BOUNDARY OF THE GLACIATED AREA. 135 



significance of the extension of the Missouri coteau in Brit- 

 ish America, hopes were at once raised that a distinct line 

 of terminal moraines might be traced across the continent. 

 With this theory in mind, the late Professor H. Carvill 

 Lewis and myself began the survey of Pennsylvania in 1881. 

 But, upon crossing the Alleghanies and pursuing the investi- 

 gations in the Mississippi Yalley, I was compelled to aban- 

 don this view, and to be content with finding marginal de- 

 posits more evenly spread over the country, ending, in some 

 cases, in an extremely attenuated border. And, upon reflec- 

 tion, the fallacy in our original theory, that there must be a 

 terminal moraine — that is, a noticeable ridge of glacial ac- 

 cumulations to mark the farthest extent of the ice — is easily 

 seen. 



The extent of a glacial deposit at any particular point 

 will be determined by three factors, namely : 1. The amount 

 of accessible loose material in the line of glacial movement 

 which the ice can seize upon and transport. It is evident 

 that, if the rocks over which the ice moves are hard and 

 smooth and already denuded of loose material, there may 

 be much motion of ice with little transportation of soil. 

 2. The length of time during which the ice-front remains 

 at a given point, since time acts as a multiplier. 3. The 

 exemption of the deposit from the action of denuding agen- 

 cies. When a glacier melts, the torrents of water arising 

 may, in a short time, tear down and distribute as sediment 

 to distant valleys the material accumulated by the slow 

 movement of centuries. Indeed, it has been questioned by 

 some whether the larger part of the grist of the glacier has 

 not been thus transported far beyond the extreme limits 

 reached by the ice itself. This transportation by water from 

 the front of glaciers is certainly of immense extent. Every 

 subglacial stream is surcharged and milky-white with sedi- 

 ment as it emerges from the ice-front. As before stated, a 

 traveler in the State of Washington, from Portland to 

 Seattle, can detect the presence of glaciers in the Cascade 

 Mountains, scores of miles away, simply by the milky color of 



