C. R. Keyes — Aggraded Terraces of the Rio Grande. 467 



Art. XLII. — Aggraded Terraces of the Rio Grande • by 

 Charles R. Keyes. 



The Rio Grande is probably the most remarkably terraced 

 drainage-way in the world. Bordering the stream is an exten- 

 sive succession of high-level mesas that constitute the most 

 striking feature of the great valley's surface relief. 



These escarpmented plains, or mesas, inclining strongly 

 towards the river, are abruptly cut off as they near the banks 

 of the stream. The different series of terraces are at levels 20 

 to 250 or 300 feet above the flood-plain. The areal shape of 

 the plains is roughly that of a parallelogram, very long in pro- 

 portion to the width, the longer axis disposed at right angles 

 to the stream. Viewed from the side, the relationships of sev- 

 eral mesas is as represented in diagram below (fig. 1) : 



Relationship of Rio Grande mesas. 



In their wide extent, in their marked inclination towards 

 the river, in their isolated character, and in their smooth and 

 even surface, these high-lying plains present notable accentu- 

 ations of physiographic expression that are rarely recognizable 

 outside of the semi-arid regions. These elevated plains of the 

 broad river valley have long attracted the attention of trav- 

 elers. Many have been the explanations of their origin. No 

 solution of their genesis has yet appeared that is at all satisfac- 

 tory, or that takes into account the unusual climatic conditions 

 of the region, and the novel geologic agencies at work. 



The Rio Grande is one of the great rivers of the American 

 continent. It is as long as the Mississippi. Unlike the case of 

 the latter stream, it has, for a long river, a very high gradient. 

 For the first thousand miles, from its headwaters, the average 

 fall is over five feet to the mile. In times of flood, when the 

 snows are melting in the mountains, the waters are almost of 

 torrential nature. At other times, as in the months of July 

 and August, the stream is often very nearly dry, although 

 there is always a strong underflow beneath the sandy bottom. 



After leaving the headwaters region, a distance of perhaps 



-Fourth Series, Vol. XXIV, No. 144. — December, 1907. 



Am. 



Jour. Sci.- 

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