278 MORRIS M. LEIGHTON 



side. To the south is undoubted IlHnoian drift heavily overlain 

 with loess (see Fig. i). For 6 miles west of the Bloomington 

 moraine, the drift laps upon the highland and is bounded on the 

 south by a belt of kames. Farther west to beyond Geneseo the strip 

 is mostly low-lying and the border is marked by a thick accumu- 

 lation of sandy loess which is known to have a thickness in excess 

 of 35 feet. Over the Illinoian area south from here the loess thins 

 somewhat and becomes finer, but maintains a considerable thick- 

 ness, while over the strip of drift of youthful aspect it is scanty. At 

 one place along the border, at the edge of the thick loess,^ a shallow 

 road-cut exposure was found which showed fresh till with Hmestone 

 pebbles overlying a loess-like deposit, which resembles the loess 

 southward from here, and which at various places is known to rest 

 on weathered lUinoian drift. 



Twenty-six borings on the strip of relatively fresh drift revealed 

 an average of 1.2 feet of soil, 3.5 feet of non-calcareous loess-like 

 silt, and 1.6 feet of leached till overlying calcareous till, making a 

 total of 6.3 feet of non-calcareous material. It will be noted that 

 this is so nearly like that of the strip on the north side of the Green 

 River basin that they may properly be considered to be of the same 

 age. Reference to Figure i will reveal that the exposures and auger 

 borings which show positive evidence of an unconformity between 

 the loess and the underlying till are situated almost entirely outside 

 of the boundary of the relatively fresh drift, on both the north and 

 the south sides of the basin. 



THE AGE OE THE GREEN RIVER LOBE • 



It is to be kept in mind that the evidence for the differentiation 

 of the Green River lobe from the lUinoian drift is based not only 

 upon differences in leaching but also upon the occurrence of gum- 

 botil, old soils, old loess-like silts and weathered zones between the 

 Illinoian drift and the overlying loess and the general absence of 

 such evidences in the Green River lobe. But the depth of leaching, 

 based upon numerous and well-distributed auger borings, affords 

 some measure of the comparative ages of the two drifts. In Table 

 I, it wiU be seen that the average depth of non-calcareous materials 



' About the center of the south line of Sec. 30, T. 16 N., R. 6 E., Bureau County. 



