1894.] . Quaternary Time Divisible in Three Periods. 985 
the time of final melting of the European ice-sheet. The ex- 
‘planation of this continuance of the ice accumulation and 
advance after the depression of the land began and until the 
maxima, beth of the land subsidence and ice extension, were 
attained, with a low altitude and even less descent of the lower 
Mississippi than now, has been well given by LeConte.” The 
subsidence was doubtless slow, even though probably many 
times faster than the preceding uplift. It may have occupied 
only 5000 years, being at a yearly rate of a half a foot to one 
foot; but possibly it was two or three times as long. While 
the slow sinking of the land was taking place, the accumula- 
tion of the ice by snowfall may have proceeded at a somewhat 
more rapid rate, so that the thickness of the ice-sheet and the 
altitude of its surface were increasing up to a maximum 
nearly coincident with that of the subsidence. Finally, how- 
ever, the subsidence brought a warmer climate on the south- 
ern border of the ice, causing it to retreat, and giving to it in 
the region of the marginal moraines a mainly steeper frontal 
gradient and more vigorous currents than duringits growth 
and culmination. 
The time of general retreat of the ice-sheet in North Amer- 
ica, with low altitude of the land and marine submergence of 
the coastal borders of northeastern New England, northward 
from Boston, and of the eastern provinces of Canada, with 
ingress of the sea along the valleys of the St. Lawrence and 
Ottawa Rivers and the basin of Lake Champlain, has been 
named by Dana the Champlain epoch. It was the final stage 
of the Glacial period, and was characterized by the rapid de- 
position of the glacial and modified drift, whose materials had 
been contained in the lower part of the ice-sheet. 
Tue POSTGLACIAL, RECENT, OR PRESENT PERIOD. 
Closely following the deposition of the modified drift as 
wide and deep flood-plains in the principal river valleys 
draining away from the departing ice, these beds were deeply 
eroded by the streams as soon as the ice-front had so far 
"Bulletin Geol. Soc. of America, Vol. ii, 1891, pp. 329, 330. Elements of 
Geology, third edition, 1891, p. 589. 
