236 The American Geologist. October, i894 
forming the eastern border of the plains. The altitude of 
these valleys and of the Manitoba lake region ranges from 1,- 
000 to 750 feet above the sea : and the escarpment, which, as 
viewed from the lowlands on the east, is named in its succes- 
sive portions from south to north the Coteau des Prairies, the 
Pembina, Riding, and Duck mountains, and the Porcupine and 
Pasquia hills or mountains, rises from 200 or 300 feet to 1,000 
feet within a few miles, its crest being mostly 1,500 to 2,000 
feet above the sea level. Thence westward the expanse of the 
plains, broken here and there by eroded valleys and tracts of 
sometimes very irregular denudation, has nevertheless for the 
greater part a very uniform nearly flat or moderately rolling 
surface, which rises on the average four or five feet per mile, 
to a bight somewhat exceeding 4,000 feet above the sea at the 
foot of the Rocky mountains in Montana and Alberta on the 
opposite sides of the United States and ( anadian international 
boundary. 
The geologic strata of this northern portion of the great 
plains are the Dakota, Colorado, Montana and Laramie for- 
mations of late Cretaceous age, whose deposition took place 
during the closing part of the Secondary or Mesozoic era. 
Southward, in the United States, the plains comprise exten- 
sive deposits of Tertiary lacustrine beds, representing the 
continuation of the brackish water and finally lacustrine con- 
ditions whi(d) prevailed over large areas of the plains during 
the Laramie period; but in the northern region considered by 
this paper no Tertiary beds are found. Since the beginning 
of the Tertiary era this region has been a land surface under- 
going denudation. When its marine and lacustrine deposits 
were first raised to be dry land they had a monotonously flat 
surface. A very long cycle of baseleveling ensued, beginning 
as soon as this northern part of the plains was uplifted at the 
end of Cretaceous time and continuing nearly or quite to the 
end of the Tertiary era. During this time the surface was 
gradually lowered by the action of rains, rills, rivulets, creeks 
and rivers, until it was mostly reduced to a baselevel of sub- 
aerial erosion. 
Alll.AI. AND VERTICAL EXTENT OF THIS BASELEVELING. 
Across an area 700 or 800 miles wide from east to west on 
the international boundary, and of much greater extent from 
