6 NEW YORK STATE MUSEUM 



water forming temporary lakes, at the close of the ice age, are of 

 quite different character from the unassorted materials left from 

 the melting ice. Some of the glacial deposits are also of economic 

 importance in other ways, especially the clays extensively used for 

 making bricks and the sands for building and molding purposes. 



It may be noted that in other ways the industrial life of the 

 people is dependent upon factors and conditions resulting from 

 the changes wrought upon the country during the Ice Age. The 

 largest city, Cohoes, owes its growth as a manufacturing center to 

 the source of power afforded by the falls in the postglacial gorge 

 of the Mohawk near its mouth. The villages of Schaghticoke and 

 Valley Falls are similarly related in location and industry to the 

 rapids of tlie Hoosic river where in its lower course it has carved 

 a channel in rock, since the close of the glacial period. A' further 

 instance of the relation of human interests to conditions deter- 

 mined by glacial agencies is that of the restoration of an extinct 

 glacial lake — Lake Tonihannock, on the Cohoes quadrangle — in 

 order to form a storage reservoir for the water supply of the city 

 of Troy. 



The history of the drainage of the area, especially of the streams 

 tributary to the Hudson, as deduced from the facts gathered in the 

 present work, is of exceptional interest and throws light on some 

 of the larger problems of the postglacial history of the Hudson- 

 Champlain valley. The data bearing on these questions and a dis- 

 cussion of them are given in the body of this report. 



PHYSICAL GEOGRAPHY AND GENERAL GEOLOGY 



The Cohoes quadrangle is intersected by the Hudson river, which 

 enters the area at about the middle of the northern border (latitude 

 43°) and flows southwestward and southward crossing the southern 

 border (latitude 42° 45') about i mile above the head of navigation 

 of the river at Troy. The elevation of the river at the northern 

 margin of the sheet falls between the 80 and 100 foot contour lines 

 and at the southern border is less than 20 feet above sea level. The 

 segment of the Hudson river here included belongs, therefore, to 

 the upper or what may be termed the river portion proper, in dis- 

 tinction to the lower, or estuarine portion, which is within the 

 influence of the ocean tidal movements. 



In considering the geology of this portion of the Hudson valley 

 it is helpful to distinguish at the outset between the preglacial 

 valley which is now largely filled with sand and clay deposits of 

 Pleistocene age and the present valley which belongs to the recent 



