Tertiary mirf Quaternary Baselevelfyig. — Upham. 245 
Comparative Estimates of the Amount of Erosion in this re- 
gion during Tertiary time, and duking the Lafayette, 
Glacial, and Recent periods of Quaternary time. 
Along the distance of about 800 miles from the Lake of the 
Woods west upon the international boundary to the Rocky 
mountains, the depth of the Tertiary denudation ranged from 
500 feet to 3,000 feet or more. Its average amount may there- 
fore be estimated as 1,500 to 2,000 feet, or about a third of a 
mile, so that the cross-section of the strata removed along this 
line would be represented approximately by 267 square miles. 
During the early Quaternary or Lafayette period of re- 
newed epeirogenic elevation, the denudation on a width of 
about 100 miles, extending across the Red river valley to the 
Pembina mountain, ranged probably from 100 feet or less to 
400 or 500 feet, with increase from east to west, and its aver- 
age was apparently as much as 250 feet, giving a cross-sec- 
tion approximately equivalent to five square miles. On the 
area of the plains, for the 700 miles from the Pembina escarp- 
ment to the mountains, the contemporaneous erosion may be 
estimated to average 25 feet in depth, giving about two and a 
half square miles as the measure of its vertical section. 
The material eroded during these baseleveling cycles was 
borne far away into the North Atlantic ocean ; but the erosion 
effected by the ice-sheet in the Glacial period subtracted 
nothing from the land surface as a whole. The ice-sheet sim- 
ply wore off the superficial beds of preglacial alluvium and 
residuary clay and portions of the bed-rocks over which it 
was amassed, bore these commingled materials onward with its 
motion, and deposited them at various distances southward 
from their preglacial sources. In this process many small 
hillocks and tower-like masses of the Cretaceous and older 
strata, similar to the fantastically eroded hills, plateaus, and 
pinnacles of the driftless area in Wisconsin, were worn down 
and leveled by the overriding ice. and the deep valleys of 
preglacial erosion were partially or wholly filled with the gla- 
cial drift. The mean thickness of the drift along these nod 
miles of the international boundary may probably be about 50 
feet, its maximum thickness, which is deposited in tin- Ned 
river valley, being 150 to 300 feet. Taking its average as 50 
feet on an extent of SOU miles, the vertical section of the gla- 
