DRIFT SHEETS OF NORTHWESTERN ILLINOIS 277 



west into the lowland area southeast of Morrison. The valleys of 

 Rock River and Elkorn Creek are the chief interruptions in the 

 continuity of the belt. A low but definite ridge of thick drift marks 

 the margin for about 2 miles west and south of Eldena and some 8 

 miles to the east and north, as shown in Figure i . To the west the 

 boundary is much less definite than could be desired, but the weath- 

 ering of the drift and the relations of the drift to the loess indicate 

 that the Kne which has been drawn marks approximately the limit 

 of a relatively young drift. Ninety-three widely distributed borings 

 along this strip showed an average of 1.2 feet of soil, 3.1 feet of non- 

 calcareous loess, and 1.3 feet of leached till, making a total of 5.6 

 feet of leached or non-calcareous material. In repeated cases where 

 the loess was thick enough to be calcareous, the till below was found 

 to be calcareous and to contain limestone pebbles to the top, showing 

 that no interval of weathering intervened between the deposition 

 of the loess and the till of this strip. North of the margin of this 

 strip, thick loess lies unconformably on the till. That is to say, 

 either an old soil or humus muck or old non-calcareous loess-Hke silt 

 or gumbotil lies between the two, or the calcareous zone of the loess 

 rests upon leached and oxidized till. No exposure of gumbotil is 

 known in the belt of younger drift except at one place along the east 

 line of the southeast quarter of Sec. 33, T. 21 N., R. 10 E., south of a 

 low portion of the marginal moraine, where there appears to be an 

 absence of the younger drift. In the territory immediately north 

 of the line, the total average thickness of leached till and loess is 

 nearly twice as great as south of the line. The writer knows of no 

 other adequate explanation for these phenomena than difference in 

 age. As in the case of the Belvidere lobe, there is a paucity of out- 

 wash related to the younger drift , and a corresponding lack of kames 

 in the moraine. Southeast of Dixon the Rock River appears to 

 have been blocked by this ice lobe, as indicated not only by relatively 

 fresh drift northeast of Sterling but by an old channel scar skirting 

 the margin and comparable in size to the Rock River. 



The strip along the south side.- — ^South of the Green River sand 

 plain and extending west from the Bloomington moraine at Sheffield 

 to Geneseo, there is another strip of drift with thin loess, which 

 appears to be of the same age as the strip of drift along the north 



