44 THE GLACIAL LAKE AGASSIZ. 



Saskatchewan River aud parallel with it to the Birch Hills and the South 

 Saskatchewan. They are the northern escarpment limiting the irregularly 

 eroded country, which is here considered as an extension of the great plateau 

 of North Dakota and southern Manitoba and Assiniboia, thus holding the 

 same relation to the valley of the Saskatchewan that the Tiger Hills 

 sustain to the Assiniboiue Valley. 



Great Bear Hills. — On the north side of the Saskatchewan the Great 

 Bear Sand Hills, extending in a north-northwest course to the east end of 

 Lac la Rouge and to the Churchill River, are geographically a continuation 

 of the line of highlands thus described from the Coteau des Prairies and 

 Pembina Mountain to the Pasquia Hills, and they will prol)ably be found 

 also to belong to the same geologic age. If this be true, they differ from 

 this great Cretaceous escarpment south of the Saskatchewan by being out- 

 lying remnants, separated from the l)r(3ad Cretaceous area on the west by 

 a belt of Devonian limestones where these overlying beds have been eroded. 

 The amoimt of erosion west of the middle portion of this escarpment, 

 through North Dakota and in southern Manitoba, since the cycle of base- 

 leveling which spared the Turtle Mountain area, has been inconsiderable, 

 so that in general the surface is a great plain with a g-radual ascent west- 

 ward. On the south the Cretaceous strata are deeply eroded west of the 

 Coteau des Prairies, exposing the underlying red quai'tzite, probably of 

 KeweenaAvan age, at the celebrated Pipestone quarry in southwestern Min- 

 nesota and at many localities thence westerly to the James River. Again, 

 in western Manitoba aud northwestward, between the Assiniboiue and Sas- 

 katchewan rivers, the Cretaceous strata are much denuded, though not worn 

 through, west of the highlands that form their eastern escarpment. Still 

 farther north, between the Saskatchewan and Churchill rivers, the denuda- 

 tion appears to have cut through the Cretaceous beds and to have left 

 remnants of their eastern portion. 



FOREST AND PRAIRIE. 



The area of Lake Agassiz is crossed by the southwestern boundary of 

 the forest that overspreads the greater part of British America and nearly 

 all of the eastern half of the United States. This boundary between forest 



