lO NEW YORK STATE MUSEUM 



but flows in a shallow trench cut chiefly in glacial deposits, with 

 rock seldom showing, and with rapids in the stream wherever it' is 

 on bedrock. 



The St Lawrence, in its course across the Brier Hill sheet, seems 

 to be occupying an old valley ; at least there is a well-defined valley 

 rock wall on the New York side. But below Ogdensburg it has the 

 same characteristics as the lower Oswegatchie, no well-defined 

 valley, low banks of drift with no rock showing, and rapids in the 

 river whenever it discovers rock ledges in its bed, as at the Galop 

 rapids. 



GLACIAL DEPOSITS 



All the surface of the district is below the levels of the bodies 

 of standing water which existed in the region during the closing 

 stages of the Glacial Period, and practically all of it is beneath the 

 level of the marine waters which invaded the St Lawrence valley 

 when the final melting away of the ice sheet cleared a way for 

 their passage. On the Brier Hill sheet the wave action of these 

 water bodies swept away most of the glacial deposits from the higher 

 rock surfaces and into the old depressions, which are deeply filled 

 with them. The low grounds which comprise all the north half of 

 the Ogdensburg are so drift-covered that rock outcrops are exceed- 

 ingly scarce, but the drift has comparatively little surface relief, 

 was probably all laid down under standing water, and has since all 

 been wave-washed. 



A well-defined morainic belt extends across the Ogdensburg and 

 Red Mills sheets, parallel to the river. It has a breadth of several 

 miles and seems to be broadening eastward, with the widening of 

 the Paleozoic plain. But the moraine has a very subdued relief, 

 the knobs being low and of gentle slope, with seldom a relief of 

 as much as 40 feet above the low grounds. Nor does the material 

 appear to be particularly thick; we doubt if it would average 40 

 feet thick. 



This moraine does not extend far west on to the Brier Hill sheet, 

 on which rock exposures abound and drift is heavy only in the pre- 

 glacial valleys. On the south half of the Ogdensburg sheet, also, 

 there is no widely spread drift cover. The rock relief is greater 

 there than to the north and the drift is solely in the depressions. 

 The depressions are often marshy. 



The morainic belt is, to a considerable extent, drowned beneath 

 the clay plain formed by deposit in the standing waters, so that the 



