BOLIAN SOIL IN COLORADO 55 



This analysis shows that more than 88 per cent of the sample consists 

 of particles less than 0.25 millimeter in diameter. According to the 

 investigations by IJdden^ as to size of grains in wind deposits, about 86 

 per cent of dune sand measures between one-half millimeter and one- 

 eighth millimeter in diameter; from 70 to 95 per cent of "lee sand" is 

 between one-fourth millimeter and one-sixteenth millimeter in diameter, 

 and in atmospheric dust a great majority of the particles measure be- 

 tween one-eighth millimeter and one-thirty-second millimeter. Certain 

 volcanic dust deposits examined by Udden gave results upon diametrical 

 analysis quite near that of the red soil. So far as the mechanical consti- 

 tution of the red soil is concerned, it may well be chiefly atmospheric dust 

 derived from a great distance. 



The geographic situation of the supposed eolian soil deposits on the 

 eastern border of the Plateau province, in conjunction with the preva- 

 lence of strong winds from the west during much of the year, serves to 

 indicate the probable derivation of this fine sand from the desert area of 

 the plateaus. Opportunity to trace the deposits to the general region of 

 their apparent source was given me in 1905, during a reconnaissance 

 from Mancos, Colorado, to Moab, in Grand Eiver valley, Utah.* The 

 route of travel led over the broad arid plain between Montezuma creek 

 and Dolores river, which is called the Great Sage plain on the Hayden 

 topographic map of Colorado and eastern Utah. This plain is underlain 

 by the Dakota Cretaceous sandstone, with remnants here and there of the 

 soft Mancos shale resting on that sandstone. 



That a red sandy soil as a coating is not in fact peculiar to the gravel 

 plains already discussed, is shown by the occurrence of the same material 

 at many points on the Great Sage plain, covering sandstone or shale, as 

 the case may be. It is held in place by the same kinds of desert vegetation 

 which grow on it in the zone nearer the San Juan mountains. No greater 

 thickness of the sand was found on the broad plateau than on the terraces 

 of the La Plata and other tributaries of the San Juan river to the east. 



The method of transportation of the red sand or soil is visibly demon- 

 strated to the traveler riding over the Great Sage plain on a hot summer's 

 day, for on every hand he sees huge columns of dust, rising as whirlwinds, 

 sweep over the arid waste. This dust is thus borne to higher strata of 

 the atmosjjhere, where it is caught by strong currents and carried to 

 varying distances. The prevalent winds of the present time blow from 



° J. A. Udden : The mechanical composition of wind deposits. Augustana Library 

 publication no. 1, Rock Island, Illinois, 1898. 



* Whitman Cross : Stratigraphic results of a reconnaissance in western Colorado and 

 eastern Utah. Journal of Geology, vol. xv, 1907, pp. 634-679. 



