98 GLACIAL AND SURFACE GEOLOGY OF THE PACIFIC COAST. 



exaggeration.* The ftict is that the forms of valleys depend on a much 

 more complicated series of events than Avoiild be indicated by the use of the 

 simple phrases " aqueous " or " glacial." There is the original form to be 

 considered, in the first place. Something determines the passage of ice or 

 Avater along a certain line, or in a certain direction. This something is of an 

 orographic nature. Whoever has studied the structure of the earth's crust 

 as revealed in the many sections afforded by great mining operations, espe- 

 cially those made in the Carboniferous rocks on so gi-and a scale in Europe, 

 must have been impressed with the manner in which portions of the earth's 

 crust have been thrust up or down along vertical or nearly vertical planes. 

 Here is nothing of theory : the facts are of too much importance, from an 

 economical point of view, to be ignored. To assume, as some geologists do, 

 that great chains of mountains, made up as many are in large part of thor- 

 oughly squeezed, folded, and overturned rocks, had originally perfectly uni- 

 form surfiices, which have since been eroded by water or carved by ice into 

 their present forms, is to occupy a position emphatically at variance with the 

 great body of facts developed by geological investigations. It is only under 

 peculiar and exceptional circumstances that water has done most of the 

 work, as for instance in the case of the canons of the Colorado River and its 

 side valleys. As a general rule it is safe to assume that where U-shaped 

 valleys exist, the perpendicular walls have an orographic origin, and that 

 those of V-form have had that shape given to them by the debris piles which 

 have accumulated against their sides. The farther we descend the mountain 

 slope, the less the gi'ade, and consequently the less the carrying power of the 

 stream: hence, the valley which is U-shaped in the upper part of its course 

 acquires more and more of a V-form as it approaches the plain at the base 

 of the range in which it heads. 



But glaciers have been and are now limited in their occurrence chiefly to 

 the higher portions of the mountains, hence their association in the mind with 

 the U-form of the valley. And this leads superficial observers to conclude 

 that the U-form is the work of the ice itself; Avhile the truth is, that the 

 material from which the glacier has been formed has simply been gathered 

 together in a pre-existing depression, all observations showing that great 

 mers de glace can only accumulate where the topographical conditions are 



* Mr. J. F. Campbell exiiresses the same idea, but in a form much more limited and nearer the truth. He says: 

 "Every agent that wears rock leaves a dilferent mark. A glaciated valley has a rounded section — ' ; a stream 

 cuts a V or a Y-" 



