THE GEOLOGICAL HISTORY OF NEW YORK STATE I05 
ridge on which it is located. It is almost completely surrounded by 
walls of hard Shawangunk conglomerate, while the lake basin itself 
is in the soft underlying Ordovicic shales. This lake does not appear 
to owe its origin to a dam of glacial drift, but rather to ice erosion 
in the soft shales at a place where they had already been exposed 
to view before the oncoming of the ice. Such patches of shale 
occur at several places on the mountain. 
Near the very western end of the State lies another lake remark- 
ably situated. This is Lake Chautauqua, famous as the great center 
of Chautauqua assemblies. The altitude of the lake is 1338 feet, 
and its northern end is near the edge of the steep front of the 
Southwestern plateau province where it overlooks the low Erie 
plain. The drainage is southward into the Allegheny river, but the 
narrow place near the middle of the lake strongly suggests a pre- 
glacial divide there. As Tarr says: “If this view be true, Chau- 
tauqua lake is made up of parts of two valleys, one north-sloping, 
the other south-sloping, and each dammed by heavy morainic 
accumulations.” * 
Extinct glacial lakes. Hundreds of extinct glacial lakes are 
known to be scattered over the State. Some of these existed only 
during the time of the ice retreat, while others persisted for a 
greater or lesser length of time after the Ice age. Lakes Warren, 
Iroquois etc., already described, were fine examples of the first type. 
North-sloping valleys were particularly favorable for the develop- 
ment of glacial lakes during the retreat of the ice because the ice 
front always acted as a dam across such valleys, thus allowing the 
waters to become ponded. When the ice front stood across the 
northern ends of the Finger Lakes valleys, the waters in those 
valleys were ponded at much higher levels than they now are, and 
the ancient water levels are more or less clearly marked by the 
old beach lines. i 
Perhaps the finest example of a large, wholly extinct glacial lake 
is Black lake, which occupied a good portion of the Black river 
valley on the western side of the Adirondacks. This lake, small at 
first, was formed by ponding the waters in the upper Black river 
valley around Forestport, Oneida county, in front of the waning 
(northward retreating) ice lobe in the Black river valley. Its first 
discharge was probably southward past Remsen. Further retreat 
of the ice lobe permitted an enlargement of the lake to the region 
around Boonville, and the discharge was then southward along the 
channel of the present Lansing kill. The deep, narrow gorge a 
1Tarr’s Physical Geography of New York State, p. 205. 
