﻿GEOLOGY OF THOUSAND ISLANDS REGION 1 25 



and there is little trace of the old consequent streams, though the 

 streams running westerly, out of the Adirondack^, seem to repre- 

 sent the old heads of such streams. 



Tertiary uplift 



Evidence derived chiefly from without the- district indicates that 

 our region, in common with much of eastern North America, was 

 worn down to a comparatively smooth surface (peneplain) of low 

 altitude by the close of Mesozoic time. It then experienced con- 

 siderable uplift, erosion was renewed and streams cut and widened 

 considerable valleys in the weaker rock belts, while the more 

 resistant rocks retained in considerable measure their original alti- 

 tude, and give us the remnants of the old plain. Elevations of over 

 1500 feet are found on the Watertown sheet, immediately south 

 of our map. On the Port Leyden sheet, next south, the altitudes 

 reach almost 2000 feet, the district there forming a low plateau, 

 capped by the resistant Oswego sandstone, between the Ontario low- 

 land to the west and the broad valley of the Black river to the 

 east. East of the valley the levels rise within a few miles to 2000 

 feet, in the westerly edge of the x^dirondack platform, and from 

 there continue to slowly rise eastward. The Adirondack highland, 

 and the Oswego sandstone plateau, are regarded as remnants of the 

 old peneplain surface, which as uplifted, was given a slight tilt 

 toward the west, while the deep valleys of the region have been cut 

 since the uplift and give some measure of its amount. Unless later 

 rocks in considerable thickness have been worn away from the 

 surface of the Oswego sandstone plateau, the amount of wear 

 there has been very slight ; yet this small thickness of removed rock 

 represents the general erosion over the entire region from the close 

 of the Ordovicic to the close of the Cretaceous, a wear so slight 

 as to be only compatible with low altitude of land when the length 

 of the time interval is considered. 



Tertiary drainage 



The Tertiary uplift of the region gave to the land an altitude 

 in excess of that of the present. A partial measure of this ex- 

 cess is the difference in level between the Tertiary valley bottoms 

 and those of today ; but we do not know the depth of valley filling 

 in this district and hence can not state the excess. Even before 

 the uplift the streams had likely become adjusted to much their 

 present relation, namely consequent streams flowing westerly 



