THE GLACIAL LAKE AGASSIZ. 405 



When the ice-barrier was so far melted back as to give out- 

 lets northeastward lower than the River Warren, other 

 beaches marking these lower levels of the glacial lake were 

 formed; and finally, by the full departure of the ice, Lake 

 Agassiz was drained away to its present representative, Lake 

 Winnipeg. 



The rate of the northward ascent of the originally level 

 highest beach, within the area of my leveling, varied from 

 about six inches per mile near its southern end to about one 

 foot per mile along the greater part of its extent to southern 

 Manitoba. On the east side of the Red River Valley the old 

 shores are higher than on its west side, the rate of ascent from 

 west to east being about half as much as from south to north. 

 The direction of maximum ascent of the planes of the former 

 lake levels is therefore toward the north-northeast. Farther 

 north several beaches of the series mapped by Tyrrell along 

 the bases of the Riding and Duck mountains have a northward 

 rise of two or three feet per mile. The changes of level were 

 in progress and were nearly completed during the existence 

 of Lake Agassiz, as is shown by the gradual diminution in 

 the northward ascent of the successive lower beaches, until 

 the latest and lowest differs only slightly from perfect hori- 

 zontality. 



Gravitation of Lake Agassiz toward the ice-sheet accounts 

 for a small part of the present inclination of the beaches. 

 Changes in the temperature of the earth's crust due to the 

 glacial period and its termination produced a still smaller 

 effect, but this tended to give the opposite slope, or a descent 

 toward the north. Upward movement of this great land area, 

 resulting from its being unburdened by the departure of the 

 ice-sheet, was the chief element in the causes of the differential 

 changes in the height of this basin. Flow of the plastic inner 

 part of the earth's mass, restoring its equilibrium or isostasy, 

 uplifted first the southern half of the area of Lake Agassiz, 

 from Lake Traverse to Gladstone in Manitoba; next it raised 



