THE GEOLOGICAL HISTORY OF NEW YORK STATE 103 
cases of the two largest lakes, Seneca and Cayuga, there is, how- 
ever, strong evidence that the preglacial channels were notably 
deepened by ice erosion.. As Professor Tarr says: “ They offered 
broad channel ways, along which the ice streams moved much more 
easily than upon the neighboring irregular hilltops. Not only was 
the movement more rapid, but the depth of ice was greater. The 
position of the rocks, dipping southward, and the nature of the 
friable shales conspired toward rapid erosion; and so these north 
and south preglacial valleys were markedly deepened. Evidence of 
this comes from the side streams. The rock bottoms of the pre- 
glacial valleys of these tributary streams are not now below the 
level of the lake water in the southern part of the valley (Cayuga). 
If all the drift could be removed and the streams be allowed to 
flow along the line of the course of the preglacial valleys and enter 
the valley of Lake Cayuga as it now stands, excepting that it be 
robbed of water, they would tumble between 300 and 400 feet in a 
distance of about a mile, commencing their descent near the present 
lake margin, a most unnatural condition for mature tributaries 
near their mouth.”? Thus it appears quite certain that the pre- 
glacial Cayuga and Seneca valleys, at least, were notably deepened 
by ice erosion below the level of the mouths of the preglacial 
tributary streams. 
Most of the numerous Adirondack lakes have certainly been 
formed by irregular damming of preglacial valleys by glacial drift. 
It is quite the rule to find the outlets of these lakes flowing through 
such loose materials. By ice erosion many of the favorably situated 
valleys were no doubt somewhat modified, but up to the present 
time we have no good example of a lake basin produced by that 
agency. The hard Precambric rocks were not so easily eroded by 
the ice. Attention is called to the prominent lake belt in the middle 
of the Adirondack province, and running in a north-northeast by 
south-southwest direction. This belt comprises many well-known 
lakes as Placid, Saranac, Tupper, Long, Blue Mountain, Big Moose, 
and Fulton Chain lakes. Sometimes small lakes or ponds are situ- 
ated well toward mountain tops because of favorably located drift 
deposits. A good example of such lake lies at an altitude of 2620 
feet, and well toward the top of Crane mountain in Warren county. 
i The surfaces of Seneca and Cayuga lakes are respectively 444 and 381 
feet above sea level, while their deepest places are respectively 186 and 119 
feet below sea level. 
2 Tarr’s Physical Geography of New York State, p. 181-82. 
