F. W. Sardeson — What is the Loess f 59 



sediments upon the land, and these were in part modified by 

 seolian agent into " bluff loess." 



In theory, the Chamberlin view may be essentially correct. 

 The loess particles came from the drift, and very probably 

 from washed drift largely. If the theory be applied, it should 

 bring into recognition the following kinds and relations of 

 deposits : the unassorted glacial deposit, or till ; this, when 

 assorted by agency of water, becomes stratified gravel, sand 

 and silt; the two latter may be made by seolian agent into 

 dune sand and " bluff " loess ; and they, after erosion, may 

 become again respectively stratified sand and silt ; under the 

 action of humic acids the silt of both kinds may be reduced to 

 ■a loam in which there is no lime ; and likewise the u bluff " 

 loess may be reduced entirely, or to a degree, to "loess" loam. 

 The till is easily recognized, as are also the stratified products, 

 the gravels and sands. Dune sand lacks the assorted and strati- 

 fied structure, and is easily distinguishable from the sedi- 

 mentary patches associated with or in it. It seems strange, 

 therefore, that silt and asolian deposit, — that is mud and dust, — 

 should not be distinguishable. 



Indeed, it is not difficult to discriminate between glacial silt 

 and loess, and I believe that they are rarely if ever confused by 

 geologists. The difficulty lies in the discrimination of theo- 

 retical sedimentary loess and seolian loess. There are two 

 kinds of loess in the Mississippi basin, and they are distinguish- 

 able at a glance, but as said, they have not been proved separ- 

 able on the basis of sedimentary and seolian origin. The two 

 kinds are very intimately associated. They are respectively 

 the characteristic calcareous, porous, "bluff" loess and the 

 oxidized portion of the same, which may be called loess loam, 

 residuary loess, or ferretto. Difference in color, or the appli- 

 cation of hydrochloric acid, serves as a ready test of them.* 

 Chamberlin and Salisbury, in their monograph, recognized the 

 loess loam of the "Driftless Area" as residuary clay although 

 they failed to connect it with the loess, as I have once before 

 indicated, f and in fact Chamberlin has practically admitted 

 the emendation by criticising other parts only of my paper.J 

 The loess loam being due to oxidation by humic acids, its 

 depth is determined by the vegetation upon its surface and by 

 the time that this agent has acted, provided that the loess be 

 not too shallow and surface erosion does not equal or exceed 

 the rate of oxidation. • On the Missouri and Mississippi River 

 bluffs, the loess was originally deepest ; it is also little covered 

 with vegetation and highly exposed to erosion. On the higher 



*This distinction is not new, for example see Geologischer Fuehrer d. Umge- 

 bung v. Freiburg, p. 78, Steinmann u. Graeff, 1890. 



f Am. Geol., vol. xx, No. 6. % Jour. Geol., vol. v, No. 6. 



