THE MAKE OP THE DESERT 



39 



is losing at the bottom by the under blocks 

 grinding away to stone and gravel. The flat- 

 tening out at the bottom, the breaking up of 

 the blocks, and the push-out of the mountain 

 foot upon the plain is the second stage of the 

 talus. In almost all the large valleys of the 

 desert the depressed talus extends, sometimes 

 miles in length, out from the foot of the moun- 

 tain range. When it finally slips down into the 

 valley and becomes a flat floor it has entered 

 upon its third and last stage. It is then the 

 ordinary valley-bed covered with its cactus and 

 cut by its arroyos. Yet this valley-floor instead 

 of being just one thing is really many things — 

 or rather made up of many different materials 

 and showing many different surfaces. 



You may spend days and weeks studying the 

 make-up of these desert-floors. Beyond Yuma 

 on the Colorado there are thousands of acres of 

 mosaic pavement, made from tiny blocks of 

 jasper, carnelian, agate — a pavement of pebbles 

 so hard that a horse's hoof will make no im-. 

 pression upon it — wind-swept, clean, compact 

 as though pressed down by a roller. One can 

 imagine it made by the winds that have cut 

 and drifted away the light sands and allowed 

 the pebbles to settle close together until they 



stages of 

 the taUiS. 



Desert- 

 floors, 



