THE PLEISTOCENE OR GLACIAL PERIOD. 



341 



seen on those which have lain long at the surface, and they are much 

 more common on the less resistant sorts of rock, such as limestone, 

 than on more resistant ones, such as quartzite. Locally, distinctly 

 striated stones are rare even in the unstratified drift, and they are 

 generally rare on the rock fragments of the stratified drift. 



No depositing agent except glaciers habitually marks the stones 



Fig. 475. — A perched bowlder of Triassic sandstone on the trap-rock of the Palisade 

 ridge east of Englewood, N. J. Size 12X8X8 feet. This bowlder was probably 

 carried up by the ice something like 200 feet. (N. J. Ceol. Surv.) 



which it deposits in this way. Bowlders dropped by icebergs some- 

 times have such markings, but icebergs are born of glaciers, and the 

 marks of the striated stones of icebergs were put upon them while 

 they were still in, or under, the land ice. Water never striates stones 

 in this way. 



(3) Structure of the drift. — The larger part of the drift is unstrati- 

 fied, but a very considerable part is stratified, often irregularly. The 

 unstratified drift or till (for some of it the name bowlder-clay is appro- 



