CRUSTAL CHANGES IN TEMPERATURE. 491 



remaining three-quarters, or a larger part, amounting at least to about 130 

 to 300 feet, from south to north, in southwestern Manitoba, belongs to a 

 difterential elevation of the land. 



CHANGES IN THE TEMPERATURE OP THE EARTH'S CRUST. 



Among the conditions producing changes in the height and slopes of 

 the land on which Lake Agassiz lay are the cooling and contraction of the 

 earth's crust by the ice-sheet and glacial waters, and the subsequent warm- 

 mg and expansion owing to the amelioration of the climate. The superficial 

 portion of the earth's crust in the Red River Valley has a temperature of 

 47° to 42° F., as shown l>y the water of artesian wells situated respectively 

 at Ada and Donaldson, Minn. ^ But during the time when this district was 

 covered by the ice-slieet the temperature of the underlying land surface 

 was reduced to the freezing point, 32° F., and a similar lowering of tem- 

 perature may have affected the crust to a considerable depth, largely through 

 the influence of percolating water, causing a slight depression of the isogeo- 

 therms, with consequent contraction of the rocks and lowering of the land 

 surface. By comparison with the present mean annual temperature of the 

 Red River Valley, ranging approximately from 41° at Lake Traverse to 33° 

 at Winnipeg, ^ it is evident that the artesian waters before noted receive part 

 of their heat from the earth's interior. In like manner probably the interior 

 heat kept the superficial portion of the earth's crust beneath the ice-sheet 

 as warm as 32°, at which temperature the earth's heat would be continually 

 melting the ice, though certainly at a very slow rate. 



The differences in the temperatures of the earth's crust, due to the 

 ice-slieet and to water permeating downward from it, would not, therefore, 

 probably exceed 15° from that of the present time in the southern part of 

 the basin of Lake Agassiz, and would decrease to 10° at Donaldson, in 

 Kittson County, the most northwestern 'in Minnesota, and to even a less 

 amount at Winnipeg. The extent to which these slight changes in the 



I Geol. and Nat. Hist. Survey of Minnesota, Eleventh Annual Report, p|). 147, 148. Detailed 

 descriptions of these wells are given in the next chapter. 



-C. A. Schott in Smithsonian Contribntions to Knowledge, Vol. XXI, 1876; Atlas of the Tenth 

 Census of the United St.ates; Report of the Department of Agriculture and Statistics of M.initoba 

 for 1882, p. 318. Also see Chapter XI for statements of the monthly and mean annual temperature of 

 this district. 



