54 



W. CROSS WIND EROSION IN THE PLATEAU COUNTRY 



system of the San Juan river and clearly antedate the recent glaciation of 

 the mountains. A discussion of these gravel terraces and plains, as to 

 their origin and relations, has been published by Mr Howe- and myself, 

 and the only feature of present interest is a peculiar fine, reddish, sandy 

 soil which occurs almost everyTvhere as a mantle to the gravel deposits. 

 The invariable character of this soil and its presence on terraces of differ- 

 ent elevations attracted the attention of A. C. Spencer, who, in 1896, was 

 my assistant in the survey of the La Plata quadrangle, Colorado, and was 

 particularly charged with the study of Quaternary problems. Mr Spen- 

 cer believed this red soil to be of eolian origin, and the correctness of that 

 opinion has become more and more evident with extended observations. 



The thiclcaess of the deposit in question is commonly several inches, 

 while in some places it exceeds 2 feet. The red soil has seldom been 

 observed on other surfaces than the gravel plains and terraces. Viewing 

 it as of eolian origin, it is natural to suppose that the sand first lodging 

 between gravel grains and pebbles became a soil permitting a vegetable 

 growth, which in turn served to catch and hold more sand. The light, 

 wind-blown particles do not seem to have accumulated on inclined sur- 

 faces, which woiild be more exposed to the degrading action of wind or 

 water than the level plains. It is, however, quite possible, as Mr Spencer 

 has suggested to me, that eolian soil may exist on certain old mountain 

 slopes covered by forest gro-n^th and changed in color by the action of 

 humus acids. Observations directly bearing on this have not been made. 



This red soil seems wonderfully constant in character, and it is be- 

 lieved that its average mechanical constitution is represented by the sub- 

 joined analysis of a sample collected by myself from a terrace adjacent to 

 the La Plata river, at a point in New Mexico very near the Colorado line. 

 At this locality the soil was more than 1 foot in thickness, and the speci- 

 men obtained did not appear to have been disturbed by the plow. Through 

 the courtesy of Dr Milton Whitney, in charge of the Bureau of Soils, U. 

 S. Department of Agriculture, Mr Lyman J. Briggs made the following 

 mechanical analysis of this red soil from La Plata valley, New Mexico : 



No. 



78642. 



Organic 

 matter. 



Per cent. 

 0.79 



Gravel, 

 2 milli- 

 meters 

 to 1 mil- 

 limeter. 



Per cent. 



1.74 



Coarse 

 sand, 

 1 to 0.5 

 milli- 

 meter. 



Per cent. 



5.66 



Medium 

 sand, 0.5 

 to 0.25 

 milli- 

 meter. 



Per cent. 



3.52 



Fine 

 sand, 

 0.25 to 

 0.1 milli- 

 meter. 



Per cent. 



9.72 



Very 



fine 

 sand, 0.1 

 to 0.05 

 milli- 

 meter. 



Per cent. 



38.00 



Silt, 

 0.05 to 



0.005 

 milli- 

 meter. 



Per cent. 



28.40 



Clay, 

 0.005 to 

 0.0001 

 milli- 

 meter. 



Per cent. 



12.50 



2 Ernest Howe and Whitman Cross : Glacial phenomena of the San Juar( mountains, 

 Colorado. Bull, Geol. Soc, Am., vol. 17, 1006, pp. 351-874, 



