DENVER & RIO GRANDE WESTERN ROUTE. 



159 



seen on the right in the cut edge of a well-developed terrace.** The 

 lop of this terrace, when seen from a high point, appears to be a part 

 of what was once the floor of the valley. Remnants of a similar 

 though higher terrace may be seen in the foothills on the left at a 

 much greater elevation. (See PI. XLIV, p. 90.) 



The traveler is now near the high mountains, and he may look up 

 on the left to lofty peaks on which the snow banks of the preceding 

 winter linger well into the 

 summer and on which a 

 fleecy mantle falls dur- 

 ing the first snowstorms 

 of early autumn, or even 

 occasionallj^ during a 

 cold midsummer storm. 

 The commanding summits 

 which may be seen from 

 time to time are Ouray 



-r» 1 /li'i 1 -ion-*- FiGCEE 40. — Formation of a i-ock terrace. 



Peak (altitude, 13.9oo 



feet), near Marshall Pass (altitude, 10,856 feet), and Mount Chipeta 



on the left, and a group of peaks known as Mount Shavano (altitude, 



''* The mode of formation and hence 

 the meaning of terraces is of great 

 interest to the geologist who is at- 

 tempting to unravel the history of the 

 land. Terraces are mainly the work 

 of water, either running, as in streams, 

 or standing, as in a lake or ocean ; 

 but the present surface of Colorado 

 has not been modified by the ocean 

 and very little by lakes, so that most 

 of the terraces here were formed by 

 ■running water. 



Streams may form terraces of two 

 kinds, known as cut terraces and built 

 terraces. A stream may flow against 

 a bluff of solid rock and cut it away 

 above a certain line and thus produce 

 a flat which, when the stream has fur- 

 ther excavated its valley, may be re- 

 vealed as a terrace or bench. Such a 

 terrace is represented in figure 40. A 

 stream, a, has cut a valley in solid 

 rock, represented by the diagonal-ruled 

 lines. After the stream has formed a 

 flood plain it begins to meander or 

 swing from side to side across the flood 

 plain. In the course of such a swing 

 it may flow against the slope on the 



right and then, if the stream is ac- 

 celerated by uplift, it will clean out 

 its old valley and cut a trench (c) in 

 its rock floor, leaving the part at ft un- 

 touched. The part at b is then a rock 

 terrace on the sidfe of the valley and 

 merely a remnant of the old valley 

 formed when the stream was flowing 

 at a higher level. Such terraces are 

 called cut terraces and are rather rare. 

 The second kind of terrace is known 

 as a built terrace because it is built 

 of waste rock material by waves or 

 running water. Most if not all of the 

 terraces in a mountain region are built. 

 A terrace is generally not built up di- 

 rectly by a stream but is the result of 

 the filling or partial filling of the val- 

 ley and of its partial excavation by the 

 stream. If a mountain stream, such as 

 the Arkansas, which is now able to 

 carry along nearly all the sand and 

 boulders swept into it by its tribu- 

 taries, should be dammed by a flow of 

 lava or other obstruction, it would be 

 unable to carry its load of this ma- 

 terial, which would be dropped in the 

 pond above the obstruction. In time 



