578 C. R. KEYES — RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



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controlled by the gradients of the lateral ar- 

 ro3^os and their varying heights to be due to 

 the changes from time to time in the position 

 of the river's channel. In the course of its 

 wide meandering the river cuts rapidly into its 

 banks on the convex side of its broad swings. 

 In a distance of 2 or 3 miles of lateral change 

 of channel a cliff 100 to 200 feet in height may 

 be formed. Arroyos entering the stream from 

 the bowed side quickly shorten their paths^ as- 

 sume new and higher gradients, and scoop out 

 canyons in the old detritus accumulations. 

 Thus between the adjacent arroyo courses there 

 is left a "high-level" terrace, or mesa, bordering 

 the main waterway. On the opposite side of the 

 bow the arroyos lengthen their beds, lower their 

 gradients, and build out new extensions of their 

 fan plains to the water's edge. The phenom- 

 enon is best indicated by diagram of an actual 

 cross-section of the grand valley (figure 3). 



The phenomena described are repeated again 

 and again along the whole course of the river. 

 As a final result there are found sloping terraces 

 at many difi^erent levels. The effects are appar- 

 ently as unique as they are striking. The typi- 

 cal characters of the high-level plains along the 

 Eio Grande are well displayed at many points 

 (figure 4). At San Felipe another interesting 

 factor comes into play. At different times dur- 

 ing the latest geologic epoch great basalt flows 

 have moved down the inclined plains toward the 

 river. These have preserved the surfaces of 

 the old mesas at several different levels. In 

 some instances the stream has quite recently 

 cut through the lava cap. The best description 

 of the high-level terraces is given by Herrick.^^ 

 Although they were thought by this observer to 

 be striking enough to deserve the distinguishing 

 title of clino-plains, he did not hint at their 

 real origin. 



^ American Geologist, vol. xxxiii, 1904, p. 376. 



