556 CENOZOIC ERA— AGE OF MAMMALS. 



this period are detectable, also along the South Atlantic and Gulf coast. 

 During the early Quaternary (Glacial epoch) the continental elevation 

 is shown by the severe erosion of the La Fayette (Orange sand) forma- 

 tion, referred by McGee to late Pliocene, and by Hilgard to early Qua- 

 ternary. During the period of subsidence, the coastal plains were again 

 covered by the sea, and the shore was again at the fall-line. The deposits 

 of this time form the Columbian formation of McGee. Finally, from this 

 subsided condition the land rose to its present level during the Terrace. 



2. Flooded Lakes. — All the lakes in the region affected by drift show 

 unmistakable evidences of a far more extended and higher condition 

 of the waters than now exists. About all these lakes is found a suc- 

 cession of terraces or old lake-margins. The highest of these marks 

 the highest water-level, and is the oldest ; the lower ones mark succes- 

 sive steps in the draining away or drying away of the waters. 



For example, about Lake Ontario successive margins are found up 

 to 500 feet above the present lake-level ; about Lake Erie up to 250 

 feet ; about Lake Superior, up to 330 feet ; and similar margins are 

 found about Lakes Michigan and Huron. It seems not improbable 

 that the retreating ice-front acted as a barrier, against which accumu- 

 lating water formed one or more enormous lakes, over which floated 

 icebergs loosened from the Canadian ice-foot. These lakes drained 

 southward into the Ohio and Mississippi until the barrier was removed 

 by the final retreat of the ice-sheet ; and then northeastward, as now, 

 through the St. Lawrence. 



Lake Agassiz. — Another great glacial lake in the region of Lake 

 Winnipeg, probably formed in the same way, was first discovered and 

 figured by General (then Lieutenant) Warren, but recently traced out 

 with accuracy by Upham. The retreating ice-front acted as a dam, 

 against which the waters of the melting ice-sheet, together with the 

 natural drainage of this region, accumulated to form a lake of enormous 

 dimensions — greater than all the present Great Lakes put together. 

 This great glacial lake drained southward through the Minnesota into 

 the Mississippi. With the final retreat of the ice-sheet, the drainage 

 was reversed, and its dwindled remains — Lake Winnipeg — drained, as 

 now northward, into Hudson Bay. The outlines of this ancient lake 

 have been accurately mapped by means of its still existing terraces and 

 it has been named Lake Agassiz, in honor of the great champion of 

 land-ice as the cause of the Drift. In map, Fig. 931, we have given the 

 outlines, taken from Upham, of the southern portion of this ancient 

 lake. It has been traced by Tyrrell 150 to 170 miles northward in Can- 

 ada,* and more recently its whole outline has been mapped by Upham. f 



* Bulletin of the American Geological Society, vol. i, p. 404. 

 f Geological Survey of Canada, vol. iv, E, p. 10, 1890. 



