PLEISTOCENE GEOLOGY. 275 



expresses himseli' without reserve in the following unqualified terms: "The 

 deposit represents the finer grist of the ice mill laid down in ice-bound 

 lakes and gorges as the Pleistocene glacier shrunk by surface melting aud 

 retreated northward." Chamberlin and Salisbury, as a resull of their 

 extremely careful and exhaustive studies of the loess of the Upper 

 Mississippi Valley, are less decided in the expressions of their views. 3 

 While admitting the advantages of the seolian theory in certain respects, 

 they find that it dues not account for the conspicuous stratification of the 

 thicker parts of the deposit along the great waterways, for the occasional 

 presence of aquatic shells, etc., and conclude thai the loess is an assorted 

 variety of glacial silt directly derived from glacial waters, and that the 

 time of deposit was during the closing stages of the second episode of the 

 first ( rlacial epoch. After mature consideration of the difficulty of accounting 

 for such a body of water as would admit of the deposition of so finely 

 divided a silt over practically the whole length of the Mississippi Valley, 

 and with the peculiar distribution that the present deposits possess, they 

 conclude that, owing to crustal deformation, the present slope of the land 

 toward the ocean was so reduced that the water was in an intermediate 

 condition between a broad river and a lake. 



The deposits considered by the above writers an- confined to areas which 

 were within the drainage system of the great northern ice sheet, and in 

 general occupy broad belts along tie- general waterways represented at 

 present by the valleys of the Mississippi and Missouri rivers. Westward 

 across the plains through Nebraska and Kansas stretch similar deposits 

 which have not yet been carefully studied, but from a perusal of existing 

 descriptions and somewhat hastv personal observations the writer has little 

 doubt that they are part of and were once continuous with the deposits 

 that have been described above. According to Aughey, 2 the loess of 

 Nebraska has an average thickness of 4<> to 60 feet, and in place- is found 

 LOO, 150, and even 200 feet in thickness:* • Hay, 3 who for some reason not 

 apparent in his report, designates it the Tertiary marl, describes the loess 



■Drift less area of the Upper Mississippi Valley : Eighth Ann. Kept. I'. S. Geol. Survey. 1885, pp. 

 286-307. 



» [Eighth] Ann. Kept. U. S. Geol. and Geog. Surv. Terr, for 1874 I Haydeu . I876,p.245. 



" A geological reconnaissance in southwest Kansas: Bull. U. S. Geol. Survey No. 57, 1890, p. 35. 



