DRIFT SHEETS OF NORTHWESTERN ILLINOIS 273 



failed to reach it in the Belvidere area. The average depth of leach- 

 ing for the Illinoian given above is therefore something less than 

 the actual. 



Significance of these differences. — -It is pertinent to inquire 

 whether or not the differences between the Belvidere drift and the 

 IlHnoian drift can be explained on some other basis than on differ- 

 ences in age. That the drift is more weathered in one area than in 

 another may be due to differences in the character of the material, 

 or to differences in climate, or to differences in age. If the drift of 

 the Belvidere area were composed of a tighter clay than that of the 

 Illinoian area, such as might be derived from the belt of Maquoketa 

 shale east of Belvidere, the leaching of the one would proceed at a 

 less rapid rate than the leaching of the other; likewise the depth 

 of oxidation, but less so the degree of oxidation at the surface. The 

 geologic map of IlHnois and the glacial map reveal that there is no 

 correspondence between the belt of Maquoketa shale and the area 

 of Belvidere drift. Besides, it is not possible on this basis to account 

 for the difference in erosional expression of the moraines of the two 

 areas, or the difference in degree of oxidation of the surficial portion, 

 or the difference in concentration of the residual pebbles, or the 

 difference in the stratigraphic relations of the buff loess, or the 

 presence of the old loess-like silt and old soil and gumbotil in one 

 area and not in the other. There are no differences in climate and 

 no reasons for thinking that there have been between these two 

 adjoining areas. A greater age for the lUinoian drift than for the 

 Belvidere drift is the only explanation which satisfactorily explains 

 all the facts. 



The age of the drifts. — Table I summarizes the depth of leaching of 

 various drifts, including the Belvidere drift, Bloomington drift, and 

 Illinoian drift, based on the number of borings. Figure 3 shows 

 graphically the differences in the depth of leaching of these drifts. 

 All of the data indicate that the drift outside of the Belvidere Lobe 

 is Illinoian in age. Comparing the Belvidere and Bloomington 

 drifts, it will be seen that there is a slight difference in favor of the 

 greater age of the Belvidere drift. But in the opinion of the writer, 

 the major divisions of the Pleistocene cannot be made on such 

 ■ slight differences, and since there are Early Wisconsin moraines in 



