GLACIAL DRIFT. I79 



by the drift have been described. Clay slate is very readily broken ; and 

 the simplest cases of fracture are those noticed in that rock, as in the 

 quarries at Guilford and Northfield, Vt. Sometimes the fractures have 

 been produced by the expansive force of freezing. Water, penetrating 

 the seams, freezes, and thus, by expansion, wedges apart considerable 

 masses. These, if on a precipice, fall to the base, and accumulate in 

 large amount. Nearly every precipice in the state exhibits more or less 

 of this work. It will be seen, also, in the flumes about the White Moun- 

 tains, and has been described in Volume II, page 158. 



The frontispiece of Volume I exhibits the condition of the ledges in 

 a certain stage of decomposition, between the upper limit of trees and 

 the snow line. Doubtless many square miles of surface were thus cov- 

 vered by angular blocks before the ice-movement commenced. 



The most striking evidence of the action of ice breaking ledges is 

 where the stone has been fractured by a lateral thrust. These are com- 

 monly seen about large quarries. A single example will suffice for many 

 that have been observed. Fig. 49 is 

 a sketch of this phenomenon at the 

 Amoskeag granite quarry in Manches- 

 ter. It is upon the north-west slope 

 of a hill, in a position where it could 



be struck by the ice descending the Fig. 49. 



Merrimack valley. The surface is smoothed and striated. For eight 

 or ten feet in depth the granite is broken into irregular pieces, so that it 

 cannot be used for underpinning. Beneath these pieces, the stone is 

 sound at least to the depth of thirty feet, or as far as the excavation has 

 proceeded. One can readily see the line between the broken and sound 

 stone upon the east side of the quarry. That the fracture was occasioned 

 by the ice-movement is proved by the fact of the smoothness of the sur- 

 face fragments. The pieces can be taken out and matched together 

 again, just like a pavement. Had they been fragments prior to the ice- 

 action, they would have been moved away by it. The same phenome- 

 non occurs, though less noticeably, at Bodwell's quarry, close by. If 

 the ice had reappeared in the Merrimack valley, these broken frag- 

 ments would have been removed, and a new smoothed and striated sur- 

 face would have been planed upon the solid granite. 



