THE GEOLOGICAL HISTORY OF NEW YORK STATE 107 
In many cases where the edge of an ice lobe extended across the 
mouth of an east, or west, or even south-sloping valley, glacial 
lakes were formed. A fine example of a glacial lake (now extinct) 
formed in a south-sloping valley has been called glacial Lake Sacan- 
daga which covered many square miles of the bottom of the broad, 
deep valley in which Johnstown, Gloversville, and Northville are 
located. Through this valley, which has a width of several miles 
and a maximum depth of over a thousand feet, the preglacial 
Sacandaga flowed southward into the Mohawk. During the gen- 
eral ice retreat, but when the Mohawk glacial lobe was still present, 
morainic deposits along the margin of the ice lobe formed an 
effective barrier across the mouth of the valley thus ponding the 
waters over the valley bottom and causing the Sacandaga to find 
an outlet northeastward over the low divide at Conklingville. The 
altitude of the lake corresponded approximately with the present 
780 foot contour line, though it is quite certain that the land was 
then somewhat lower. This lake persisted for a good while after 
the disappearance of the ice because of the effective drift dam, 
and even today, in the spring of the year, a number of square miles 
of swamp in the lowest part of the valley are flooded. The lake 
was drained by cutting down the divide at Conklingville. It is 
interesting to note in passing that the construction of the proposed 
Sacandaga reservoir, by means of a dam at Conklingville, would 
almost exactly restore this former glacial lake. 
Many other glacial lakes are known to have been formed by 
ponding of water alongside the waning Mohawk ice lobe. During 
the melting of the ice tongue from the Hudson and Champlain 
valleys, many small glacial lakes are also known to have been 
formed in the tributary valleys because of ice dams across them. 
New York State fairly abounds in such extinct glacial lakes, and 
though comparatively few have yet been described, they are usually 
easily recognizable by means of the typical, flat-topped, delta 
deposits of crudely stratified sands, gravels and clays. 
Drainage changes, gorges, and waterfalls. Along with its 
lakes, New York State is also famous for its numerous gorges and 
waterfalls, which are also largely due to the great Ice age. Asa 
result of the very long preglacial erosion period, it is perfectly 
clear that typical, steep-sided, narrow gorges and true waterfalls 
must have been very uncommon, if present at all. Like lakes, such 
features are ephemeral because, under our conditions of climate, 
gorges soon (geologically) widen at the top and waterfalls dis- 
appear by retreat or by wearing away the hard rock over which 
they fall. 
