GLACIAL GEOLOGY OF THE COHOES QUADRANGLE 9 



The segment of the Mohawk valley included in the southwestern 

 part of the sheet belongs to the portion of the river (from Aqueduct 

 near Schenectady, to its confluence with the Hudson) which occu- 

 pies a postglacial valley.^ The evidence of this is that the valley 

 of the river, for a distance of 5 miles east of Aqueduct,, is a gorge 

 cut into the rocks and extending like a trench across the face of 

 the country. There is no buried old valley, as in the case of the 

 Hudson, lying outside the present valley. These features, which 

 are. well marked in the upper portion of the preglacial valley, are 

 considerably modified in the lower portion. The course of the 

 stream becomes irregular, the valley broadens and its slopes are 

 less steep. These differences are due in part, especially in the 

 segment of the valley falling within the Cohoes sheet, to structural 

 features of the underlying rocks which here have their strata 

 steeply inclined, dipping to the east, and in part to the fact that the 

 Mohawk here enters the region of the old eroded rock valley of 

 the Hudson. The river has, however, lowered its bed into the floor 

 of the rock valley and in the last 3 miles of its course occupies a 

 rock gorge. This portion of the river is marked by the well-known 

 falls at Cohoes. 



On the north (northeast) side of the river, stretching back from 

 the summit of the gorge, there is an area, several square miles in 

 extent, consisting of rock with a thin covering of clayey soil. This 

 area has evidently been swept by the currents of the Mohawk when 

 the river flowed at a level about 80 feet higher than its present bed, 

 above the falls. The Pleistocene clays were removed and sub- 

 sequently the existing clays were formed as a residuum from the 

 weathering of the exposed rock surface. On the south side of the 

 gorge only a narrow strip of the Pleistocene clays was removed by 

 river erosion and the surface topography of the clay formation 

 exhibits terraced forms, normal to the Hudson valle}', as above 

 described. See figure 7, page 36. 



About three- fourths of a mile below the falls at Cohoes the 

 Mohawk waters divide into several divergent streams which enter 

 the Hudson by as many channels, thus forming a group of rock 

 islands. These multiple mouths of the Mohawk are interpreted as 

 originally delta distributaries which became intrenched in the under- 

 lying weak rocks after the removal of the delta deposits from their 

 beds by erosion. 



^Stoller. Glacial Geology of the Schenectady Quadrangle. N, Y, 

 State ^Tus. Bui. 154, p. 11, 



