62 OSCAR H. HERS HEY 



tendency of the extreme marginal portion to turn up, producing 

 a base of a form somewhat as in the accompanying figure. 



There was a struggle for the mastery between the advancing ice 

 and the accumulating debris. The incipient morainic or marginal 

 deposits of Stephenson county, including the transported rock 

 ledges, belong to the extreme outer border of the ice and were 

 transported least. Those which are most thoroughly kneaded 

 belong to a position at about b. The drumlin (?) area south of 

 Freeport may be placed at c, while much of the ground moraine 

 was formed and deposited still farther back under the ice. By 

 recession and repeated slight readvances of the ice-front, with a 

 partial reworking of old deposits, the apparently indiscriminate 

 distribution of the drift of this county may be accounted for. 

 When the ice melted away entirely, the englacial bowlders which 

 had become superglacial through ablation of the border portions 

 of the glacier, and are so represented in the figure, came to rest 

 upon the surface of all the other drift of the district and the 

 areas of nearly bare rock represented at a. 



If the advance was continued far, the ice overrode the coarse 

 angular gravel at ^, crushed it finer, brought up "semi-residuary" 

 till and a little foreign drift from farther back, and combined 

 them into typical till. This gradually fell behind the advancing 

 border of the ice, and was redeposited as ground moraine, 

 locally accumulating into drumlins and smoothly undulating 

 drift areas, while the extreme outer portion of the glacier was 

 forming a new deposit of angular local limestone debris. 



Oscar H. Hershey. 

 Freeport, III., 

 January 5, 1897. 



