28 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



All the terraces point to a single series of events. They indicate that, on a small scale, 

 the streams have repeatedlj' passed through what may well he called cycles. Let us assume 

 that a cycle begins at a time when the streams are engaged in deepening and widening their 

 channels, and let us suppose that the process has gone so far that the rivers are cutting into 

 bed rock more or less rapidly. The next step, no matter what our theory may be, is a 

 change which causes deposition. This proceeds until the valley has been filled to an 

 appreciable depth with alluvial deposits which naturally vary in texture from time to time 

 and place to place. Next there ensues another change which reverses the process. It 

 compels the streams to deepen their channels at first and then to widen them. Constant 

 repetition of these two processes on an ever-diminishing scale has produced the terraces 

 ^\•hich are now so common. Sometimes the streams have shifted far to one side of the valley 

 or to the other, and have completely undermined the older terraces, which in many cases 

 have entirely disappeared or have been reduced to mere fragments. In almost every 

 main valley, however, some trace of older terraces can be found by careful search, and in 

 all valleys, large and small, the younger ones are visible, provided the region is sufficiently 

 arid and mountainous. 



NATURE AND DISTRIBUTION OF TERRACES ACCORDING TO THE TECTONIC HYPOTHESIS. 



We are now prepared to test the two chief theories of the origin of terraces in arid regions. 

 As has been said, neither theory has yet been carried to its logical consequences and ade- 

 quately tested in reference to America. In respect, however, to the Gila conglomerates, 

 which are the gravels of the terraces along the Gila River, Lee* has advocated the theory 

 of tectonic origin, basing his conclusions on personal observation, while Barrell,t on the 

 basis of wide reading and most careful reasoning, has come to the conclusion that they are 

 of climatic origin. 



Let us take up the tectonic theory first, and see exactly what tj^ie of movements of 

 the earth's crust would be required to produce terraces such as those which we find all over 

 the Southwest. The primary process in the formation of terraces may be considered as 

 deposition. After the valleys have reached a certain topographic stage, which may be 

 roughly defined as mature, there must ensue some change which causes erosion to give 

 place to deposition. The amount of deposition may vary from tens to hundreds of feet, 

 but this is immaterial. Finally, it must come to an end, and must be succeeded by erosion, 

 thus giving rise to terraces. The problem before us is simply this: would it be possible 

 for repeated movements of the earth's crust to give rise to the succession of terraces which 

 we find so frequentlj^ in widely separated regions? Alternate periods of uplift and quies- 

 cence undoubtedly cause terraces, such as those of the gorge of the Rhine, but do they 

 give rise to innumerable terraces which not only occur in valleys of every type, but are often 

 composed entirely of gravel without a trace of solid rock throughout their entire extent? 



Let us follow out the process of terrace-making by earth movements, and see what 

 results are obtained under specific circumstances. For the sake of simplifying the problem, 

 let us assume that the streams of a region are engaged in broadening and shghtly deepening 

 their valleys. Let us further suppose that the processes of weathering and erosion have pro- 

 ceeded so far that the main streams have flood-plains, although the minor ones have none. 

 In such a case, the first step toward terracing, according to the ordinary form of the tec- 

 tonic hypothesis, would be either an uplift of the mountains at the heads of the streams 

 in such a way as to cause excessive erosion with consequent deposition farther downstream, 

 or else a tilting of certain portions of the beds of the streams in such fashion as to lessen 

 the grade and thereby induce deposition. Other possibilities, such as the capture or 



* W. T. Lee: Underground Waters of Salt River Valley, Arizona. Water Supply and Irrigation Paper No. 136, U. S. 



Geol. Survey, p. 115. 

 t Joseph Barrell: Relations between Climate and Terrestrial Deposits. Journal of Geology, vol. xvi, 1908, pj). 



173-170. 



