GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK I41 



PHYSIOGRAPHY 



Since the West Point quadrangle lies almost wholly in the High- 

 lands, with crystalline rocks of various sorts making up its whole 

 substructure, it does not furnish such complete illustration of the 

 physiographic history cha.racterizing the region as would an 

 area containing a greater variety of rocks. The features are, of 

 course, intimately related to those of adjacent districts where pro- 

 nounced and critical physiographic features are represented. 



Here as elsewhere the fundamental factor in the production of 

 surface form is rock quality and geologic structure. It is, for 

 example, due to the crystalline condition and resistant character of 

 most of the rocks of this locality that the topography is so rugged 

 and the surface elevated so much above the surrounding region. 

 And ridgelike forms with their decided trend in a northeast-south- 

 west orientation for both ridges and valleys result from the rock 

 structure. It is also chiefly due to peculiarjities of rock structure and 

 lines of weakness caused by regional deformation that the valleys 

 are narrow and that so many of them are straight. 



Two structural conditions probably control nearly all the depres- 

 sions. The most general is the occurrence of crush zones following 

 fault lines; the other is the occurrence of belts of rock which are 

 naturally less resistant to weathering and erosion than is the average 

 country rock. The most pronounced of these latter are the lime- 

 stone belts and next to them is the occurrence of Grenville rocks 

 with their varied facies. It is very noticeable indeed that the only 

 broad valleys in the whole d,istrict are developed either on limestones 

 or on Grenville belts carrying occasional limestone bands 



Other topographic expressions are related to glacial influences. 

 Heavy deposits of glacial drift cover many depressions, and, in the 

 southeast quarter of the quadrangle, the drift entirely obscures the 

 rock floor topography and the detail of formational distribution and 

 structure. Glacial lakes are common because of the filling of 

 old outlets and the general obstruction of original drainage lines. 

 Some of the accumulations are morainic and others are modified 

 drift quite free from any other control than that of ice deposition. 



Another feature that is somewhat independent of structure is 

 related to the steps of physiographic history, represented by periods 

 of peneplanation and rejuvenation. Thus it happens that there are 

 plain traces of two attempts at base levelling and two or three periods 

 of rejuvenation. Perhaps still other minor steps are represented 

 which may account for some of the physiographic peculiarities. 



