58 W. CROSS WIND EROSION IN THE PLATEAU COUNTRY 



Holmes received similar impressions on viewing Dry valley from the 

 Abajo peaks.'' The "cave-like holes" noted by Peale form, in fact, one of 

 the most peculiar characteristics of Dry valley. It is common to find the 

 exposures of the upper La Plata sandstone sculptured with huge recesses 

 or alcoves, both in the more exposed ends of ridges and in long lines of 

 rounded cliffs. Figure 2, plate 3, represents the "Casa Colorado," as named 

 by Newberry, an isolated remnant of sandstone near the upper end of Dry 

 valley. From the colored sketch of this point given in Newberry's 

 report,^" it seems certain that a photograph by W. H. Jackson, in the files 

 of the Geological Survey, represents the same sandstone monument, and 

 this photograph is reproduced here as figure 2. The character of the 

 alcoves is here well illustrated and needs no further description. 



Such erosional remnants are not uncommon in Dry valley. One ob- 

 served by our party seems, from its position, to be the Casa Colorado of 

 the Hayden topographic map. This is shown in figure 1, plate 4, and it is 

 manifestly not the one so called by Newberry. In this case the alcoves are 

 lacking, but the rounded faces suggest the modeling of wind action. 



Another interesting illustration of these peculiar cave-like forms is 

 afforded by Looking-glass rock, near the southwest base of the La Sal 

 mountains, of which a view is given in figure 2, plate 4. In this case a 

 huge alcove on the southern face actually penetrates the rear wall, and 

 there is a large opening through it. The scale of the whole is shown by 

 the minute figures of two men standing in the window at the rear and by 

 the mule on the plain in front of the alcove. 



At the time of visit the walls of this alcove had a thin coat or crust of 

 disintegrated sandstone, crumbling at a touch. This is clearly the com- 

 mon meal of disintegration, due largely to the great diurnal change of 

 temperature characterizing the region, which is particularly effective 

 upon just such rocks as the La Plata sandstone. That the alcove is not 

 wholly the result of excessive local decay of that sort is plain from the 

 coarse debris upon the floor, representing the breaking up of huge scales 

 of rock detached en masse from the ceiling. The fragments are in process 

 of disintegration just as are the walls. The swirling eddies of fierce 

 blasts sweep this loose dust and sand out of the alcoves. 



Our observations were not extended enough to demonstrate fully the 

 process by which these alcoves in the La Plata sandstone have been made, 

 but it seems to me that it must have been chiefly through the disintegra- 



° W. H. Holmes : V. S. Geological and Geographic Survey of tbe Territories, etc., 

 Tenth Annual Report for 1876, p. 191. 



" J. S. Newberry : Report of expedition from Santa Fe, New Mexico, to the junction 

 of the Grand and Green rivers of the Great Colorado of the West, In 1859. Washing- 

 ton, 1S7G, plate vl. 



