(Jrifjiii <i>i:I A;/c of N/e Lunrenf la n Lalcc^. — I'plimyi. 171 
ries on one side irom the basins of lakes Huron, Erie, and 
Ontario, and on the other side from the basin of lake Superior. 
Changes bringing on the Ice Age. 
When the early Quaternary epeirogenic uplifts heaved the 
northern half of our continent 2.000 feet to probably in large 
part 5,000 feet above its present hight, as is known by fjords 
and the continuations of valle3''s on the submarine slopes of 
the continental plateau, it seems not improbable that the 
warping and deformation of the land surface enclosed and 
changed into land-locked basins some parts of the Tertiary 
river valleys of the Laurentian lakes area, so that these lakes 
may have begun to exist in some form before the Ice Age. 
About the same time, during the epeirogenic movements 
which finally brought on the Glacial period, the Ohio river 
and the upper Mississi])pi river, as suggested by Chamberlin, 
Leverett, and Hershey. assumed nearly their present relative 
importance and have since channeled large valleys, although 
wholly disproportionate with the great Tertiary vallcAJ's which 
are now the beds of the Laurentian lakes. 
At length the high elevation northward caused snowfall to 
prevail there, instead of i-ain, during all seasons of the year; 
and in several or many thousand years it enveloped all the 
coLintr}' from the Arctic sea to the Missouri and Ohio rivers, 
and from the Atlantic to the Pacilic, with a thick sheet of 
snow and ice like the ice-sheets now covering the Antarctic 
continent and the interior of Greeiiland. 
Kecession of the Ice-sheet. 
Under the vast weight of the ice accumulation this part of 
the earth's crust, after bearing the burden for a long time, 
isostatically sank, so that when the ice-sheet, exposed thereby 
along its borders to a warm temperate climate, melted away, 
the land on which it had lain was uncovered with a less alti- 
tude than now. The depression of tiie land beneath the sea, 
from which it has since risen, extended from lioston and from 
Nova Scotia, where the relative liights of land jind sea were 
jiearlv the same as now, inland to a vertical extent of 560 feet 
at Montreal and 500 to 600 feet thence westerl}' to the areas 
of the Laurentian lakes, the glacial lake Agassiz. and the ba- 
sin of ILuUon l)av. 
