690 W. C. A'LDEN PRE-WlSCOisrSIN GLACIAL DRIFT IN MONTANA 



Topography of the Plain 



The terminal moraine of the Keewatin ice-sheet was traced northward 

 across the Cut Bank quadrangle to the international boundary, in longi- 

 tude 112 degrees 20 minutes west, by Mr. Calhoun^ in 1902. From this 

 point the margin of the Keewatin drift trends westward, in part north 

 and in part south of the 49th parallel, to Waterton River. Going west- 

 ward from the terminal moraine across the Blackfeet Indian reservation, 

 one finds the area characterized by the topography which is delineated 

 on the Cujt Bank, Blackfoot, and Browning topographic sheets, and 

 which has been described by Willis^ and Calhoun. It is a great treeless 

 plain, in places nearly flat, but mostly rolling to hilly, rising gradually 

 toward the mountains. Standing above the plain here and there in the 

 eastern half of the reservation are butte-like eminences or small elevated 

 plateaus and elongated ridges, with abrupt, incised marginal slopes. 

 These have reliefs of several hundred feet above the valley bottoms. 

 Farther west such elevated tracts gradually coalesce into broader pla- 

 teaus or larger ridges which are particularly characterized by smooth, 

 nearly flat surfaces. Approaching the mountain front, bulky ridges 

 stand at either side of the principal valleys heading in the range and 

 lead directly up to the bases of the bold salient mountains which guard 

 the portals to the park. Adjacent to the mountain front these ridges 

 attain elevations ranging from 5,900 to 6,400 feet above sealevel and 

 stand 800 to 1,G00 feet above the intervening valley bottoms. Taken 

 together, the flat-topped ridges and plateaus have the appearance of being 

 remnants of a continuous high-level plain which once sloped gradually 

 eastward from the base of the mountains, but which has been so reduced 

 by long-continued stream erosion that but a small part of the original 

 surface now remains. 



Describing these features in his paper on the "Stratigraphy and struc- 

 ture of the Lewis and Livingston ranges," Mr. Bailey Willis writes: 



"The highest surfaces of the plains are limited in extent, constituting accord- 

 ing to field estimate not more than one-fiftieth of the total area. Nevertheless 

 their profiles fall into a uniform line that represents an ancient plain, due to 

 the erosion of the Cretaceous shales and sandstones of uneipial hardness. The 

 extent and uniformity of the plain are very marked, and it is the initial 

 physiographic fact of the region. It is herewith designated the Blackfoot plain, 

 after the Indian tribe whose name is associated with the region. On this 

 ancient surface there is a widely distributed thin layer of gravel wjiich is 

 supposed to antedate the Pleistocene deposits." 



8 Op. cit., p. 22. 



" Bailey Willis : Stratigraphy and structure of the Lewis and Livingston ranges, Mon- 

 tana. Bull. Geol. Soc. America, vol. 13, 1902, p. 310, 



