i72 The American GeoloytsL sci)tcmbor, i896 
Molting on its margin, because of the land depression and 
restoration of a warm climate, the ice-sheet waned and grad- 
ually withdrew, although with many vicissitudes of temporary 
halts, marked by marginal moraines. Its great readvance 
after the Aftonian interglacial retreat, however, seems refer- 
able to an earlier time while yet the high preglacial altitude 
continued, though perhaps in a diminished degree. 
Glacial Lakes in the St. Lawrence basin. 
With the retreat of the continental glacier, all hydrographie 
basins which had a descent toward the receding ice border 
Avere temporaril}^ occupied by glacial lakes, held by the ice 
barrier until its continued recession gave avenues of discharge 
and at last permitted drainage to become established in its 
present courses. Over the great Laurentian lakes, as thej 
exist to-day, such glacial lakes of much larger extent rose to 
maximum hights from 50 feet to 600 feet above the present 
Jake levels, as shown b}'' the deserted beaches and deltas and 
by old channels of outliow. More correctly, liowever. we 
should say that the lowering of the glacial lakes, by erosion 
and changes of outlets, to their present representatives, to- 
gether with the uplifts of the northern and northeastern parts- 
of the old lake basins, as compared with their less viplifted 
southern portions, have a combined maximum of about GOO* 
feet in each of the three l)asins of lakes Suj^erior, Huron, and 
Ontario. 
Lake Wltrrcn. The first in the order of time, and the 
largest in area and depth, among the three glacial lakes to be 
here specially noticed, was lake Warren, so named by Spencer 
froni his exploration of its shores in the basin of lake Erie. 
Its shore lines, mapped by Spencer, Gilbert, Lawson, Taylor,, 
and others, show that in its maximum extension this glacia! 
lake stretched from the west end of the basin of lake Ontario- 
west and northwest over the four upper great lakes, attaining^ 
an area nearly equal to that of the contemporaneous lake 
Agassiz in the basin of the Red river of the North and of lake 
Winnipeg. The latest southern beaches of lake Warren., 
traced by Gilbert eastward to Crittenden, N. Y., are there 
found by Leverett to have been contemporaneous with the 
Lockport marginal moraine. The course of the boundary of 
the retreating ice-sheet at the time of greatest extent of lake 
