GEOLOGY OF OGDENSBURG 9 



GENERAL TOPOGRAPHY 



The mean level of Lake Ontario and of the St Lawrence through 

 the Thousand Islands is 246 feet above tide. At Ogdensburg it is 

 only 2 feet lower than this, but on the 12 miles of river across 

 the Red Mills quadrangle there is a rapid drop at the Galop rapids, 

 from 244 feet above the rapids to 228 feet at Rockaway point, 16 

 feet in 8 miles. The levels of the river are the lowest levels in the 

 area mapped. 



The highest land on the Ogdensburg quadrangle is around Dekalb 

 Junction in the extreme southeast corner where an elevation of 470 

 feet is reached. This gives for the quadrangle an extreme of 

 relief of only 225 feet, a very small amount; and the bulk of the 

 quadrangle lies between 300 and 400 feet altitude. On the Brier 

 Hill quadrangle it is even less, since 400 feet is exceeded in only 

 two places, and 440 feet is the highest point, only 200 feet above 

 the river. 



The minor relief on the Brier Hill sheet is considerable, much 

 of the surface consisting of slopes ; there are many rock exposures, 

 and hence very little guess work is necessary in working out the 

 geology. But the entire northern half of the Ogdensburg quad- 

 rangle consists of a plain with little relief, and so covered with 

 glacial drift that rock exposures are very infrequent, and the map- 

 ping of formation boundaries is a very uncertain matter. 



Drainage. The mapped area drains entirely into the St 

 Lawrence, the chief stream being the Oswegatchie river, which is 

 one of the five good-sized streams which drain out from the Adiron- 

 dacks to the northwest, the others being the Black, Grass, Raquette 

 and St Regis. Five miles south of Ogdensburg, Black lake, which 

 is nothing but an expansion of the waters of the Indian river into 

 a lake, empties into the Oswegatchie. The Indian river is a con- 

 siderable stream, but not comparable to the others in size. 



The general slope of the district is to the northwest, toward the 

 St Lawrence. This is, however, at right angles to the geologic 

 grain of the country which trends northeast. Most of the preglacial 

 valleys of the district have this northeast trend. Black lake lies 

 in a preglacial valley; and the adjacent valleys of Fish creek and 

 Beaver creek have the same trend and are small preglacial valleys. 

 South of Rensselaer Falls the'Oswegatschie is now occupying the 

 preglacial valley of a small stream ; but below that point its course 

 seems entirely postglacial, and it has no valley worthy of the name, 



