PAPERS ON GEOLOGY 113 



and Wisconsin drift sheets as is present over the surface of the 

 older Ulinoian and Kansan till. The maximum loess deposition 

 did not occur until a long time after the Ulinoian till was de- 

 posited as shown by the fact that (1) before the loess was de- 

 posited sufficient time elapsed for the development, on the sur- 

 face of the Ulinoian till, of erosion slopes and relief quite sim- 

 ilar to those of the present surface; and (2) the interval be- 

 tween the deposition of the Ulinoian till and of the overlying 

 loess in Illinois was sufficiently long for the development in 

 many places upon the surface of the Ulinoian drift of the San- 

 gamon soil horizon and beds of peat, and in others of an oxi- 

 dized and weathered zone, and in still others of a zone of con- 

 centrated pebbles on the slopes by the removal of the finer 

 constituents of the till, such as are found above the Ulinoian 

 till and below the loess in many places in Sangamon, Fulton, 

 Menard, Champaign and other counties in Illinois. 



Some loess-like material was doubtless deposited by the 

 winds during each of the interglacial stages, for thin deposits 

 of such character are known between the Kansan and the 

 Ulinoian and the Kansan and the Iowan drift sheets at num- 

 bers of places in the Mississippi Valley. However, the thickness 

 of the loess between the Kansan and Ulinoian till or between 

 the Kansan and the Iowan is not nearly so great as that over 

 the surface of the Kansan and the Ulinoian till sheets. The pre- 

 Iowan loess appears to have been limited in its distribution, 

 compared with the present loess deposits, as is indicated by the 

 following facts : ( 1 ) The loess is thin, only one to three feet, 

 and of relatively rare occurrence between the Ulinoian and the 

 Kansan or between the Kansan and the Iowan drift sheets, 

 while a thickness of several feet is common between the Uli- 

 noian and the Wisconsin till. (2) Masses of typical fossil- 

 iferous loess, in size up to 15 feet long and 4 feet thick, are 

 often found incorporated in the Wisconsin till, but they are 

 rarely or never found in the till of earlier age, in which masses 

 of sand of corresponding size are not uncommon. 



The relations of the loess to the drift varies with the valley 

 slopes. In many places where the banks bordering the valleys of 

 the larger streams are steep, the loess breaks off abruptly near 

 the top at the uppermost exposed level of the till. In many other 

 places where the bordering slopes are gentle and are favorably 

 situated with respect to areas of flood plain and the direction of 

 prevailing winds, a mantle of normal loess covers the slope to 

 a thickness of from two or three to six or eight feet, often ex- 

 tending down almost or entirely to the level of the flood plain. 



