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GEOLOGY OF THE FOURTH DISTRICT. 
Potholes—Effects of artificial dams. 
These evidences of running water exist in several places in the Fourth District; but in all 
that I have seen, they are near the margin of some stream, and in a situation easily accounted 
for by supposing the water at a higher elevation. In the rapids at Rochester, small potholes 
are formed in the limestone, and also in the sandstone and shales of Portage on the same 
river. In the bed of the river at the Rapids above Niagara falls, large and deep potholes are 
formed. These probably exist over the whole extent of the rapids, as they are visible in all 
accessible places. On the western side of Goat island, there are remains of some of these in 
situations higher than the water usually flows, proving therefore that the elevation is less, and 
that at the time they were made, the margin of the fall was farther north than at present. 
Potholes similar to these are frequently worn in rapids below artificial dams. Mr. Wilder, 
of Hoosick-falls, informs me, that on turning the water to repair a dam which had been 
standing twelve years, potholes of four feet in diameter, and several feet deep, were found in 
the sloping rock below. Dr. Ambler, of Ogdensburgh, informs me of still more important 
effects produced by the falling of water over dams. The dam at Brownville, Jefferson county, 
which was erected forty or fifty years since, in a narrow place in the river, bounded by per¬ 
pendicular walls of limestone, and flowing over a bed of the same, had a perpendicular fall of 
twelve or fifteen feet. From the time of its erection till 1841, when it was carried off by a 
freshet, the falling water, with all the fragments of rock, pebbles, etc. had been wearing upon 
the limestone bed of the river, and had produced a cavity or pothole estimated at fifty or sixty 
feet in depth. Sawmill logs of fourteen feet in length, passing over this dam, would plunge 
beneath the water, disappear for a considerable time, and on coming to the surface, would rise 
more than half their length above water. From this circumstance it was considered that the 
depth of sixty feet was not too great an estimate. The rock is of hard grey and black lime¬ 
stone of the Black river and Trenton limestone formations. 
In another case, of the dam at Ogdensburgh on the Oswegatchie, with a fall of twelve feet, 
the water has removed large slabs of limestone of a foot thick, and from ten to twenty feet 
in diameter, and carried them several rods down the stream. 
Neither are these isolated instances. I could extract pages from my note books, showing 
the modern effects of running and falling water, freezing, etc.; but examples enough have 
already been presented. 
