WORK OF WATER IN ARID REGIONS 



579 



All parts of the valleys of the through-flowing rivers traversing the 

 desert region are deep. That they are not all narrow chasms instead of 

 open valleys is due to the character of the substructure. With the single 

 exception of the Grand Canyon, the valleys are all excavated in relatively 

 soft rocks. These are mainly Carboniferous and Triassic shales, friable 

 Cretaceous sandstones, and Tertiary marls, aggregating a very great 

 thickness. Nowhere outside of the Grand Canyon region have any of the 

 streams mentioned yet excavated down to the thick Paleozoic limestones 

 and pre-Cambrian crystallines. 



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Figure 3. — High-level Plains of the Rio Grande 



The influence of the hard rocks upon canyon formation is, however, 

 suggested at a number of points other than on the Colorado. At one 

 point, where the Eio Grande crosses from one bolson to another, the hard 

 mountain rock between two ranges is touched. The narrows thus formed 

 made the spot notable, for it was called by the early Spanish explorers of 

 the region El Paso del Norte, or the North Pass. So, too, the Canadian 

 Eiver, where, in northeastern New Mexico, it has cut through the great 

 so-called Dakota sandstone lying between thick shale beds, is bounded by 



Figure 4. — Relationship of Rio Grande Valley Terraces 



the magnificent Carrazon escarpment, ;<J,000 feet high, which forms the 

 side of the valley for many miles. 



The Grand Canyon region is the only place in all of the vast desert 

 country wheie there has been extensive bowing up and removal of the 

 overlying soft strata (10,000 feet in thickness). It is the only place 

 where any one of the great streams has had opportunity to attack the 

 crystalline basement. Were the Eio Grande or the Eio Pecos excavated 



