18 PROCEEDINGS OF BROOKLYN MEETING. 



Minnesota and Red river vallej^s and from the flat country inclosing lakes Winni- 

 peg, Manitoba, and Winnipegosis, is mainly by an abrupt escarpment eroded in 

 the Cretaceous strata 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 successive 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 irreg- 

 ular 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 height 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 Slates and 

 Canadian international boundary. 



The geologic strata of this northern part of the great plains are the Dakota, Colo- 

 rado, Montana and Laramie formations of late Cretaceous age, whose deposition 

 took place during the closing part of the Secondary or Mesozoic era. Since the 

 beginning of the Tertiary era this region has been a land surface undergoing denu- 

 dation. 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 subaerial 

 erosion. 



Areal and vertical Extent of the Baseleveling. 



Across an area 700 or 800 miles wide from east to west on the international 

 boundary and of much greater extent from south to north the processes of base- 

 leveling were at work through the vast duration of Tertiary time ; but here and 

 there isolated areas of hills and even mountains remain, consisting of remnants of 

 the horizontal Cretaceous strata which elsewhere have suffered erosion. The most 

 noteworthy eastern highland area of this kind is the Turtle mountain, lying in the 

 north edge of North Dakota and the south edge of Manitoba, its extent on the 

 boundary being about 40 miles, with two-thirds as great width. Its altitude above 

 the surrounding country is 300 to 800 feet, the summits of its highest hills being 

 about 2,500 feet above the sea. Beneath a veneering of glacial drift, which is in 

 large part morainic and generally strewn with many bowlders, averaging perhaps 

 50 to 75 feet in thickness, Turtle mountain consists of nearly horizontally bedded 

 Laramie strata, chiefly shales, w^ith very thin seams of lignite. At or below the 

 base of this highland the fresh- water Laramie formation rests on the marine series, 

 which comprises the Fox Hills sandstone and Fort Pierre shales, the two great 

 shale formations being separated by a sandstone stratum which outcrops in North 

 Dakota on Ox creek and Willow river and on the Souris river between Minot and 

 its most southern bend. A thickness of not less than 500 to 1,000 feet of the Lara, 

 mie and Montana (Fox Hills and Fort Pierre) strata has been carried away from 

 the surrounding eastern part of the plains. 



