﻿GEOLOGY OF THOUSAND ISLANDS REGION 23 



The Pleistocene 1 



During the geologic periods of the Devonic, Carbonic and 

 Permic, and the Mesozoic and Cenozoic eras, each millions of 

 years in length, our area was doubtless always above the sea and 

 subjected to the wasting processes of atmospheric erosion. 



Closing the immensely long time of erosion and bringing the 

 history down to the present time, three geologic episodes are 

 conspicuously recorded in the existing surface features. The 

 first of these episodes was the burial of the entire area for some 

 scores of thousands of years under the Labradorian ice sheet with 

 its grinding flow. The second was the burial for further thou- 

 sands of years under glacial and marine waters that immediately 

 succeeded the latest of the ice bodies. The third episode is the 

 present time, a restoration of the subatmospheric conditions of 

 erosion, which has endured, probably, some 10,000 or 20,000 

 years. 



It is now comparatively certain that during the long geologic 

 history great changes of climate have occurred. The idea, once 

 prevalent, that there had been during all geologic time a steady 

 lowering of temperature and refrigeration of climate from a 

 primitive condition of excessive heat and moisture is wholly an 

 error. The oldest rocks of sedimentary origin contain records of 

 glaciation. In the Permic, ice work was great and wide-spread, 

 and glaciation was probably frequent during past time in elevated 

 regions now eroded. The warm climate of the middle Tertiary was 

 followed by glacial cold in northern lands, and all of New England, 

 New York State and the basin of the Great Lakes was deeply 

 buried under successive sheets of ice which had their origin or 

 centers of accumulation in Canada and Labrador. The peculiar 

 effects of the glacial invasions will be described in a later chapter. 



Following at least the latest of the ice sheets the entire area 

 under description was buried for some thousands of years be- 

 neath waters held up to high levels by the glacier acting as a 

 barrier across the St Lawrence valley. The shore features and 

 deposits characteristic of lake action are found over the region. 



During the time of the ice retreat this portion of the continent 

 was lower, or nearer ocean level, than at present, and when the 

 ice barrier melted away in the St Lawrence valley, the glacial 

 waters (Lake Iroquois) were drained down to sea level, and the 

 north and west sections of our area were long swept by oceanic 

 waters, a branch of the Champlain (Hochelagan) sea called 



1 By H. L. Fairchild. 



