66 C. R. KEYES INTEEMO^'T PLAIN'S OF THE AEID REGION 



the Jornada del Muerto, New Mexico,"^ "Geotectonics of the Estancia 

 plains,"* and other shorter papers, to which reference may he made for 

 man}" details bearing upon the subjects here discussed. 



As early as 1903 I endeavored to show,^ in a brief article on the "Geo- 

 logical structure of ISTew Mexican bolson plains," that (1), so far at least 

 as the Jornada del Muerto and Estancia plains were concerned. Hill's in- 

 terpretations were wholly incorrect; (2) instead of being structural val- 

 leys deeply covered by mountain waste, the rock surface of the two basins 

 referred to was worn out on the beveled edges of Cretacic and Carbonic 

 strata, representing a total thickness of more than 10,000 feet; and (3), 

 for the most part, the rock-fioor of these plains was covered by only a 

 thin veneer instead of b}' great thiclaiesses of loose materials. 



A geological cross-section of the Jornada del Muerto presents some- 

 thing of the structure represented below (figure 1), tlie length of the sec- 

 tion being about 30 miles. To all appearances the plain is a simple, very 

 shallow s}-ncline,the ends of which are clearly visible in the lofty Sierra de 



ilER»A Ot LOS CAeAU.O& StERftA DE SAN ANDREAS . feef 



7O0O 



Itio Grai^de/^^^iiuyZ:,^,^^^ j o R n A o a 



sooo 



Figure 1. — BeceleO liocli-floor of the Jornada del Muerto, New Alexico 



los Caballos and the Sierra San Andreas at a distance of a score of miles 

 from the railway wliich traverses the middle of the plain. From the train 

 the surface of the plain, as far as eye can reach, appears floored by gravels 

 and coarse mountain waste; in reality it is covered by soft, porous loam 

 and sands, and so even is the surface that the railway grade in a distance 

 of 60 miles scarcely cuts or fills the ineqiialities. 



Closer examination of the ground and a comparison of the drill-well 

 records, shallow dug-well records, and the few outcrops clearly show that 

 the beds do not lie in a simple syncline of which the mountains are the 

 upper protruding limbs. The strata are disposed at much higher angles 

 than the general dips of the simple synclinal trough would require. jMore- 

 over, near the mountains, especially on the west side of the plain, is a belt 

 3 to 4 miles wide where the surface, so level when viewed from a distance, 

 is found to l)e trenched by intricately ramifying canyons, often several 

 hundreds of feet deep. The geologic formations are here everywhere well 

 exposed. While the general slope of the plain toward its center is only 

 from 2 to 3 degrees, the dips of the strata are frequently as high as 30 



' U. S. Geological Survey, Water Supply and Irrigation Paper, no. 123, 1905, 42 pages. 



* Journal of Geology, vol. xvi, 1908. 



» American Journal of Science (4), vol. xv, 1903, pp. 207-210, 



