7 o THE POPULAR SCIENCE MONTHLY. 



ing eastward from the base of the Rocky Mountains, the surface, at 

 first more than four thousand feet above the sea-level, descends by- 

 successive steps to twenty-five hundred feet, and is based on creta- 

 ceous and Laramie rocks, covered by bowlder clay and sand, in some 

 places from one hundred to two hundred feet in depth, and filling up 

 pre-existing hollows, though itself sometimes piled into ridges. Near 

 the Rocky Mountains the bottom of the drift consists of gravel not 

 glaciated. This extends to about one hundred miles east of the moun- 

 tains, and must have been swept by water out of their valleys. The 

 bowlder clay resting on this deposit is largely made up of local debris 

 in so far as its paste is concerned. It contains many glaciated bowl- 

 ders and stones from the Laurentian region to the east, and also smaller 

 pebbles from the Rocky Mountains ; so that at the time of its forma- 

 tion there must have been driftage of large stones for seven hundred 

 miles or more from the east, and of smaller stones from a less distance 

 on the west. The former kind of material extends to the base of the 

 mountains, and to a height of more than four thousand feet. One 

 bowlder is mentioned as being forty-two by forty by twenty feet in 

 dimensions. The highest Laurentian bowlders seen w r ere at an eleva- 

 tion of forty-six hundred and sixty feet, on the base of the Rocky 

 Mountains. The bowlder clay, when thick, can be seen to be rudely 

 stratified, and at one place includes beds of laminated clay with com- 

 pressed peat, similar to the forest-beds described by Worthen and 

 Andrews in Illinois, and the so-called interglacial beds described by 

 Hinde on Lake Ontario. The leaf -beds on the Ottawa River and the 

 drift-trunks found in the bowlder clay of Manitoba belong to the 

 same category, and indicate that throughout the glacial period there 

 were many forest oases far to the north. In the valleys of the Rocky 

 Mountains opening on these plains there are evidences of large local 

 glaciers now extinct, and similar evidences exist on the Laurentian 

 highlands on the east. 



Perhaps the most remarkable feature of the region is that immense 

 series of ridges of drift piled against an escarpment of Laramie and 

 cretaceous rocks, at an elevation of about twenty-five hundred feet, 

 and known as the " Missouri Coteau." It is in some places thirty 

 miles broad and a hundred and eighty feet in height above the plain 

 at its foot, and extends north and south for a great distance ; being, 

 in fact, the northern extension of those great ridges of drift which 

 have been traced south of the Great Lakes, and through Pennsylvania 

 and New Jersey, and which figure on the geological maps as the edge 

 of the continental glacier an explanation obviously inapplicable in 

 those Western regions where they attain their greatest development. 

 It is plain that in the North it marks the western limit of the deep 

 water of a glacial sea, which at some periods extended much farther 

 west, perhaps with a greater proportionate depression in going west- 

 ward, and on which heavy ice from the Laurentian districts on the 



