60 W. CROSS AVIND EROSION IN THE PLATEAU COUNTRY 



about the San Juan mountains. Such gravel deposits are distinct from 

 the coarser debris scattered over the entire valle}'. 



The rocks occurring in these large fragments are all of sedimentary- 

 origin, and familiarity with the section of the region soon suggests their 

 source. The conglomerate mentioned as a component of this debris is 

 relatively rare, and it is unmistakably derived from the Dakota sand- 

 stone. It is mainly composed of small chert pebbles of various colors and 

 shades, and is typical of the basal conglomerate of the Dakota as known 

 all through Colorado and in other states. The conglomerate was foimd 

 to be particularly well developed at the base of the Dakota in the southern 

 arm of Dry vallev, called "Cold Spring canon" on the Hayden map. 



The rusty indurated sandstone, which is more abundant than the con- 

 glomerate in the valley debris, is likewise probably derived largely from 

 the Dakota. A cementation of the Dakota sandstone by silica and iron 

 hydroxide is much more common than in the McElmo and La Plata for- 

 mations, and many fragments of the valley floor are very similar to strata 

 visible in the Dakota sections about the valley. 



The cherty fragments and the limestone were not so readily referred to 

 their proper horizon, but an examination of the upper surface of certain 

 La Plata ridges, like those of figure 1, plate 3, showed an almost contin- 

 uous deposit of fragments, and exposures of the zone between the McElmo 

 and La Plata formations revealed the presence, just alcove the La Plata, 

 of a stratum several feet thick made up chiefly of impure cherty material. 

 Where best exposed, on the road from Jloab to Monticello, as it descends 

 from the divide north of Canes spring, this stratum is made up of a con- 

 fused brecciated mass of dense siliceous rock with a sandy calcareous 

 cement. The limestone was not seen in place, but is believed to occur 

 at or very near the horizon of cherty material. 'No fossils were found in 

 it, though repeatedly looked for. 



My conception as to the origin of these blocks scattered over Dry valley 

 is that they are the residue of the harder rocks in the section of sediment- 

 ary beds out of which the valley has been carved mainly Ijy deflation, 

 using this term as proposed by Walther" for wind transportation. They 

 appear to have been let down gradually through the disintegration and 

 transport of the soft clays, shales, and sandstones of the Dakota, McElmo, 

 and upper La Plata formations. 



The fine desquamation products of sandstone and shale in Dry valley 

 have of course been subject to some degree of transportation by water. 

 The features of the valley already mentioned indicate, however, notable 

 aridity and rarity of floods. The wash of occasional rains must transport 



" J. Walther : Das Gesetz der Wustenbildung, p. 33. 



