EXTINCT GLACIERS OF COLORADO 4I 



is an open question. In the same basin a very important deposit of 



fine silt or loess is found. ' Similar deposits in other parts of the world 



have been variously ascribed to subaerial, glacial, or glacio-aqueous 



origin. The Denver Basin loess is apparently continuous with that 



which extends across Nebraska and Kansas practically to the Missouri, 



and perhaps with that of Iowa and adjoining states. It overlaps glacial 



drift in Lancaster County, Nebraska.^ There is some difficulty in 



assuming a body of water of sufficient size to furnish a settling reservoir 



for the detritus brought down by streams from beneath the ancient 



mountain glaciers or the continental glacier and sp^'ead it over such a 



large territory. Formation by dust blown across the mountains from 



the arid Great Basin is equally objectionable, as in such case many 



of the mountain valleys would have caught and held similar deposits. 



To one familiar from long association with the windstorms of the Great 



Plains and foothill region, seolian processes seem fully efficient to produce 



such formations on the largest scale. Every such storm which sweeps 



down from the mountains carries out to the plains many tons of finely 



divided materials, to be again picked up by the next storm and borne 



farther away, and being continually ground finer and finer. This 



process has been going on for ages, and the aggregate in the course of 



even a few centuries is of considerable importance ; but of much greater 



moment is the work the winds have been performing for ages, assisted 



by streams, frost, chemical solution, alternating heat and cold, and 



other forces, in the disintegration, destruction and redeposition of the 



Tertiary formations of the plains. Since those formations were first 



exposed to atmospheric influences there has been time enough for the 



rocks to be disintegrated, and their component materials to be worked 



over and over by wind and water, until left in their present condition. 



The^exact origin of the loess may never be definitely known, but surely 



the solution of the problem is well worth striving for. Whatever may 



be the ultimate conclusion, it cannot affect the statement that the glaciers 



of the Colorado Rockies did not extend to the plains, so far as the 



evidence goes. 



' Ibid., pp. 41, 258, 278. 



» Geology and Underground Waters of the Central Great Plains, sad Prof. Paper, U. S. Geol. Sur., p. 138. 



