470 C. R. Keyes — Aggraded Terraces of the Hio Grande. 



A more recent description of what appears to be a very 

 similar phenomenon, but upon a miniature scale and in the 

 humid region of Vermont, is that of Fisher.* This account 

 is important in this connection in showing that the general 

 features are also recognizable beyond the arid districts. 



The original surface of the line of bolson plains, through 

 which the Rio Grande now treads its way, appears to be from 

 700 to 1000 feet above the present bed of the stream. At the 



a-tec Cca-veU . 



Basalt flows cut by the Rio Grande at San Filipe. 



present time there are remaining only few traces of these old 

 bolson surfaces. Most of these remnants havebeen preserved 

 only on account of being covered by extensive lava sheets. 

 The Albuquerque volcanoes and basalt sheets, six miles west 

 of the city, on the divide between the Rio Grande and the Rio 

 Puerco, seem to rest on the old bolson surface. The basalt 

 fields near Santa Fe have similar relations. The elevated 

 plain, southwest of Socorro, also appears to belong to the same 

 class. Farther south at Paraje, at old Fort Selden and at 

 El Paso, the same remnantal levels are noted. All of these 

 mesas are to be carefully distinguished from the so-called 

 clinoplains of the Rio Grande valley proper, that are seldom 

 found rising more than 200 to 300 feet above the water-level 

 in that stream. 



There is still another class of plains which might be easily 

 taken for terraces bordering the Rio Grande, that should be 

 clearly separated from all the others. These may be in a 

 sense regarded as lake-benches. They are due to the damming 

 of the river for a brief period. Evidences are abundant 

 indicating that the Rio Grande has been in very recent times 

 and at various points dammed by the crossing of volcanic 

 flows. At old Fort Selden, at San Acacia above Socorro, at 

 Algodones above Albuquerque, at the White-Rock canyon 

 west of Santa Fe and at the Black canyon south of San 

 Antonio, basalt-flows have covered the course of the river and 

 produced a temporary choking up of its waters. These bar- 

 riers thrown across the river were from 200 to 500 feet in 

 height. 



*Proc. Boston Soc. Nat. Hist., vol. xxxiii, pp. 942, 1906. 



