208 C. JR. Keyes — Geological Structure of Bolson Plains. 



As the type of the bolsons, the Jornado del Muerto may be 

 taken — that vast plain in southern New Mexico which from 

 the time of the early Spanish explorers of the region up to 20 

 years ago was truly, as its name signifies, a "journey of death." 

 This bolson is more than a hundred miles long and thirty miles 

 wide. On the west are the Sierra de los Caballos and the 

 Sierra Fra Cristobol, rising 3000 feet above the plain. On the 

 east are the lofty Sierra San Andreas and Sierra Oscura. The 

 railroad traverses the middle of the plain from north to south. 

 The mountain ranges are monoclinal blocks made up chiefly 

 of Carboniferous limestones 1500 feet in thickness and resting 

 directly on quartzites, granites and gneisses. The limestones 

 of the eastern ranges slope westward ; those of the western 

 ranges dip eastward. To all appearances, the valley is a simple 

 synclinal trough, the surface of which is covered by gravels. 

 This is the impression that one gets from the train when pass- 

 ing through the region. 



Closer examination of the rocks clearly shows that the beds 

 do not lie in a simple syncline of which the mountain ridges 

 are the upward protruding limbs. The strata lie at much 

 higher angles than the general dips of a simple synclinal trough 

 would require. Moreover, near the mountains, especially on 

 the west side of the plain, is a belt three to four miles wide 

 where the surface, so level when viewed from a distance, is 

 found to be trenched by intricately ramifying canyons, often 

 several hundred feet deep. The geological formations are 

 here everywhere well exposed. While the general slope of the^ 

 plain towards the center is only 2 to 3 degrees, the dips of the 

 strata are often as high as 30 degrees in the same direction, and 

 in places they are even vertical. On the beveled edges of the 

 steeply inclined beds the plain-gravels are laid down ; and also 

 broad sheets of basaltic lava, the latter spreading out from 

 numerous low cones. 



The geological formations represented are essentially as 

 follows : 



Plains gravels (Pleistocene and Tertiary) 5 to 100 feet. 



Yellow sandstones (Cretaceous) 4000 feet. 



Red Beds (Upper Carboniferous) 1000 feet. 



Blue limestone (Middle Carboniferous) 1500 feet. 



Gray quartzite 50 feet. 



Crystalline complex (exposed) 1000 feet. 



A somewhat generalized geological cro§s-section of the Jor- 

 nado del Muerto bolson is given below : 



