CHAPTER VIII. 

 THE LOESS AND ASSOCIATED SILTS. 



GE^STEEAL STATElVrElVT. 



On the borders of the Mississippi and its main tributaries there is a 

 very porous silt which overhes the Sangamon soil and weathered surface of 

 the Illinoian drift sheet, and which has long been known as loess. It is 

 commonly calcareous to a marked degree, though its main ingredients are 

 siliceous and argillaceous particles. A series of chemical, mineralogical, 

 and mechanical analyses appear in Monograph XXXVIII of this Survey. 

 Portions of the loess are highly fossiliferous, with a fauna composed chiefly 

 of terrestrial species of mollusks, but containing also species which inhabit 

 ponds, and occasionally a fluviatile mollusk. Lists of the fossils are also 

 given in Monograph XXXVIII. 



On the uplands back from the Mississippi and its main tributaries, and 

 also along the minor tributaries, there are silts of more compact texture 

 than those which border the valleys. Mechanical analyses of samples col- 

 lected in Illinois, also appearing in Monograph XXXVIII, show that there 

 is a larger proportion of very fine particles in these compact silts than in 

 the loess bordering the valleys, but that in many respects they are similar. 

 The porous loess does not contain coarser particles than are found in the 

 compact silt. 



DISTRIBUTION. 



The compact silt extends eastward in a practically continuous sheet 

 from Illinois over southern Indiana, southern Ohio, and neighboring portions 

 of Kentucky and West Virginia, and is the surface deposit as far north as 

 the border of the Wisconsin drift sheet. It is known to underlie the Wis- 

 consin drift, numerous exposures having been found beneath that drift. 



This silt has long been recognized in the glaciated districts of south- 

 western Ohio and southeastern Indiana. In the Ohio reports it is referred 

 to as the "white clay," and in the Indiana reports as "slash land." It has 



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