350 HALL AND SARDESON — EOLIAN DEPOSITS OP MINNESOTA 



seen of such accumulations, the inference is that they have been regarded 

 by the many glacialists who have worked in this region. as glacial in 

 origin and therefore described as modified drift; that is, as a water de- 

 posit rather than dune sands of eolian origin. One exceptional case is 

 the description by Warren Upliam * of an area of dune sand where the 

 sand is still shifted by the wind and the nature of the deposit is for that 

 reason unmistakable. The area referred to is in Anoka county, from 12 

 to 15 miles north of Minneapolis. The description is as follows : 



" Dunes of sand, gathered from the modified drift by the wind and heaped up 

 in mounds and ridges 10 to 20 feet high, occur in the south part of sections 34, 

 35, and 36, Grow, Anoka countJ^ Thej' are blown into frequently shifting forms, 

 like drifts of snow, and are too unstable to give a foothold to vegetation. It seems, 

 most probable that they were gathered from the coarser sand and gravel of the 

 surrounding area soon after the deposition of those beds, before they became cov- 

 ered and protected from wind erosion by grass, bushes, and trees." 



This is doubtless the same belt of wind-driven sands which can be seen 

 in many places as far southeastward as Saint Paul. 



One of the authors of this paper has already pointed out, although in- 

 cidentally, that very much of the " modified drift " mapped in eastern 

 jNIinnesota is reall}^ dune sand, but not having traversed the entire region, 

 the full extent of the dune sand deposits was unknown. f 



A. H. Elftman X has more recently identified the same kind of deposit 

 in northeastern Minnesota. At one locality — 



"are deposits of unstratified sand above the till and modified drift. These de- 

 posits present an uneven surface similar to that of the moraines. The material is 

 composed entirely of fine sand. These dune-like hills are referred to wind deposits 

 which are derived from extensive deposits of modified drift in the immediate 

 vicinity." 



Definitions of Loess, Dune Sand, and Lag Gravel 



Before describing some typical occurrences of this kind it may be re- 

 called that the recognition of dune sand, so far as cited, is based on 

 structural peculiarities of the sand and its relation to the till and modi- 

 fied drift. 



Loess, so far as here observed, is an eolian deposit. It is accumulated 

 b}^ long continued winds bloAving prevailingly in one direction. This ac- 

 counts for its leeward position wherever found. The material com[)osing 

 it undoubtedly springs from glacial deposits, since these have effectuall}^ 

 covered all older rock formations within a wide region. 



* Geology and Nat. Hist. Survey of Minnesota, vol. ii, 1888, p. 418. 



fF. W. Sardeson : On gUujial deposits in the driftless area, .\merican Geologist, vol. xx, pp, 

 302-403. 



t The geology of the Keweenawan area iu northeastern Minnesota, pt. 1. American Geologist, 

 vol. xxi, pp. 90-109. 



