Modified Drift and the Champlain Epoch. — Up ham. 323 
The Glacial and Champlain Epochs. 
Dana, in his presidential address before the American As- 
sociation in 1855. announced the three epeirogenic move- 
ments, as they are now termed, which characterized the Gla- 
cial period, namely, (i) high uplifts of the countries that be- 
came enveloped with snow and ice; (2) depression of these 
countries until mainly they were lower than now by vertical 
amounts ranging, in North America, to about 500 feet; and (X) 
a moderate reelevation to the present hight. The fossiliferous 
beds of marine modified drift in the St. Lawrence and lake 
Champlain valleys, at first named Lawrentian deposits by De- 
sor, were described by C. H. Hitchcock, in 1861, under the 
name Champlain clays in the Geology of Vermont (vol. i., pp. 
156-167), whence the name Champlain epoch was soon applied 
by Hitchcock and Dana to the time of depression and coastal 
submergence of the drift-bearing northern part of our conti- 
nent terminating its glaciation. 
With the progress of observation and study of the drift for- 
mations by Dana, N. H. Winchell, the present writer, and 
others, it was learned more than twenty years ago that the ice- 
sheet inclosed much drift in its lower part, which during its 
final melting became exposed on the ice surface, thence to be 
partly washed away and laid down as modified drift, while the 
remaining part fell loosely to the groimd as an upper till. The 
recession and disappearance of the ice-sheet, brought about by 
the Champlain subsidence and consequent restoration of a 
temperate climate on the ice border, were attended with de- 
position of the previously intraglacial drift, marking the Cham- 
plain epoch as the especial time of abundant and rapid accum- 
ulation of drift, made possible by the much longer time of its 
erosion and transportation during the oncoming and culmina- 
tion of the Ice age. 
Epeirogenic movements, first of great uplift and later of 
depression, are thus regarded as the basis of the two chief time 
divisions of this period. Each of these epochs is further divid- 
ed in stages, marked in the Glacial epoch by fluctuations of the 
predominant ice accumulation, and in the Champlain epoch 
by successively diminishing limits of the waning ice-sheet, 
which, however, even then sometimes temporarily readvanced. 
