QUATERNARY AGE. — CHAMPLAIN PERIOD. 555 



deposits in this and other valleys may yet give the actual amount of 

 depression. 



2. Conditions due to the supply of Fresh Water. TJie Final Flood 

 from the melting of the Glacier. — That the melting of the glacier 

 should have ended in a great flood may be inferred from the common 

 observation that in cold latitudes floods terminate ordinary snowy win- 

 ters. 



The subsidence of northern lands would have brought on the con- 

 ditions of a warmer climate ; and, as the melting went slowly forward, 

 this amelioration must finally have become very decided. Conse- 

 quently, there was melting, not merely along the southern edge of the 

 glacier, but over its wide surface ; and, when the thickness of the ice 

 was at last reduced to a few hundreds of feet, and it had become rot- 

 ten throughout, the melting must have gone forward with greatly aug- 

 mented rapidity; and a flood, filling rivers and lakes to an unwonted 

 height, must inevitably have followed. 



The fact that such a flood, vast beyond conception, was the final 

 event in the history of the glacier, is apparent in the peculiar stratifi- 

 cation of the flood-made deposits, described above (p. 548) ; and it is 

 strikingly manifested in the spread of the stratified Drift southward 

 along the Mississippi valley to the Gulf, as first made known by Hil- 

 gard. Only under the rapid contribution of immense amounts of sand 

 and gravel and of water from so unlimited a source, could such de- 

 posits have been accumulated. 



There is direct evidence, as already implied, that the flood reached a 

 maximum just before the close of the melting. In some of the New 

 England estuaries of the Champlain era, as that of New Haven, and 

 along many of its river valleys, the stratified deposits are mainly of 

 sand and small pebbles until within fifteen or twenty feet of the top ; 

 but above this limit there is often a sudden change to gravel and some- 

 times very coarse gravel or cobble-stone beds; a change which indi- 

 cates that, when the flood was at its height, the torrent bore away most 

 of the sand, leaving the stones. The coarse upper stratum is usually 

 overlaid by a yard or two of finer material. 



The sand deposits which succeed the " Erie Clays," in the region 

 of the Great Lakes, may be evidence of the flood over those regions. 

 The logs and vegetable debris, which in some spots top the clay beds, 

 may be additional proof of the loosened grasp of the ice. The de- 

 positions of Orange sand along the Mississippi vallev probably took 

 place at this time of maximum flood. 



The flood would have continued long into the Alluvian era, on ac- 

 count of the ice to the north, yet with much abatement of its violence. 

 Even till near its close, the melting glacier about the northern margin 



