﻿GEOLOGY OF THOUSAND ISLANDS REGION 21 



over 3000 feet of rock thickness has been removed, and likely con- 

 siderably less. Since the overlying rock has been worn away down 

 to the Precambric over only a small portion of the whole district, 

 it follows that in the remainder the erosion has been less than the 

 above amount by the remaining thickness of such overlying rock. 

 The character of the district to the south of the map limits however 

 indicates an uplift of the land of comparative recency to the amount 

 of several hundreds of feet, and the present-day stream valleys 

 of the region have been worn down below this old level in this 

 comparatively recent period. This relatively considerable recent 

 elevation and erosion makes still more emphatic the necessity for 

 assuming slight elevation of the region during the much longer in- 

 terval which preceded it. As compared with much of the district 

 surrounding it this area has been one of but slight changes of level 

 during its past history. While in their early history these surround- 

 ing districts were submerged and subsiding, allowing thick accumu- 

 lations of deposits, this area subsided less and received but scanty 

 deposit. Only during middle and late Lower Siluric time, during 

 Lowville, Trenton, Utica and Lorraine deposition, was it a dis- 

 trict of considerable subsidence and deposit. In its subsequent 

 history as a land area it seems to have been one of but small 

 uplift as compared with much of the adjacent region. 



As has been stated, in the comparatively recent past the district 

 experienced uplift to the amount of several hundred feet. Prior 

 to this it had been worn down to a surface of comparatively slight 

 relief. The uplift gave the streams power to deepen their valleys 

 by an equivalent amount, and the processes of wear which have 

 given the present relief to the region were set in motion. Then, 

 as now, the Black river was the chief stream of the neighborhood, 

 and perhaps turned west into the Ontario lowland as it now does ; 

 but the lake was not in existence then, nor was the drainage of the 

 lowland to the eastward, but the Black river flowed through it in a 

 westerly direction, receiving many tributaries from the north and 

 the south. There were also easterly flowing waters in the district, 

 however, the beginnings of streams which drained down the St 

 Lawrence valley. But the St Lawrence of the time had its sources 

 in the immediate region, and contained no waters coming from 

 farther west, the divide between the easterly and westerly flowing 

 waters being here, crossing the present St Lawrence in the Thou- 

 sand Island region on the hard rock barrier which the Precam- 

 bric rocks furnish. On the New York side the divide can be traced 

 across the Clayton, Alexandria and Theresa quadrangles in a south- 



