34 NEW YORK STATE MUSEUM 



area is therefore essentially preg^acial and represents, in fact, a 

 portion of the ancient rock valley of the Hudson stripped of its 

 Pleistocene covering and somewhat modified by postglacial erosion 

 and weathering. 



The same area shows in its minor surface features the action of 

 strong currents laden with fragments of hard rock as cutting tools. 

 There are numerous depressions marking ancient potholes, now 

 partially or wholly filled with the residual clays. Some of these are 

 exposed where the clay is thin or has been removed ; for example, 

 on the left bank of the river near the falls. ^ 



The surface materials of this area consist mainly of residual 

 clays derived from the underlying shale (or slate) rocks. The 

 section of the state barge canal which extends from Waterford 

 to the Mohawk, at a point about i mile above Cohoes falls, crosses 

 this area and is everywhere cut in bedrock. The layer of residual 

 soil overlying rock is thus clearly exposed. Along the gorge of 

 the Mohawk the weathered rock surface, forming a dark-colored 

 clay soil, is also well shown. The thickness of the clay mantle is 

 nowhere great, varying from a few inches to perhaps 5 or 6 feet. 

 These clays have certainly originated as the products of rock- 

 weathering since the end of the epoch of flooded Mohawk waters. 



While the covering of this area consists predominantly of residual 

 clays, there occur in places materials of the glacial till which escaped 

 removal by flood erosion. These are scattered boulders and cob- 

 bles ; also, in places, especially in the eastern portion of the area, 

 some of the less coarse materials of the till still persist in the cov- 

 ering of the bedrock. 



Areas of residual clays also occur on the floor of the Hudson 

 valley. The largest of these forms a broad tract lying west of the 

 curve of the river between Bemis Heights and Stillwater and 

 extending in narrower development southward to near ]\Iechanic- 

 ville. This area includes many patches of outcropping or thinly 

 covered rock, the distribution of which corresponds in general with 

 the highest parts of the surface. In that portion of the area which 

 lies east of the abandoned canal (which follows a line of drainage 

 depression) the parts of highest elevation are shown by the looped 

 contours marking the loo-fcct level. But vv^est of the road that 

 runs northerly from Stillwater thei'e is an elongated hill of rock 

 with thin covering of detritus which is 160 feet in elevation. There 



* For a full description of these potholes with measurements, see paper 

 by G. K. Gilbert (Deposition of the Mastodon at Cohoes), N. Y. State 

 Cabinet Nat. Hist., 21st Annual Report, 1871. p. 129-48. 



