28o MORRIS M. LEIGHTON 



23 per cent of the 150 borings in the lUinoian area did not pass 

 through the leached zone of the drift, while only 4 per cent failed 

 to do so in the Green River area. 



From this and the other data, there is no question in the mind 

 of the writer but that the lUinoian drift is distinctly older than the 

 Green River drift. The question now remains as to whether the 

 Green River drift is lowan or Early Wisconsin in age. 



Table I gives comparative data for the leaching of the Green 

 River drift and the lowan drift in Iowa. The average thickness 

 of non-calcareous materials resting upon calcareous till in the two 

 areas is 5.7 feet for the Green River lobe and 5.2 feet for the lowan 

 drift. This would at first glance make it appear that the Green 

 River drift is at least as old as the lowan drift, but this non- 

 calcareous zone includes thicker loess-like silt and thinner till in the 

 former case than in the latter. Loess leaches much more rapidly 

 than till because of its porosity and absence of pebbles, probably 

 twice as fast according to the writer's data. If this ratio is used 

 and the data evaluated correspondingly, the leaching of the Green 

 River drift would compare with that of the lowan drift in the ratio 

 of 3.5 to 4.3, and with the Belvidere drift, 3.5 to 2.9. The data 

 for the age of the Green River lobe are, therefore, not decisive. 

 The Green River lobe is surrounded by thick loess, which, it is to be 

 noted, is the same relationship as holds for the lowan drift in Iowa. 



THE TERMINUS OF THE GREEN RIVER ICE 



Previous to the Green River ice invasion, the area involved was 

 even more of a lowland than now, and invited the ice protrusion. 

 The lowland became such in pre-IUinoian times by the erosion of 

 the old Mississippi River and its tributaries when the master stream 

 left its present valley near Cordova and flowed southeastward to the 

 big bend of the Illinois River.^ The Illinoian ice reversed the drain- 

 age to the west by way of a tributary valley, now the lower course of 

 the Rock River, and thence over a divide to the west and south. 

 That the Cordova gorge was not cut at this time is indicated by the 



^ Frank Leverett, "The Illinois Glacial Lobe," U.S. Geological Survey Monograph 

 XXXVIII (1896), overprint of Plate VI; and "Outline of Pleistocene History of Mis- 

 sissippi Valley," Journal of Geology, Vol. XXIX (1921), pp. 615^26. 



