THE QUATERNARY PERIOD 351 



Well within the glaciated region of the interior of the continent 

 the history of Lake Winnipeg is of special interest, but since this 

 lake is merely a remnant of a former much larger body of water, it 

 will be described in connection with extinct glacial lakes. 



Extinct Glacial Lakes 



Thousands of extinct glacial lakes are known to be scattered 

 over the glaciated area. 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 development of 

 glacial lakes during the retreat of the ice, because the ice front 

 always acted as a dam across such valleys, thus causing the 

 waters to become ponded. Among the best criteria for the recog- 

 nition of these extinct glacial lakes are typical, flat-topped, delta 

 deposits formed by inflowing streams and distinct beaches. 



A very fine example of many large, wholly extinct glacial lakes 

 is Black Lake, which occupied a good portion of the Black River 

 Valley on the western side of the Adirondacks in New York. This 

 body of water, small at first, was formed by ponding the waters 

 in the valley by the waning (northward retreating) ice lobe. Its 

 earlier discharge was southward. Further retreat of the ice front 

 allowed Black Lake to expand greatly until it had a width of from 

 5 to 10 miles and a length of from 25 to 30 miles. Finally the ice 

 withdrew far enough northward to permit a discharge of the lake 

 waters northward and the lake soon drained away entirely. A 

 great delta deposit, formed by the coalescence of smaller deltas 

 produced by the streams which drained into Black Lake from the 

 Adirondack Mountains, may now be seen in a remarkable state of 

 preservation on the west side of the valley (Fig. 212). It is some 

 30 miles long, several miles wide, very flat-topped except where 

 trenched by post-Glacial streams, presents a steep front toward 

 the west, and shows a depth of from 200 to 250 feet along its 

 western edge. 



A fine example of a very large glacial lake in the interior of 

 North America, and now represented only by remnants (e.g. 

 Lake Winnipeg), has been called Lake Agassiz in honor of the dis- 

 coverer of the fact of the Quaternary Ice age. This lake, fully 



