EROSION OF TUE TEOUGH OF LAKE AGASSIZ. 105 



tiaiy baseleveliiiy and this subsidence a widelj- extended epeirogenic uplift 

 of North America intervened. To this perjod of late Pliocene and early 

 Quaternary elevation belong the erosion of the canyons of the Colorado 

 and its tributaries, of the canyons on the slopes of the Sierra Nevada, and 

 much river channeling of the j)lains east of the Rocky Mountains. 



The eastern margin of these plains, which probal)h' extended,' as before 

 noted, over the Avhole area of Lake Agassiz, was then subjected to renewed 

 erosion, removing the mostly soft Cretaceous strata upon a width of a 

 hundred miles or more and to a depth westward of several huucbed feet. 

 Previous to this new cycle of active work by the streams, Riding and 

 Duck mountains stood above the general level, like Turtle Mountain and 

 other isolated high areas farther west; and the maximum depth of the late 

 stream-cutting by which the trough of the Red River Vallev and Lake 

 Agassiz was formed is a])proximately measured by the height of the Pem- 

 bina Mountain escarpment, which rises 300 to 400 feet from its base to its 

 crest along" its extent of about 80 miles. The greater part of this erosion 

 we must attribute to the probably long time of elevation preceding, and 

 finally at its climax producing, the ice-sheet of the Glacial period. So 

 far as can be discerned, the entire hydrographic basin of Lake Agassiz 

 may have continued, tlu'ough all these vicissitudes of changes of levels, 

 excepting when it was wholly or partially ice-covered, to be drained in 

 the same north and northeast direction as during tlie Tertiary era and at 

 the present day.' 



In the progress of denudation by the Tertiary l)aseleveling and by 

 the later erosion of the hollow which was to hold Lake Agassiz, some of 

 the Cretaceous . strata liave ])roved more durable than tli(.)se next above 

 and below, and conse(punitlv ha-s'e liad a more important influence on the 

 topography. This is especially noteworthy in the case of the Fort Pierre 

 formation, wliicli forms llie upper and main part of tlie great escarpment 

 that boi'ders the west side of Lake Agassiz from the Coteau des Prairies 

 north-northwest to the Saskatchewan River. East of the Red River Valley 

 in Minnesota the similiir but less {prominent ascending slope from the flat 



I Am. Geologist, Vol. XI\', i)i>. 235-246, Oct., 1894. Bulletin Geol. Soc. of America. Vol. VI, pp. 17-20, 

 'Nov., 1894. 



