﻿GEOLOGY OF THOUSAND ISLANDS REGION 141 



Physiography 



Glacial diversion of the Black river. The history of the Black 

 river is not only the most interesting problem connected with the 

 evolution of the physiography of the region but specially important 

 as it may supply the key to Tertiary drainage of the entire area. 



In only the middle portion of its course has the present Black 

 river any pronounced valley. The headwaters and upper section, 

 about 30 miles long, lie on the crystallines of the southwest slope 

 of the Adirondacks, with no conspicuous valley. The lower section, 

 below Carthage, has only a shallow postglacial channel. The great 

 valley begins at about Forestport and extends northwest to Car- 

 thage, a distance of more than 40 miles, and steadily deepens and 

 widens northward. At Glenfield or Lowville, near the middle part 

 of the valley, the altitude of the river is 740 feet, while the great 

 ridge on the west, separating this valley from the Ontario, rises to 

 2000 feet, and the breadth of the valley is at least 10 miles on 

 the 1300 foot contour. 



Former writers have regarded the Black river as the trunk stream 

 of the early drainage which headed the Ontario valley. 1 It appears 

 to the writer that that view is a mistake and that quite the oppo- 

 site is the fact, that the Black river was the headwater of the St 

 Lawrence drainage, at least for New York State. 



Plate 43 shows the present hydrography of the region and the 

 divide between northward and southward streams of the Ontario- 

 St Lawrence valley. Plates 44 and 47 show portions of the divide 

 on the larger scale of the topographic sheets. On plate 43 the 

 heavy, broken line south of the Black river marks what was the pre- 

 glacial divide between Ontario and St Lawrence drainage before 

 the Black river was forced by the interference of the ice sheet across 

 the divide. The light, continuous line indicates the present and 

 shifted divide. It is apparent that below Great Bend the river has 

 peculiar and anomalous relationship, and that the divide leading 

 east up the Adirondacks slope is newly established. 



In discussion of this problem the theoretic evolution of the drain- 

 age will be considered first and then the recent history and the 

 present features. 



The Black valley was initiated and developed, at least as early 

 as the Tertiary uplift, along the contact or overlap of the Ordovicic 

 sedimentaries on the ancient crystallines. The west wall of the 



1 Specially the paper by A. W. G. Wilson, Trent River System and the 

 St Lawrence Outlet. Geol. Soc. Am. Bui. 15:211-42. Pages 236-38 refer 

 to our district. With the entire article excepting the point of the Black 

 river relationship we are in hearty accord. 



