EARLY COMPLETION OF TOE UPLIFT. 487 



between the stages luiu-kcd h}- the formation of definite beaches, which 

 doubtless belong to times when these changes advanced very slowly or 

 were interrupted by intervals of repose. Great as were the combined 

 epeirogenic uplift and modification of the geoid surface of level, producing 

 a differential rise of tlie higliest western shore of the lake in Manitoba to 

 the extent of 175 feet at the international boundary, 266 feet at the latitude 

 of Gladstone, and about 400 feet at the latitude of 51° 52' north on the 

 east side of Duck Mountain, 200 miles north of the international boundary, 

 in the relation of the land to the water level, as compared with the vicinity 

 of Lake Traverse, they were yet almost or perhaps quite completed before 

 the ice-sheet was so far withdrawn that it was no longer a- l^arrier to prevent 

 free drainage from the basin of the Red River and Lake Winnipeg. 



During the subsequent postglacial period, to the present time, only 

 very slight changes have taken, place in the relative elevations of the part 

 of this area- where the heights of the beaches of Lake Agassiz have been 

 determined in Minnesota, North Dakota, and Manitoba; and these small 

 changes of level, shown by the Niverville beaches, have been merely a 

 continuation of the movements which accompanied the recession of the 

 ice-sheet and are recorded by the successive shore-lines of this lake. 



CAUSES OF THE CHANGES OF LEVELS. 



In attempting to discern the causes of the changes of levels shown 

 by the shore-lines of Lake Agassiz, three diverse agencies, which certainly 

 uuist have been factors working together to produce the observed results, 

 are to be studied with respect to the proportion contributed by each. They 

 are considered in the following order: (1) Gravitation of the water of Lake 

 Agassiz toward the ice-sheet; (2) changes in the temperature of the earth's 

 crust due to the ice-vsheet, or, in other words, to the cold of the Glacial 

 period and the return of the warmer climate now enjoyed; (3) epeirogenic 

 movements, or downward and upward bending, often more or less accom- 

 panied with the formation of faults, affecting large areas of the earth's 

 surface, which may be due (a) to the imposed weight of the ice-sheet 

 and to its removal, or (h) to conditions and stresses of the earth's crust and 

 interior originating otherwisn.^ as by secular cooling and contraction. 



