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University of California Publications. [Geology 



dinately by the wind. The sheet and slope water, as well as 

 that concentrated in more definite channels, having had its excess 

 of load removed when its velocity was first checked, is able to 

 erode farther down the slope. Thus it is that one stream, in 

 the desert as well as in regions of abundant rainfall, may have 

 many places along its course where contemporaneous degradation 

 at the expense of former aggradation is taking place. Coupled 

 with this process is the fact that the drainage area of the stream, 

 up to the stage of maturity, is being constantly increased ; while 

 from maturity onward the drainage basin is being worn lower 

 and lower, so that in the first part of the erosion cycle the amount 

 of water available for erosion, climatic conditions being fixed, 

 is constantly being increased, while in the second part of the 

 cycle the amount of material for the water to transport, pro- 

 vided the area is not undergoing diastrophic changes, is con- 

 stantly being diminished. This gives the underloaded water 

 a chance to erode materials from surfaces where formerly the 

 overloaded water was forced to deposit them. Hence much 

 material strewn in earlier stages over surfaces contiguous to 

 topographic unconformity will later be dissected and removed. 

 The fact that, as long as the slopes remain steep, vertical down- 

 cutting will be much greater than lateral planation will account 

 for the deep, narrow canons excavated in the bedrock of the 

 higher portion of an area exhibiting topographic unconformity. 



The processes of terrace formation and the dissection of 

 alluvial slopes which are salient features of the normal cycle 

 of erosion in humid climates are none the less perfectly normal 

 phenomena in regions subjected to the arid cycle. This appears 

 so obvious that it should not be necessary to do more than to 

 call attention to the process in order to have it generally ac- 

 cepted, yet there has been a notable tendency in recent years 

 to explain recent dissection of alluvial slopes on the desert as 

 caused by renewed uplift or change of climate. Occasionally 

 such dissection has been taken as proof of recent renewed uplift 

 or change of climate without taking the pains to prove such 

 uplift or climatic change by other data bearing more directly 

 on the problem. That renewed uplifts or recent climatic changes 

 have taken place is indeed quite probable, but they cannot be 



