GLACIAL AND POST-GLACIAL HISTORY 



421 



The clay overlies till (Fig. 4), gravel and sand, or rock surfaces, 

 which are frequently striated. It fits into the irregularities of the till, 

 or the stratified gravel and sand which sometimes appears to have the 

 form of kames. In the upper Hudson the underlying stratified sand 

 and gravel often has a high angle of dip, generally southward, but 

 sometimes in other directions. These layers are interpreted as repre- 

 senting deposits made by the ice waters in the standing body of water 

 as the ice was retreatmg. This structure can frequently be seen from 

 the car windows of the New York Central Railroad. The marginal 



Fig. 3. — Ripple marks in the clay at New Windsor. 



deposits of gravel and sand have the form of plateaus of two distinct 

 classes : 



Class I. — Gravel terraces and plateaus with undulatory topography 

 on the edge toward the Hudson, which sometimes assumes a more or 

 less kame-like or morainic form, or with the edge next to the Hud- 

 son higher than the edge next to the valley wall, and with the dip 

 of the layers of gravel and sand toward the valley w^all and down- 

 stream. This phase of the drift is the characteristic phase in the 

 Highlands, and is not accompanied by a clay plain. 



Class 2. — The second phase of the stratified drift consists of 

 gravel plateaus and terraces with the undulatory edge toward the 



