COMPOSITE MORAINIC BELT OF NORTHERN ILLINOIS. 303 



this may account for its greater ruggedness. The undulatory tract leading 

 southward from this sharply moraiuic portion might by this interpretation be 

 accounted for as a spur extending backward from the sharp bend in the 

 margin. The bulk of the Bloomington system is not greatly different from 

 that of the composite belt west of Fox River and presents no obstacles to 

 this interpretation.' The topography of the composite belt is much sharper 

 in expression than that of the Bloomington system, but changes of topog- 

 raphy have been found to occur in other belts to as marked a degree as 

 in this instance. As pointed out by Chamberlin, in the Third Annual 

 Report, the Kettle moraine changes from a very sharp knob-and-basin 

 topography in southern Wisconsin to a comparatively smooth swell-and-sag 

 topography in northeastern Illinois and northwestern Indiana, and again 

 assumes a sharp knob-and-basin topography when traced into Michigan. 



By the second interpretation this portion of the composite belt is thrown 

 into the late Wisconsin series of moraines and its continuation found in the 

 bowlder belts and feebly developed morainic tracts lying outside the Valpa- 

 raiso morainic system in Kane, Kendall, Grundy, and Kankakee counties. 

 These belts can be traced into a reasonably close connection with the 

 southern end of the undulatory belt in southern Kane County. There 

 seems to be, therefore, no formidable gap to bridge in making this corre- 

 lation. The greatest obstacle to the interpretation appears to be found in 

 the abrupt change in bulk which the moraine presents in the district east 

 of Elburn. From this point southward a thickness of only 20 or 25 feet is 

 presented by the moraine, where best developed, while to the north the 

 thickness averages more than 100 feet. The expression also is much 

 stronger north than it is south from this line. 



Perhaps by combining these interpretations a solution may be found. 

 The great bulk of the portion north from Elburn and the very strong 

 expression of the portion immediately northeast of Elburn may be a result 

 of the invasion which formed the Bloomington morainic system. At the 

 late Wisconsin invasion this region may have been partially overridden by 

 the ice, but without greatly modifying its appearance and without deposit- 

 ing a greater amount of drift than is found in the gently undulatory belt in 

 southern Kane County and the feeble moraines in the district to the south. 

 In this case the gently undulating tract in southern Kane County ma}- be 

 largely the result of the late Wisconsin invasion, though the interpretation 



