1 84 STUDIES FOR STUDENTS 



as erosion, subjects which are treated in the following chapters. 

 A partial statement, in so far as erosion is involved, may, however, 

 be made at this place. 



It has been seen that climatic variations are potent causes of 

 changes both in the absolute rate of erosion and in the ratio of erosion 

 to the forces removing the waste. While erosion is dependent for 

 its existence upon initial tectonic movements, it has been seen that 

 its varying rate is as dependent upon climatic variations as upon 

 those secondary crustal movements by which occur further uplift 

 or depression or distortion. 



Criteria for the distinction of these tectonic and climatic factors 

 are discussed by Davis, in so far as erosion at the headwaters and 

 aggradation of the middle slopes are concerned, and the conclusion is 

 reached that in general the terracing in Central Turkestan seems to 

 be due to climatic variation.' The recognition of the importance 

 of climate in building river terraces is in fact a feature in this volume 

 in both the papers of Davis and Huntington. It is only necessary 

 in consequence to summarize briefly certain points which distinguish 

 the upstream terraces built as a result of climatic from those formed 

 by tectonic oscillations. First, the development of terraces on the 

 upper portions of the streams, terraces which die out lower down in 

 the valleys, implies a change in the stream gradients not due to a 

 raising or lowering of the mouth. Regional uplift or depression is 

 excluded in this way. Such a change in gradient in the upper portion 

 of a stream may be due to a local uplift, to a regional warping or a 

 change in the ratio of erosion to transportation brought about by a 

 climatic change. Second, the universality of an epoch of aggradation 

 or degradation in all of the streams of a region is a strong indication 

 of a climatic change, since, as Davis has noted, the crustal bendings 

 necessary to rejuvenate all streams flowing in various directions and 

 finally frequently to bring them back to the initial profile would be 

 extremely complicated and involve an adjustment of subsurface 

 movement to detailed surface form such as is not known to occur and 

 is in fact unthinkable. Although a general similarity in the action of 

 various streams would be expected to occur as a result of climatic 



I Explorations in Turkestan, Publications of the Carnegie Institution, No. 26, 

 1905, p. 203. 



