﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 23 



During the remainder of the Paleozoic we know but little concern- 

 ing the region here, except by comparison with other regions more 

 or less remote from it. It may have been somewhat submerged dur- 

 ing the Siluric, but certainly, for most of the time, it was a land 

 area, and the small amount of wear which it experienced indicates 

 that, for most of the time, its altitude was low. 



Amount of erosion 



The total amount of rock thickness which has been worn away 

 since the region became a land area, can not of course be exactly 

 determined, though it is thought that it can be approximated. To 

 the south the Trenton limestone is overlaid by the Utica and Lor- 

 raine shales, and these by the Oswego sandstone and Medina shale 

 and sandstone. These are all sufficiently near to make it in high 

 degree probable that they were laid down over our district, espe- 

 cially since the source of their sediment must have been to the north 

 and east. It is regarded as unlikely that they had any greater thick- 

 ness here than they now show toward the south, but they may have 

 been as thick. We have no evidence that any formations later than 

 the Medina were ever deposited here, and even if so, the thickness 

 would seem to have been small and the submergence brief. If 

 therefore we allow to these formations the full thickness which they 

 show to the south, we are likely exaggerating their thickness here 

 and allowing a margin to account for any possible later formations 

 which may have existed. 



The deep wells which have been drilled at various points between 

 this district and the Syracuse region, give the data desired. In the 

 Monroe well at Baldwinsville the drill went through 1740 feet of 

 sandstone (Medina-Oswego) and shale (Lorraine-Utica), reaching 

 the top of the Trenton at 2240 feet. If we assume them to have 

 been deposited over our district in the same thickness, and add the 

 thickness of underlying rock (Potsdam-Trenton) we get 2600 feet 

 as an outside measurement of the Paleozoic thickness here origi- 

 nally. In all probability this is considerably too high. There were 

 1200 feet of sandstone and 500 feet of shale above the Trenton in 

 this well, and the full thickness of both was passed through by the 

 drill. In the wells further north, as in Orwell and Central Square, 

 less sandstone appears but the shales thicken to 700 feet. Since 

 no certainty is possible our purpose is best subserved by a generous 

 estimate, and an original thickness of 3000 feet of Paleozoic rocks 

 here will be assumed. Where Precambric rocks are now at the 

 surface, 3000 feet is regarded as the outside limit of the thickness 



