74 



PLEISTOCENE OF INDIANA AND MICHIGAN. 



Indiana is but 5 to 10 feet on uplands and is even less on slopes, opportunities for observing 

 the Sangamon weathered zone are very numerous. 



POST-SANGAMON OR MAIN LOESS AND ASSOCIATED SILTS. 



EXTENT AND DEPOSITION. 



The weathered surface of the Illinoian drift and the outlying driftless territory in southern 

 Indiana both bear a thin deposit of silt which is part of a practically continuous sheet that 

 extends from Ohio westward beyond the Mississippi. In the vicinity of the main drainage 

 lines it is loose textured and is commonly termed "loess." On the interfluvial tracts it ranges 

 from a deposit readily pervious to water to one very slowly pervious. The least pervious 

 portion is of a pale color and is termed "white clay." 



The method of deposition of a large part of this silt is still in dispute and is one of the 

 difficult problems of Pleistocene geology. Certain deposits of it along the main valleys, notably 

 the marl-loess deposits of the Wabash, have been referred to fluvial action, 1 but generally 

 along valleys as well as on interfluvial tracts the loess seems best explainable by wind action. 



CHARACTER OF THE LOESS. 



Analyses of the loess show that it is composed mainly of silica, the percentage of the 

 material being from about 60 up to fully 80 per cent. Alumina, which is the next in order 

 of abundance, except perhaps in very calcareous parts such as the marl loess of the Wabash, 

 ordinarily constitutes 10 per cent or more. The amount of oxide of iron is notable, ranging 

 from about 1 per cent to more than 6 per cent in the analyses at hand. The following analyses 

 of loess and associated silts in Indiana have been taken from reports of the Indiana Geological 

 Survey: 



Analyses of the loess and white clay in Indiana. 



1 and 2. White clay near Terre Haute at depths ol 10 and 22 inches, respectively. Analyst, W. A. Noyes. 



3. Near Princeton. Analyst, Robert Lyons. 



4. Near New Harmony. Represents the marl loess. Taken from Owen's report for 1838. 



A number of samples of loess and more compact silt collected by the writer in western 

 and southern Illinois and mechanically analyzed under the direction of Milton Whitney, of 

 the United States Department of Agriculture, show that the pervious loess contains a smaller 

 percentage of very fine particles than the compact, but that it contains no more coarse par- 

 ticles. Considerable variation is shown, but the bulk of the deposit, in some samples as high 

 as 75 per cent, consists of grains which fall between 0.05 and 0.01 millimeter, or 0.002 and 

 0.0004 inch in diameter. Few grains measure more than 0.1 millimeter, or 0.004 inch. Some 

 soils in the loess contain a larger percentage of grains below 0.01 millimeter than subsoils, a 

 feature which probably results in part from the greater disintegration of soil particles and in 

 part from the constant addition of very minute particles to the soil from dust carried in the 

 atmosphere. 



The thickness of the deposit in Indiana in few places exceeds 40 feet and is greatest along 

 the Ohio and Wabash valleys. Within a short distance back from the bluffs of these valleys 



i Bull. Geol. Soc. America, vol.' 14, 1903, pp. 153-176. 



