35 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 init. 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. Winchell was referred to in Volume I. 
He describes very graphically the manner in which the Bowlder clay ac- 
cumulated on the surface of 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 
