
GLACIAL PHENOMENA AND SUPERFICIAL DEPOSITS. 255 
conditions prevailed for a very long period before that of the greatest sub- 
sidence, nor do we meet with any phenomena not easily explicable by the 
action of the waters and ice during the subsidence and subsequent eleva- 
tion. There is also reason to believe that the elevation took place rapidly 
as compared with the subsidence. 
589. The first act, then, in the sequence of the cold period, which 
can be definitely recalled by its effects, is that in which we find the 
waters gaining upon the land, and flowing up over a great part of the 
eastern plains. The wide channel thus formed along the western base 
of the Laurentian region, though it may have existed for some time in the 
form of two deep bays, one opening northward to the Arctic Sea, the 
other southward, and separated by the highland of the present second 
transverse watershed ; must soon have formed an open strait between the 
Arctic and Southern Oceans. At this period, if not before, it must have 
been invaded by the saltwaters of the sea; and it was encumbered with 
icebergs which were strewing boulders and finer detrital matter over the 
former surface of the land, and producing the lowest portions of the drift. 
There is no indication on the Laurentian plateau to the east and north, 
of a great northern ice-cap, such as-has been supposed to be necessary to 
account for the glaciation of certain regions. The slopes of the plateau, 
as already shown, are too gentle to account for the descent of 
glaciers from it in the ordinary way except on a scale altogether 
too small to explain the general uniformity and amount of its glacia- 
tion. I can therefore only suppose that this plateau, or gently sloping 
ridge, of granitic and gneissic rocks; was covered by a thickness of ice 
sufficient to form a confluent glacier, which by its own weight, rein- 
forced by the gentle inclination of the rocks, moved forward along 
its whole front into the sea. The subsidence continuing, the condi- 
tious of a sea margin passed in succession ‘westward over each part 
of the plain; then that of water deep enough for ice of small burthen 
only, and lastly deep water, bearing icebergs of great size. In conform- 
ity with this sequence, we find on the higher levels the lower portion of 
the deposit to have a decidedly local character, and that in ascending in it, 
travelled stones become larger and more frequent, while in the superficial 
layers the larger boulders are most abundant. 
590. On leaving the eastern region, we cease to find in grooving 
and striation, a guide to the direction of the action of the glaciat- 
ing force, and must trust instead to the distribution of the rocky frag- 
ments. Southward, however, as far as lat. 41° on the Mississippi River, 
