GLACIAL GEOLOGY OF THE COHOES QUADRANGLE 25 



The coarse sands and gravels making up the middle layers of 

 the delta formation and rising well toward its surface are inter- 

 preted as the fore-set beds. They are the coarser sediments w'hich 

 were deposited at first at the head of the delta and then successively 

 added to, thus building up the delta progressively toward the body 

 of the lake. 



A section of the lower and miiddle beds of the delta, together 

 with the underlying till, is shown on the right bank of the river 

 about I mile below the gorge west of Schaghticoke. At the base 

 there is a thickness of about 40 feet of materials of bluish color, 

 mixed composition, including boulders, and without evident strati- 

 fication. Above this and somewhat sharply defined from it, though 

 without evident unconformity, are beds of yellow sands with 

 approximately horizontal stratification, as seen in this section. 

 There is an exposure of perhaps 30 feet of these sands as seen in 

 nearly vertical section, and at a higher level the sands are con- 

 tinued on a sloping surface. 



The elevation of the highest terrace plain of the delta is about 

 360 feet. The extensive plain northwest of Schaghticoke, bisected 

 by the ravine, is 340 feet at its inner margin and rises to 360 feet 

 at its outer margin. Southwest of Schaghticoke there is a narrow 

 but distinct plain (crossed by the road to Schaghticoke hill) which 

 is at 360 feet elevation. It is believed that these plains represent 

 the level of the waters of Lake iVlbany. The former has an areal 

 extent of several square miles and it is scarcely open to question 

 that its materials were laid down below or at the level of the lake 

 waters. It evidently consists of the top-set beds of the delta which 

 were spread out horizontally over the fore-set beds. It will be 

 noted that this elevation is considerably above that of the general 

 or average elevation of the upper terrace of the body of deposits 

 in the lake north and south of the area of the delta. 



We have now to consider the sand and gravel deposits which 

 border the Hoosic valley for several miles eastward from Schaghti- 

 coke. At East Schaghticoke these deposits are 380 feet in eleva- 

 tion and the village of Valley Falls, one and one-half miles to the 

 east, is built on a river terrace of the same elevation. Also on the 

 north side of the river and farther to the east there is a distinct 

 terrace of sand and fine gravel. This terrace is sharply distinguish- 

 able, both in regard to materials of composition and topographic 

 form from the hill of till adjoining it to the east and which formed 

 a part of the dam of glacial debris behind which were ponded 

 the waters which formed the glacial lake north of Johnsonville. 



