FOLDING OF SUBJACENT STRATA BY GLACIAL ACTION 229 



and the thinning of the till sheet over the axis of the fold, are thought 

 to indicate that the fold was made under restraining pressure from 

 on top. Since the folding effects the glacial till (G), and the gravel 

 (/), is thinned over it, it was evidently made later than the time 

 when the till was deposited, and perhaps later than the deposition 

 of the gravel. A glacier has undoubtedly crept over this place 



Fig. 2. — Profile of a fold in stratified rocks under glacial deposits. 



within this period, as proved by its deposits, although these are 

 absent from this particular spot, presumably because the glacial 

 river Mississippi has cut away the upper sheet of till. Other sec- 

 tions in the same region show the till of the later glacier. It may 

 be assumed, therefore, that glacial ice moving over the till bed, 

 and perhaps over the gravel bed (/) gave the thrust that upfolded 

 the limestones and shales at this point. 



How the glacial ice could do such heavy crushing of crystalline 

 limestones may perhaps be explained by the friction of the glacier 

 upon the field of gravel, which for a considerable area could accu- 

 mulate great stress and transmit it to the subjacent strata suffi- 

 ciently to cause the upper stratified rocks {S 3-5) to glide along a 

 clay seam {S 2), and thus converge the power of a large portion 

 of the glacier upon a small part of the rock strata. This hypoth- 

 esis is the more plausible since in some places in this region the 

 gravel bed rests immediately on the stratified rocks. 



