Geology of the Irondequoit Region. — Dryer. 205 
length of 80,000 years to the period during which they have 
been subjected to erosion. 
Between the Irondequoit gorge and the Genesee river south 
of Rochester lies a remarkable range known as the Pinnacle 
hills. They extend from the river a little north of east five 
miles in a line directly toward the group of larger cones. The 
range is simple, straight, symmetrical, regular except for some 
serration, devoid of spurs and completely isolated on all sides 
by a level plain. Its highest point rises at the U. S. Lake 
Survey station 502 feet above lake Ontario and about 200 feet 
above its base. The lower half is composed of coarse gravel 
and the upper half of sand. Its position indicates that it 
belongs to the Irondequoit system, and its structure shows 
that it is a gigantic kame, differing only in size and direction 
from those before described, the axis of which it would inter- 
sect near the sugar loaves. 
The Irondequoit gorge undoubtedly had its origin in the 
channel of some preglacial river. Present appearances point 
to the Genesee as its original occupant, since from the Pinna- 
cle hills to its mouth the present channel of that river is post- 
glacial; but the length of the gorge and the path by which the 
river reached it are both equally buried in mystery. The 
possibility that it may have given passage to a river which 
flowed southward into the Susquehanna is not unworthy of 
consideration. This conjecture receives some support from 
the fact that the line of maximum depth in lake Ontario 
approaches the south shore with a sharp angle at a point just 
opposite the mouth of the bay." At some time in the latter 
part of the glacial period a more or less independent lobe of 
the ice sheet, intermediate between the glacier of the Genesee 
and that of the "finger lakes," pressed through the gorge, 
which it widened and deepened, and maintained itself long 
enough to form, with the help of its neighbors, the moraine 
twenty miles to the south. When the glacier retreated it left 
the valley and gorge blocked with drift which has since been 
removed to the extent described. The washing out has been 
accomplished chiefly by the Irondeciuoit river when its volume 
was much greater than at present. 
There is, perhaps, no equal area in the world where all the 
' See map opposite p. 279, Wright's "Ice Age in North America." 
