GLACIAL DRIFT. 27/ 



repose, not unstable like the rocks in the lower till. There may be 

 twenty other examples of large stones of similar origin in the 1,200 feet 

 of excavation passed through north of the ice drift mentioned. The con- 

 ditions involved by the facts seem to be the presence of thick masses of 

 ice upon the sides of Mt. Tom sliding down like glaciers, and carrying 

 detritus, together with the existence of very much water, — possibly a 

 glacial lake almost as high as the Crawford house, — into which the bergs 

 loaded with stone floated, after being dissevered from their source, and 

 dropped their burdens. The waters continuing to flow, layers of sand 

 and other debris covered up the dropped fragments until the supply 

 ceased. As these deposits are situated upon the surface of the ground, 

 they belong to the close of the ice period, and must have been formed 

 by local glaciation. If the railroad be followed to Fabyan's, three or four 

 miles further, there will be seen a constant repetition of phenomena sim- 

 ilar to those just described. The same is true of the excavations in the 

 White Mountains Railroad below the White Mountain house, and be- 

 tween Fabyan's and Ammonoosuc. A cut through "Winding hill," near 

 the "Base" on the latter route, shows coarse boulders, probably glaciated 

 at the bottom, capped by stratified layers somewhat ferruginous, suggest- 

 ing the lower and upper tills of other parts of the state. 



The sand deposits east of Fabyan's have large stones resting upon 

 them, obviously brought there at the same time and in a similar manner 

 with those just described. The valley of the South Branch also exhibits 

 fine examples of kames, which seem to have been contemporaneous with 

 the sand, and produced by the same glacial currents. 



Other examples of boulders lying in or upon sand have been mentioned, 

 upon pages 89, 105, 107, 117, 162-163, etc. Notable instances are also 

 just below the Glen house, the Bartlett boulder, at North Lisbon on the 

 new Franconia road, and east of Rock Rimmon in Manchester. 



Shapes of Glaciated Boulders. 



By far the most common shape of glaciated boulders is that of a 

 rounded trapezoidal prism, whose longer sides do not vary much from 

 parallelism to each other. One such is figured in the heliotype showing 

 boulders from Hanover. In this the ends are rather sharper than is 

 common. Perhaps half the thoroughly glaciated stones have such a 

 vol. III. 36 



