DEFLATION IN DRY VALLEY 57 



places its color has led to its mistaken correlation with the Vermilion 

 Cliff sandstone. 



Deflation in Dry Valley 



The area between Grand Eiver, the La Sal, and the Abajo Moimtain 

 groups has few deep canyons. The principal one is that named "Caiion 

 Colorado" by ISTewberry, who descended it to Grand river. The name is 

 also used on the Hayden map, but is omitted from the Survey sheets. 

 This canyon derives its principal water supply from the slopes of the La 

 Sal mountains, but its largest drainage area is a broad basin, appropri- 

 ately named Dry valley. This valley, with its several branches, occupies 

 the greater part of the area between the La Sal and Abajo mountains. It 

 approaches to within a few miles of the slopes of the Abajo group, but 

 now receives no water from that source, since the streams heading on the 

 northern slope curve either to the east to join Montezuma creek, a tribu- 

 tary of San Juan river, or west directly toward the Colorado. 



At the head of Dry valley three of its branches cut back into the Great 

 Sage plain as narrow streamless canyons several hundred feet in depth, 

 shown on the map; but the greater part of the basin is an arid expanse 

 with low ridges of gray or light red sandstone. The main floor is at or 

 very near the top of the lower division of the White Cliff sandstone and 

 approximately 1,000 feet below the level of the Great Sage plain. 



The general character of the valley floor and of the sandstone ridges 

 between its branches is shown by figure 1, plate 3. While there may be 

 local ravines cut into solid rock, where cloud-burst floods have done their 

 work, there is, in the portions of the valley I saw, no continuous water- 

 course through which debris is now transported, even by flood water. 



The sandstone ridges, such as those of figure 1, exhibit many evidences 

 of rather rapid wasting through the usual disintegration due to the atmos- 

 pheric conditions of desert lands. The sandstone is normally a very 

 massive, fine-grained, even-textured rock, friable and crumbling under 

 the blow of a hammer. In many places a dust-like coating is on the more 

 protected surfaces, while miniature talus heaps of fine sand are found 

 below some exposed faces. In many places thin coats of brown "desert 

 varnish" preserve the exterior, while the interior is rotten or cavernous. 



The topographic forms presented by the La Plata sandstone of Dry 

 valley are almost invariably of rounded contours such as suggest to the 

 trained eye the sculpturing of wind erosion. Peale records in these 

 words the impression produced on him on seeing this district from the 

 summits of the La Sal group : "The beds appear to be horizontal and in 

 many places have cave-like holes worn by rain and wind."^ 



* A. C. Peale: U. S. Geological and Geographic Survey of the Territories, etc., Ninth 

 Annual Report for 1875, p. 60. 



