33 GEOLOGY OF OHIO. 



Many instances are reported of pavements of stones embedded in the 

 Bowlder clay which are striated in a common direction, but which, when 

 lifted from their beds, are found to be worn or scratched perhaps on sev- 

 eral sides, showing that they have been previously transported in the 

 Till. These do not necessarily mean a second ice period, but are proba- 

 bly the result of the temporary advance of the glacier when the edge 

 over-rode the embankment which had been piled up before it, and wear- 

 ing away the softer portion above, formed a bed for itself on the tougher 

 mass below and the stones impacted in it. The evidence that these pave- 

 ments do not mean a second ice period seems to be furnished by their 

 being so local and by the homogeneity of the mass of Till above and 

 below them. 



Those who can not accept the view presented in our former article — 

 that the Bowlder clay accumulated along the margin of the glacier — are 

 offered the choice of two other theories, one of which they must accept : 

 first, that the Bowlder clay formed under the glacier ; second, that it 

 was formed on the glacier, and by its melting was let down into its pres- 

 ent position. 



The considerations which oppose the acceptance of the first of these 

 theories have been already alluded to. The peculiar character of the 

 erosion produced by the glacier proves that it was accurately moulded to 

 the surface over which it moved, and that the grinding it effected was 

 done by sand and stones impacted in it ; that it was, in fact, a sort of 

 great emery wheel. We sometimes see acres of rock surface not only 

 ground to a plane but polished, and sections containing fossils as nicely 

 cut as they could be done by hand. That there could have been any con- 

 siderable thickness of clay under the ice when this process was going on 

 is simply impossible. 



The theory proposed by Prof. N. H. Winch ell was referred to in Volume I. 

 He describes very graphically the manner in which the Bowlder clay ac- 

 cumulated on the surface 0/ the glacier, increasing in thickness towards its 

 edge and finally, by its melting, let down quietly on the bed rock. 

 However complete the picture Prof. Winchell has presented of the for- 

 mation of the Bowlder clay, it is necessary to say that it is impossible 

 that in the basin of the lakes or the Valley of the Mississippi it could have 

 had any counterpart in nature. No accumulations of stones and earth 

 take place on the top of glaciers except where peaks and cliffs of rock 

 overtop them. In all the country north of Ohio there are no such peaks 

 or cliffs, and this country during the glacial period was covered with a 



