PREGLACIAL EPEIROGEXIC ELEVATION. 97 



formed during a preglacial time of high epeirogenic uplift of the northern 

 part of this continent, whose culmination was attended by the accumu- 

 tion of the ice-sheet. On the high plains in the Saskatchewan part of 

 the Nelson basin similar beds underlie the glacial drift and are inti- 

 mately associated with its lower portion. In the Mackenzie valley strati- 

 fied gravel and sand likewise preceded the drift and their upper part 

 was interbedded with deposits of till. All these preglacial beds seem to 

 me more closely related to the Glacial period and the conditions pro- 

 ducing the ice-sheets than to the preceding very long Tertiary era, and 

 for the same reasons which have been well stated by Hilgard and Spencer, 

 namely, their dependence alike on the epeirogenic elevation. With the 

 Ice age we should unite the probably much longer preglacial time of 

 gradual uplift of the continent, and the postglacial or recent period in 

 which we live, to form together the three successive parts of the Qua- 

 ternary era. 



How long the early part comprising the epeirogenic uplift, represented 

 by the deposition and erosion of the Lafayette formation, may have been, 

 we can only vaguely or perhaps approximately estimate. During the 

 beginning of the uplift its effect would be probably to increase the trans- 

 portation and deposition of gravel and sand by the rivers many times 

 beyond their present action. The rate of average land erosion now pre- 

 vailing throughout the drainage area of the Mississij^pi is supposed by 

 McGee to be competent to supply in about 120,000 years a volume of 

 river gravel, sand, and silt equal to the original Lafayette formation in 

 the ^Mississippi valley. With the greater altitude and increasing slopes 

 of the land during the deposition of the Lafayette beds it may have 

 required a third or a sixth of the time here mentioned, that is, some 

 40,000 or 20,000 years. As the elevation continued, however, rapid 

 fluvial erosion of those deposits and of the underlying strata ensued, 

 which was extended over so long and broad an area of the lower Missis- 

 sippi valley, and to such depth, that, even with the high continental 

 elevation of 2,000 to 3,000 feet known from submerged valleys off both 

 the Atlantic and Pacific coasts, it must have required a long period. 

 Perhaps it may be reasonably estimated twice as long as the time of the 

 deposition, or somewhere between 40,000 and 80,000 years. Meanwhile the 

 northern ice-sheet was accumulated, doubtless fluctuating much during 

 its time of gradual growth and advance as afterward during its decline. 



Depression of the Land previous to the maximum Stage of the 



Glaciation. 



The loess, deposited by very gentle currents of broad river floods in 

 the Missouri and Mississippi valleys, attending the maximum extension 



