274 N. S. SHALER BROAD VALLEYS OF THE CORDILLERAS 



still extend in admirable continuity up to the foot of the rocky steeps. 

 Moreover, if we follow up the lines of contact of the slopes with the 

 ranges toward the head of any of the main branches of a river system 

 such as that of the Missouri, .we find that these lines rise, though it may be 

 at a less rate, with the general ascent of the valleys. To suppose that the 

 margins of the slopes marked, even in a general wa}^ the shores of ex- 

 tinct lakes would require an h3^pothesis of secondary tilting for each 

 valley such as would be quite inadmissible. Thus, Avhilehere and there 

 lacustrine deposition has undoubtedly pla3^ed some part in the devel- 

 opment of the existing forms of these troughs, it can not be held that 

 the process has been the main agent in giving them their shape. Fur- 

 ther reasons for dismissing this hypothesis appear when we examine 

 into the constitution of the deposits which underlie the approximately 

 level parts of these valleys. 



Structure of the Valley Deposits 



Owing to the fact that the streams which drain the valleys of the broad 

 type do not, to any considerable extent, attack the deposits b}' which 

 they flow, there are very few natural sections of value to the observer. 

 The surface of the beds is, in most parts of the field, covered with a soil 

 in which more or less angular pebbles abound. At many points the 

 burrowing insects and mammals have brought the finer debris which 

 originally lay between the pebbles within a few feet of the top to the 

 surface, so that it has been blown away by the wind. By the growth of 

 vegetation, a part of the decomposed rock has been brought to the air, 

 where, after the decay of the plants, it is likewise thus swept off. The 

 result of these deportative actions has been to so far concentrate the pebbly 

 material of the soil that in the more arid portions of the Cordilleras the 

 ground is often completely covered with coarse fragments. Where quanti- 

 ties of earthy matter have thus been removed in the form of dust it is 

 common to find considerable deposits of the wind-blown material along 

 the banks of the streams having a thickness of from 3 to 6 feet. The 

 deposition takes place because the speed of the wind is lessened in the 

 tangle of vegetation which grows in the humid belt near the streams. A 

 portion of this blown debris finds its way up into the mountains, where it 

 comes temporarily to rest in the forests and grassed fields. Much of it 

 appears to journe}'' on in the prevailing westerly winds until it passes be- 

 yond the eastern border of the mountain system. 



Penetrating beneath the concentration of debris produced in the man- 

 ner above noted, we find in the ordinary limited sections such as are 

 afforded by railway cuttings and irrigation ditches or the occasional shaft 



