82 NEW YORK STATE MUSEUM 
Lawrence? According to J. W. Spencer, the St Lawrence received 
those waters, but in the light of what we have said about Mesozoic 
drainage, and also in view of the fact that the St Lawrence now, 
in the Thousand Islands region, does not flow through anything 
like a distinct channel (see plate g) cut out by a great river, we 
must admit that there is little to favor such northeastward drainage 
from western New York. The St Lawrence is almost certainly 
postglacial in its course at the Thousand Islands as shown by the 
lack of any real channel, and by the presence of a belt of hard Pre- 
cambric rock extending across the river and connecting the Adi- 
rondacks with the Canadian Precambric rocks. This hard rock 
belt must have formed a preglacial divide until the recent forma- 
tion of Lake Ontario and the downwarping of the land which 
allowed the drainage to pass over the divide for the first time (see 
later page). 
Grabau’s interpretation is that the Tertiary drainage of western 
New York passed westward and southwestward into the Mississippi, 
and this view is, in the writer’s belief, far more tenable. The 
accompanying map (figure 27) gives.a good idea of the drainage 
lines according to this view. The major drainage lines were no 
doubt inherited from the Mesozoic, the southwestward courses hay- 
ing been originally determined by the tilt of the land at the time of 
the Appalachian uplift. When the Cretacic peneplain was upraised, 
these major streams as, for example, the Dundas river, again began 
very active work of erosion, and tributary streams were developed, 
during the Tertiary, along the belts of weak rocks. Thus an 
important west-flowing tributary was developed along the belt of 
soft Ordovicic and Medina shales, and formed a channel where the 
basin of Lake Ontario now is. The Rome river, with source at 
- Little Falls, became a branch of this stream while another important 
branch had its source on the Thousand Islands divide. There is 
also shown the position of the Black river, which by late Tertiary 
had already carved out that important valley and flowed into the 
Ontario depression, according to Grabau. More recent evidence, 
however (see later page), strongly favors the passage of the lower 
end of the Black river north and northeastward into the precursor 
of the modern St Lawrence, and which had its source on the 
Thousand Islands divide. 
On the accompanying map the three south-flowing streams, one 
heading near Rochester and the other two flowing through Lakes 
Seneca and Cayuga, are, in the writer’s judgment, not properly 
shown for the late Tertiary, that is, just prior to the great Ice age. 
