GLACIAL GEOLOGY OF THE COHOES QUADRANGLE 2,^ 



the rock beds and partly by the slope of the old valley floor, the 

 river eroded its present channel. Where it passed over the rim 

 of the narrower inner portion, or gorge (see page 7), of the 

 ancient rock valley, rapids were formed and, owing to the steep 

 dip of the rocks, the rapids developed into a waterfall. This was 

 the origin of the Cohoes falls. 



The height of the rock wall of the gorge below the falls as seen 

 on the east side is 120 feet. About three-eighths of a mile below 

 the falls this wall falls away to a height of 40 feet. The contours 

 of the upper portion of the wall, representing 80 feet, here become 

 continuous with the slope extending northeasterly and bounding 

 the present valley floor of the Hudson. This slope (crossed by 

 the road a short distance back from the summit of the gorge) is 

 composed of rock covered with residual clays. The continuation 

 of this escarpment is seen on the opposite side of the gorge though 

 much reduced in height and angle of slope for some distance back 

 from the river. This is due to the flooded Mohawk waters of late 

 glacial times, which laid bare the rocks on which the lower 

 (northern) portion of the city of Cohoes is built. The trend of 

 the currents southward due to their confluence with the Hudson 

 waters, caused a greater reduction of the scarp immediately south 

 of the gorge. But farther to the south, west of the railway tracks, 

 where the surface of the rock is exposed in a ravine, the scarp 

 shows at its full height. 



It is quite evident that this slope of rock is the rim of the inner 

 portion of the ancient rock valley and that the place where the 

 height of the wall of the gorge below the falls abruptly lessens 

 marks the original location of the falls of the river. The present 

 Cohoes falls is located approximately 2000 feet back from this 

 landmark, showing that a recession of the falls has taken place 

 to that extent in postglacial times and subsequent to the with- 

 drawal of the Lake Albany waters. 



This small amount of recession compared with other postglacial 

 falls (Niagara Falls has receded 7 miles in postglacial times) is 

 explained, at least as to the main factors concerned, by the 

 materials and structure of the rocks through which the gorge has 

 been cut. The rock consists of indurated shales (slates) and sand- 

 stones, dipping steeply in a general direction to the east. The 

 amount and direction of dip varies locally, due to contortions of 

 the strata resulting from compression. In general, the sandstone 

 layers predominate in the rock and layers of a thickness of from 



