C. E. Keyes — Geological Structure of Bolson Plains. 209 



v Los Caballos 



San Andreas 



Figure 1. Generalized Cross-Section of the Jornado del Muerto. 



The sequence of events in the region appears to be briefly 

 as follows : In late Tertiary times, probably, the orogenic 

 blocks began their tilting. Some crushing also took place and 

 sharp local foldings were in evidence. Daring a pause in the 

 uprisings, at or soon after the close of the Tertiary, the country 

 was beveled off to the condition of a peneplain. Near the 

 main crests of the primitive Los Caballos and San Andreas 

 sierras, where the displacements were greatest, the Cretaceous 

 and Red Beds were largely removed. With a new period of 

 elevation lava flows spread out from various centers of activity, 

 on the upturned beveled edges of the indurated beds. At the 

 Caballo cone, a low volcanic hill on the top of the plains, a 

 section shows the relationships of the various beds as given 

 below : 



Figure 2. Caballo Cone, resting on Cretaceous sandstones. 



Very recently a new cycle of stream cutting has been inaugu- 

 rated. The Rio Grande, which flows southward on the west 

 side of the Los Caballos range, has deepened into a valley 600 

 to 800 feet below the Jornado del Muerto. The side streams 

 are rapidly cutting back into the great bolson. This period of 

 erosion commenced since the volcanoes were active, for some 

 of their flows are cut in two, and deep canyons mark the paths 

 of the water-courses. 



The gravels covering the Jornado are chiefly composed of 

 crystalline pebbles — many of them apparently foreign to the 

 region. The depths of the gravels covering the bolsons appear 

 to have been greatly overestimated. One would naturally 

 expect under the circumstances great thicknesses of detrital 

 matter. The surprise has been the extreme thinness of the 

 detrital materials covering these plains. In the Jornado del 

 Muerto, the underlying Cretaceous rocks are exposed in many 

 places, protruding through the few feet of gravels. Along the 



