THE GREAT ICE AGE 373 



is that immense series of ridges of drift piled against an escarp- 

 ment of Laramie and Cretaceous rocks, at an elevation of about 

 2,500 feet, and known as the " Missouri Coteau." It is in some 

 places 30 miles broad and 180 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 

 westward, and on which heavy ice from the Laurentian dis- 

 tricts on the east was wafted southwestward by the arctic 

 currents, while lighter ice from the Rocky Mountains was 

 being borne eastward from these mountains by the prevailing 

 westerly winds. We thus have in the west, on a very wide 

 scale, the same phenomena of varying submergence, cold cur- 

 rents, great ice floes and local glaciers producing icebergs, to 

 which I have attributed the boulder clay and upper boulder 

 drift of eastern Canada. In short, we arrive at the conclusion 

 that there never has been a continental glacier, properly so 

 called, but that in the extreme Glacial period there have been 

 great centres of snow and glacial action, in the Cordillera of 

 the west, in the Laurentian plateau of the north, and in the 

 northern Appalachians, and the Adirondacks, while the lower 

 lands have been either submerged, or enjoying a climate habit- 

 able by hardy animals and plants. 



The till or boulder clay has been called a "ground moraine," 

 but there are really no Alpine moraines at all corresponding to 

 it. On the other hand, it is more or less stratified, often rests 

 on soft materials which glaciers would have swept away, some 



