Upham.j 
454 
[Jan. 1, 
as continental plateaus much above their present height. The 
fiords of these regions indicate that they had generally stood 
higher than now during probably the whole Pliocene period ; and 
the submerged fiord of the Hudson shows that when this uplift 
culminated, being as I believe the most important part of the 
causes of the ice-accumulation in the glacial period, its extent on 
our coast at New York was about 2,800 feet. But after the 
earth's crust became heavity loaded by the ice-sheets it sank until 
the coast from Boston northward was depressed below its present 
level, submerging the border of northeastern Massachusetts, New 
Hampshire, Maine, and New Brunswick, and carrying the sea 
inland along the valleys of the St. Lawrence, the Ottawa, and lake 
Champlain. This depression of the submarine Hudson fiord was 
so rapid that a bar 1,600 feet in height was formed across its 
mouth while the deep fiord behind the bar remained unfilled by the 
alluvial sediment of the river. 
The greater part of the drift deposits exposed to observation, 
including the moraines of recession, their accompanying kames 
and overwashed frontal plains, the osars and their associated plains, 
the valley drift and its terraces, the drumlins, the contour of the 
general sheet of till, and even the glacial striae, tell us far more 
of the retreat of the ice-sheet than of its advance and action dur- 
ing its maximum extension. A very great change of climatic con- 
ditions restored again prevailingly mild and warm temperatures, 
melting away the vast ice-sheets ; but the returning warmth and 
the glacial cold several times contended for the mastery, recessions 
of the ice being succeeded by stages of re-advance, spreading till 
in many places over interglacial soil and fossiliferous beds. 
From my studies of the glacial lake Agassiz for the geological 
surveys of Minnesota and the United States, I am much impressed 
with the suddenness of the departure of the ice-sheet. This lake, 
which grew to be six hundred miles long, probably as large as the 
combined areas of the Laurentian lakes, and seven hundred feet 
deep, confined in the northwardly sloping valley of the Red river 
of the North and lake Winnipeg by the barrier of the waning 
ice-slieet, is recorded by very definite beaches and occasional low 
escarpments of shore-erosion ; but the total amount of wave action 
on its shores is far less than that shown by the extensive deposits 
of dune sand and the high cliffs of till bordering lake Michigan. 
Now the whole of postglacial time, during which these effects have 
