GLACIAL GEOLOGY OF THE COHOES QUADRANGLE 7 



period, that is, has been formed since the time of the subsidence 

 of the body of glacial waters in which the sands and clays were 

 deposited. The preglacial valley is cut in rock and is somewhat 

 complex in form, consisting of a broad and open outer portion and 

 an inner portion with steeper slopes. The whole, as a physiographic 



Fig. I Diagram showing preglacial topography of the Hudson valley 



feature, may be described as a gorge within a valley. This ancient 

 rock valley represents the erosive work of the Hudson river in the 

 Cenozoic (Post-Cretacic) era of geologic time and its double form 

 is considered to be the result of an uplift of the general region fol- 

 lowing an earlier period of valley erosion. Its form and dimen- 

 sions are now evidenced by outcrops of rock that occur near the 

 lateral border of the clay and sand area and by the depths of the 

 ravines which have been cut into the filling of the ancient rock 

 valley and in places into the underlying bedrock. From these data 

 it is shown that the ancient valley has an average breadth of 

 between 4 and 5 miles and a depth in its middle portion of 200 

 feet or more. The width of the inner valley or gorge is on the 

 average about 2 miles : its depth, of course, is the same as that of 

 the broader valley. 



The present valley of the Hudson, as here considered, is the 

 broad depression on the bottom of which the river flows in a more 

 or less winding course and the sides of which are the steep clay 

 banks which rise 100 feet or more above the valley bottom. The 

 present valley lies within the old rock gorge, its bottom being coin- 

 cident with the middle portion of the floor of the latter. The 

 present channel of the river, however, in the greater part of its 

 extent, is cut into the floor of the old gorge, forming a shallow 

 rock gorge representing the erosive work of the river in the recent 

 period. The present valley bottom, threaded by the channel, has a 

 width varying from three-fourths of a mile to one and one-half 

 miles. 



Where the river enters the quadrangle and for several miles 

 southward this bottom is an alluvial plain, but from Stillwater to 



