GEOLOGY OF OGDEXSBURG 57 



Hill and again returns with increased prominence east of Ogdens- 

 burg, running at least as far as the Raquette river. How much 

 farther down the St Lawrence valley it may go, is today unknown. 

 The formation is not known in the Champlain valley, though it 

 may exist there. But it is now certain that the Tribes Hill waters 

 bordered the Adirondacks on the south, west and north sides. 



At the close of the Tribes Hill there was oscillation of the district, 

 bringing the whole above sea level following which the Champlain 

 valley region alone was depressed, and deposition of Beekmantown 

 divisions B and C began there. As Beekmantown time went on this 

 Champlain depression began also to involve the St Lawrence trough, 

 extending westerly up that trough until the Ogdensburg region was 

 reached, and deposit of the Ogdensburg formation, division D, 

 began. This depression no doubt continued through the remainder 

 of Beekmantown time, the Champlain and St Lawrence valleys 

 below, the Black and Mohawk valleys above sea level, all Beekman- 

 town formations other than the Tribes Hill being absent on the 

 south and west sides of the Adirondacks. At the close of the 

 Beekmantown the sea was withdrawn from the whole region. 



There is no direct evidence of the deposit of any of the later 

 Ordovician formations in the Ogdensburg region, but it is highly 

 probable that the thinned edges of several of them were laid down 

 here. The Pamelia formation of the Theresa region may have come 

 as far east as this ; outcrops of the Ottawa Chazy come in above 

 the Beekmantown not a great many miles down the St Lawrence 

 on the Canada side, and may well have reached farther west. Some 

 of the various Black River and Trenton formations may likewise 

 have been laid down, and this is also possible in regard to the upper 

 Ordovician shales. But the Frontenac axis was early outlined as 

 a barrier between the St Lawrence and Black river troughs of 

 deposit, and it is not believed that anv of these formations could 

 have had any large thickness in the Ogdensburg res^ion. So far 

 as now known there is not a scrap of reason for believing that any 

 Paleozoic marine rocks younger than the Ordovician were ever laid 

 down here. 



LATER PALEOZOIC HISTORY 



During Silurian, Devonian and Carboniferous times the region 

 no doubt experienced oscillations of level, as during its previous 

 history. But the downward movements do not seem to have carried 

 it below sea level. It seems to have persisted as a low altitude 

 area, so low^ that erosion of its surface Avas but slight, but not low 



