54 OSCAR H. HERSHEY 



supply of calcareous material. A few sections show it inter- 

 stratified with till, and it can sometimes be traced back to its 

 original undisturbed position/ Having been transported but a 

 score or at most a few hundreds of feet, there is no room for 

 anything but subglacial action. 



We have disposed for the present of all the loose material on 

 the surface of the solid rock, and we now come to the most inter- 

 esting part of the process. 



4. Stephenson county is a comparatively hilly region, 

 although the hills are not high, steep, or close together. A sec- 

 tion throusfh a hill would show the Galena limestone as a series 

 of practically horizontal layers, not cemented, but held in posi- 

 tion by gravity and the projections on the upper and under sur- 

 faces of the layers. The ice in moving westwardly across the 

 country, upon ascending the eastern slope of a hill from which 

 the loose material had been removed, exerted a powerful pres- 

 sure on the edges of these layers. Near the top of the hill, the 

 pressure overcame the friction, and the upper layers of the rock 

 were pushed forward, sliding on a lower unmoving stratum. In 

 many cases, these transported rock ledges were not broken up, 

 but moved forward as a solid mass, being found in that condi- 

 tion to day. They may be ten or fifteen feet in thickness, and 

 several hundred feet in length, and perfectly horizontal, so that 

 the fact of their not being insitu^owXdi not be known were it not 

 for their unusual position, partially obstructing valleys, produc- 

 ing a topography radically different from the preglacial ; and 

 from their overlying drift of various kinds, including stratified 

 water-worn gravel ; and more often than otherwise underlain by 

 loose angular gravel from ten to thirty feet in thickness.^ In 

 transportation they were clearly pushed in front of or under the 

 extreme marginal portions of the ice. None of them have been 

 removed far from their original position, and this with much 



' Small exposures of it, after being disturbed but befoi'e being combined with the 

 red clay, may be seen at nearly every outcrop of the latter, notably in the railway cut- 

 tings of the southeastern part of the county. 



^ Locality where best exposed — six miles east southeast, of Freeport, in a high 

 ridge, on the north side of the I. C. R. R. 



