GLACIAL GEOLOGY OF THE COHOES QUADRANGLE I5 



that in places accumulations of materials from the melting of the 

 ice lobe at its margins gave rise to lateral morainic deposits, thus 

 forming these ridges. 



Glacial lakes. In the middle eastern and southeastern rec- 

 tangular divisions of the Cohoes sheet there are two elongated 

 tracts conspicuous by their flatness as contrasted with the highl} 

 irregular surface of the uplands country surrounding them. The 

 more northerly of these tracts, or intervales, extending from near 

 Raymertown northwesterly and having a length of about 5 miles 

 and an average breadth of about one-half of a mile, has -recently 

 been converted into a reservoir for the public water supply of the 

 city of Troy. This was accomplished by constructing a dam across 

 Tomhannock creek at the place where this stream, after following 

 in meandering course the length of the flattened area, entered a 

 gorge about i mile south of East Schaghticoke. 



Lake Tomhannock. There is quite conclusive evidence that this 

 area, now an artificial lake, was in early postglacial times a natural 

 lake. The stream that flows past the village of Tomhannock built 

 a delta in this postglacial lake which now shows quite perfectly as 

 a sand and gravel bench or terrace at the 400-fo'Ot level and border- 

 ing that arm of the artificial lake which extends northeasterly 

 toward Tomhannock. The materials of the terrace are well 

 exposed in road gradings and show horizontal stratification. 



It is believed that this glacial lake (which may be named Lake 

 Tomhannock) originated through the gathering of waters in an 

 old stream valley across the course of which a dam was formed by 

 deposits from the ice sheet. This preglacial stream flowed north- 

 westerly from near Raymertown and then southwesterly toward 

 Melrose and the glacial dam was formed in the latter portion of 

 the stream course and near where at present the divide at the 400- 

 foot level occurs. For a time the waters of the glacial lake, held 

 back by the ice front, overflowed the dam and the lake had its out- 

 let in the stream that flows southwesterly past Melrose to the 

 Hudson river. 



When the ice front had retreated as far north as the plain south 

 of East Schaghticoke, a lower outlet for Lake Tomhannock was 

 afforded in the line of its present course. The outflow stream thus 

 established degraded its bed and eventually the waters of the lake 

 were drained off. 



The other intervale, farther to the south, also marks an extinct 

 glacial lake. This tract continues southward on the Troy sheet and 



