G R. Keyes — Aggraded Terraces of the Rio Grande. 471 



Under the conditions now existing it would be expected that 

 such dams would leave conspicuous traces of the existence of 

 the lakes. Such, however, does not appear to have been the 

 case. At least, the lakes were so ephemeral in character that 

 they left no evidences of distinct beach-levels, or elevated lines 

 of delta-fans. It may be that the fans were confluent in such 

 cases, and that the entire areas of the high plains, as those 

 existing above the White Rock canyon, at Rincon, and above 

 the mountain barrier at El Paso, represent these temporary lake 

 deposits. 



Notwithstanding the turbid character of the river waters 

 and the prodigious quantities of coarse materials carried by the 

 swiftly flowing stream, any lake that may have been formed 

 must have quickly disappeared through the cutting of the dam 

 or holding barrier. Even with some extraneous accumulations 

 of sediments the local gradient was soon lowered, and the great 

 influx of coarse sands and gravels from the lateral arroyos 

 must have rapidly destroyed all traces of the lake beds and 

 moulded them into new aggradation surfaces. In any case it 

 wo aid be exceedingly difficult to differentiate these plains of 

 old lake-basins from ancient arroyo fillings. Moreover, the 

 plains aggraded by the arroyos extend, at the distal and more 

 elevated side, very much higher than would be possible for 

 lake beds to do. If any deposits of the last mentioned class 

 were formed, they would be quickly destroyed, or so disguised 

 as to be almost unrecognizable. 



When the barriers finally disappeared, enabling the river to 

 recover its original lower level and to renew its old meander- 

 ings, the gradients of the lower courses of the arroyos were 

 greatly increased, and the high aggradation-plains were cut by 

 canyons more or less parallel to one another. This may be the 

 explanation of certain local series of close-set, parallel canyons 

 found at such places as in the Pajarito Park district west of 

 Santa Fe. 



Many erroneous notions are entertained regarding the his- 

 tory of the Rio Grande. Travelers unaccustomed to the novel 

 effects of the workings of the geologic agencies in the arid and 

 semi-arid regions are prone to explain everything in terms of 

 their native humid heaths. As a result much that is fanciful 

 has been indulged in. With the exception of W. J. McGee 

 even the trained geologists, who have traveled through the 

 region, seem to have entirely overlooked the two most vigorous 

 geologic processes of the arid regions — the eolian and the 

 planorosive, the latter more widely known among the dwellers 

 of those regions as sheet-flood erosion. Both are practically 

 unknown to people living in those parts of the world where 

 there are abundant rains. They give rise to phenomena the 



