104 THE GLACIAL LAKE AGASSIZ. 



Sierra Nevada/ Again, the same record of loiiy-continued Ijaseleveling, 

 followed by uplift and a new cycle of rapid valley erosion, is found by 

 Powell and Button in the plateaus and Grand Canyon of the Colorado.' 

 The denudation above these plateaus, when compared with the studies thus 

 noted in other regions and with the total erosion of the canyon, seems to 

 have required not only the Eocene and Miocene periods but also most of 

 the Pliocene; for the ratio of the denudation to the canyon-cutting must 

 be nearly or quite as great as that between the duration of the entire 

 Tertiary era and the comparatively short time since its close. Instead of 

 referring the division of these parts of the history of the Grand Canyon 

 district to the beginning of the Pliocene, as was done provisionally by 

 Button, it may therefore mark the final stage of the Pliocene and the 

 inauguration of the Glacial period, with high elevation of all the northern 

 j)art of this continent and of the glaciated northwestern portion of Europe.^ 



LATER EROSION OF THE TROUGH OF LAKE AGASSIZ. 



At the time of uplifting of the plains near tlie end of the Pliocene 

 period, this great liaseleveled region ajjpears to have stretched from the 

 Rocky Mountains to the Archean hills on the eastern border of Lake 

 Agassiz, and to have included also the expanse of flat or only moderately 

 imdulating ci>untry which slowly falls from Lake Winnipeg and the upper 

 part of the Nelson River toward Hudson Bay. The Tertiary drainage of 

 this district, from the j)resent sources of the Saskatchewan, Red, and Rainy 

 rivers to Hudson Bay and Strait, probably formed a great river flowing 

 through the Ajjpalachian-Laurentide mountain belt in the deep valley 

 which is now submerged to form this strait, and emptying into the Atlantic 

 between Labrador and Cape Farewell. The depression of the lower ^^art 

 of this basin Ijeneath the sea seems to me referable to the time of the cul- 

 mination and departure of the Quaternary ice-sheet. Between the Ter- 



' U. S. Geol. Survey, Eigbtli Annual Report, pp. 428-432. Compare also articles by Prof. Joseph 

 Le Conte, Am. .Jour. Sci. (3), Vol. XIX, pp. 176-190, March, 1880; Vol. XXXII, pp. 167-181, Sept., 1886; 

 Vol. XXXVIII, pp. 257-263, Oct., 1889. 



= Exi)Ioratiou of the Colorado River of the West, 1875. Geology of the eastern portion of the 

 Uinta Mountains, 1876. U. S. Geol. Survey, Monograph II, Tertiary History of the Grand Canon District, 

 1882. Am. Jour. Sci. (3), Vol. XXXII, pp. 170, 171, Sept., 1886. 



^ Am. Geologist, Vol. VI, pp. 327-3.39, 396, Dec, 1890. Am. .Jour. Sci. (3), Vol. XLI, ]>p. 33-52, Jan., 

 1891 ; Vol. XLVI, pp. 114-121. Aug., 1893. 



