GLACIAL GEOLOGY OF THE COHOES QUADRANGLE 35 



is a depression of surface between this hill and the clay bluff which 

 bounds the lower lacustrine terrace. 



The interpretation placed upon these features is as follows: In 

 the course of the lowering of the Lake Albany waters to the level 

 marked by the base of the bluff fronting the lower terrace, the 

 character of this body of waters had gradually become changed 

 from lacustrine to fluviatile. Strong currents from the north flowed 

 in the channel formed by the shrunken bed of the lake. . At Bemis 

 Heights these currents sweep southwestward along the clay bluff; 

 subsequently the line of strongest currents shifted easterly, denuding 

 the broad tract now covered with residual soils, and finally occupy- 

 ing the present channel of the Hudson. 



In a number of places farther to the south there is like evidence 

 of the erosive work of strong currents which flowed over the floor 

 of the Hudson valley. The rock surfaces thus laid bare, are now 

 covered mainly by residual clays. The attempt has been made to 

 map these areas, but in some places there are no clear delimitations 

 and the boundaries as shown on the map are to be taken as 

 approximate. 



Areas of bared rocks. The areas of expensed rock surface on 

 the floor of the Hudson valley and the two tributary valleys from 

 the west are not sharply separable from the areas of residual clays 

 described above. Both were originally swept bare by the same 

 flood currents and it is probable that, excepting the rock islands 

 of the Hudson river and other small exposures adjacent to the 

 present streams, much of the area now bare was formerly cov- 

 ered with residual clays. The larger towns of the valley, Cohoes, 

 Lansingburg, Waterford and Mechanicville, are built wholly or in 

 part on rock and it is evident that human agencies have played a 

 part in denuding these areas. 



Postglacial gorges. The Mohawk river from' near Crescent 

 to its mouth has its bed on rock and in the last 3 miles of its course 

 occupies a rock gorge marked by the well-known falls at Cohoes. 

 A dam has been built across the upper portion of the gorge, in 

 order to supply power for the industries of Cohoes. The level of 

 the water in the dam is 156 feet. From the dam to the falls the 

 river has the character of a rapids descending 20 feet in the dis- 

 tance of three-fourths of a mile. It then falls 70 feet over a preci- 

 pice of rock. The water does not descend abruptly as a vertical 

 sheet but flows over the steeply inclined rock declivity. The slope 

 of this declivity corresponds in general to the dip of the rocks but 



