670 TEE POPULAR SCIENCE MONTHLY 



PHYSICAL FEATURES OF THE COLORADO VALLEY.' 



By Majoe J. W. POWELL. 

 in. Water- Sculpture. 



THE more important topographic features in the valley of the 

 Colorado are mountains, hills, hog-backs, bad-lands, alcove-lands, 

 cliffs, biittes, and canons. The primary agency in the production of 

 these features is upheaval, i. e., upheaval in relation to the level of the 

 sea, though it may possibly be down-throw in relation to the centre of 

 the earth. This movement in portions of the crust of the earth may 

 be by great folds, with anticlinal or synclinal axes, and by monoclinal 

 folds and faults. 



The second great agency is erosion, and the action of this agency 

 is conditioned on the character of the displacements above mentioned, 

 the texture and constitution of the rocks, and the amount and relative 

 distribution of the rains. 



In a district of country, the diiFerent portions of which lie at dif- 

 ferent altitudes above the sea, the higher the region the greater the 

 amount of rainfall, and hence the eroding agency increases in some 

 well-observed but not accurately-defined ratio, from the low to the 

 high lands. The power of running water, in corracling channels and 

 transporting the products of erosion, increases with the velocity of 

 the stream in geometric ratio, and hence the degradation of the rocks 

 increases with the inclination of the slopes. Thus altitude and inclina- 

 tion both are important elements in the problem. 



Let me state this in another way. We may consider the level of 

 the sea to be a grand base-level, below which the dry lands cannot be 

 eroded ; but we may also have, for local and temporary purposes, 

 other base levels of erosion, which are the levels of the beds of the 

 principal streams which carry away the products of erosion. (I take 

 some liberty in using the term level in this connection, as the action 

 of a running stream in wearing its channel ceases, for all practical 

 purposes, before its bed has quite reached the level of the lower end 

 of the stream. What I have called the base-level would, in fact, be 

 an imaginary surface, inclining slightly in all its parts toward the 

 lower end of the principal stream draining the area through which 

 the level is supposed to extend, or having the inclination of its parts 

 varied in direction as determined by tributary streams.) Where such 

 a stream crosses a series of rocks in its course, some of which are hard, 

 and others soft, the harder beds form a series of temporary dams, above 

 which the corrasion of the channel through the softer beds is checked, 



From " Report on United States Geological and Geographical Survey of the Terri- 

 tories, Second Division," J. W. Powell in charge. 



