GEOGEAPHIC FEATURES 213 



former publication,* to which the reader is referred for more extended 

 statements than can be given in this connection. A brief summary;, 

 however, may be in place. ; 



The plain in which the craters occur is the aggraded surface of th^ 

 ancient valley of the Eio Grande. In common with the old valleys o'f 

 the southwest generally,! that of the Eio Grande was filled with san^ 

 and gravel to a great though unknown depth in recent geologic time. It 

 should be noted that the Eio Grande, as described in the paper just re- 

 ferred to, formerly flowed southward into Mexico across La Mesa west of 

 Eodadero peak (or Cerro de Mulero, as the elevation is called by the 

 residents of that region) and was deflected eastward at El Paso after the 

 old valley had been filled with sand. This deflection may have been due 

 to the building up of the valley floor until the level of the Pass was 

 reached, or it may have been accomplished, as suggested by E. T. Hill, 

 by the headward erosion of a gulfward-flowing stream which, after cap- 

 turing the ancient Eio Grande, became the lower portion of that river 

 as we know it at the present time. 



After its deflection at El Paso the river eroded a secondary valley in 

 the sand beds, leaving the aggraded surface, here about 20 miles wide, as 

 a mesa plain 300 to 400 feet above the present river bed. The material 

 composing the plain is so porous that the rain falling upon it sinks at 

 once without forming even temporary drainage courses of any consider- 

 able length. The surface is therefore practically unaffected by stream 

 erosion, but the sand at the surface is constantly shifted by winds. 



The depth of the valley filling is unknown; wells nearly 1,000 feet 

 deep do not reach through it, and in a neighboring valley near El Paso 

 no solid rock was encountered at a depth of nearly 2,300 feet. J The 

 depth to which the ancient valley of the Eio Grande has been filled is, 

 therefore, probably much more than 1,000 feet, the greatest depth 

 reached in the wells of La Mesa. 



Geologic Conditions 

 quaternary sands 



The determination of the geologic age of the sands seems to fix within 

 narrow limits the time when the craters were formed. In drilling a 

 well in the Kilburn crater a fossil was found at a depth of 70 feet — 

 330 feet below the general level of the plain — which Dr J. W. Gidley,§ 



* W. T. Lee : Water resources of the Rio Grande valley in New Mexico. Water-supply 

 and Irrigation Paper no. 188, U. S. Geological Survey, 1907, pp. 21. 



t Compare W. T. Lee : Geology of the lower Colorado river. Bull. Geol. Soc. Am., vol. 

 17, 1906, pp. 275-284. 



t G. B. Richardson : Reconnaissance in trans-Pecos, Texas. Bull. Univ. Texas, no. 23, 

 1904, p. 96. 



§ Personal communication. 



