of these lakes could not have been land formerly raised 

 higher than now, but was the receding ice-sheet, against which the land 

 shores terminated. 



On slopes descending in parallelism with the retiring ice-border, drainage 

 from it in many places flowed in channels from which the streams became 

 turned into new and more northerly courses as the ice retreated. Several 

 glacial river-courses of this kind I have observed between the Coteau des 

 Prairies and the Minnesota river.^ Others have been noted by G. M. Daw- 

 son,t McConnell,J and Tyrrell,§ in various parts of Alberta and Assiniboia. 

 But these seldom were outlets of glacial lakes of large size. It w^as only 

 when extensive hydrographic basins were inclined toward the ice-sheet, that 

 broad glacial lakes, as those named Lake Saskatchewan, Lake Souris, and 

 Lake Agassiz, and the greatly enlarged Laurentian lakes from Superior to 

 Ontario, were held between the northwardly sloping land and the waning 

 ice-sheet, with long continued outflow across the present main water-sheds of 

 the continent. 



The depth of erosion of these outlets varies from 50 feet or less to 150 feet 

 or more. So far as known to me, they are cut through the easily eroded 

 drift deposits, and sometimes beneath these, on the extension of the great 

 plains in the Canadian northwest, through Cretaceous shales or clays and 

 soft, unconsolidated sandstones, which could be easily worn away. Nowhere 

 is it found that a glacial river has channelled deeply into the harder rock 

 formations. The time required for the work observed w^as brief 



Eroded Cliffs. — This type of shore lines, denominated sea cliffs by Gilbert, 

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

 modified glacial drift or till, with no definite beach deposit. Waves and 

 currents at these places have been efficient to erode, by undercutting at the 

 base of the terrace, and to carry away, rather than to accumulate. Only a 

 small portion of the shores of Lake Agassiz examined by me consists of these 

 steep, wave-cut slopes of till ; and they nowhere form conspicuous topographic 

 features, their range in height being from five or ten to thirty feet. This is 



* Geological and Natural History Survey of Minnesota, Final Report, vol. I, 1884, pp. 508-9, 606. 

 t Report on the Geology and Resources of the Region in the vicinity of the Forty-ninth Parallel, 

 1875, pp. 263-5 ; Geological Survey of Canada, Report of Progress for 1882-83-84, p. 150 C. 

 X Geological Survey of Canada, Annual Report, new series, vol. I, for 1885, pp. 21 C and 74 C. 

 g Ibid., Annual Report, new series, vol. II, for 1886, pp. 43 E, 45 E, and 145 E, 146 E. 



