C. B,. Keyes — Aggraded Terraces of the Rio Grande. 469 



into its banks on the convex side of its broad swings. In a 

 distance of two or three miles lateral change of channel a cliff 

 100 to 200 feet in height may be formed. Arroyos entering 

 the river from the bowed side quickly shorten their paths, 

 assume new and higher gradients, and scoop out canyons in the 

 old fans. Thus, between the adjacent arroyo-courses there is 

 left a high-level terrace, or mesa, bordering the main water- 

 way. On the opposite side of the bow the arroyos lengthen their 

 courses, lower their gradients, and build out new extensions of 

 their fan-plains to the water's edge. This phenomenon is best 

 shown in diagram, by an actual section across the Rio Grande 

 at Socorro (fig. 2). 



SX 







Q 



Z 



\ -f-B. . 



V si ""■'- 







B<?fl . 



Pla-m -. 



""----.--. Ar 



royo 



High-level plains of the Rio Grande, at Socorro. 



The processes described are repeated again and again, along 

 the entire course of the river. As a final result there are 

 found sloping terraces at many different levels. The effects 

 are apparently as unique as they are striking. Closer inquiry 

 clearly shows that the same relief effects exist along the streams 

 in more humid regions : only in the last named cases the char- 

 acteristic phenomena are all but totally obscured because here 

 destructive processes are far more active than the constructive. 

 In the semi-arid districts the gradation conditions are reversed. 



The typical characters of the high-level plains along the Rio 

 Grande are well displayed at many points. At El Paso, at 

 Rincon, at Socorro, at Albuquerque and at San Filipe they are 

 finely shown. Near the last named place another interesting- 

 factor comes into play. At different times during the latest 

 geologic epoch great basalt-flows have moved down the plains 

 towards the river. These have preserved the surfaces of the 

 old mesas at different levels. In some instances the river has 

 quite recently cut through the lava-cap, as represented below 

 (fig. 3). 



The best description of these high-level terraces is given by 

 Herrick.* Although they were thought by this writer to be 

 striking enough to deserve the distinguishing title of clino- 

 plains, he did not even hint at their real origin. 



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



