lands,
and that these, in a radial manner, north and south, may have
distributed transported material, until the coercion of the more
continental mass united all the local sheets and turned them southward
in a comprehensive and generally continuous advance.
We
are required to believe that the thickness of this ice sheet was, at a
maximum, over 6,000 feet, that it crossed mountainous elevations and
undeviatingly crossed valley and river gorges also, filling them up,
but continuing its former unimpeded direction past and beyond them. As
it finally disappeared (it is still a matter of geological discussion
whether there were two such Ice Ages, with an intervening moderation of
conditions in which vegetation [trees] returned to the previously
glaciated districts) it probably underwent a differential dissolution,
retreating more rapidly at points near the seaboard than in the
interior. It thus left isolated tracts of ice upon propitious sites,
ice gorges, clefts, and wedges as well, in deeper denies, and shedding,
in the last stages of its collapse (if the assumption of a general land
depression is correct), from its impaired or ragged contour icebergs
and icefloes. These latter may have contributed, at points where they
stranded and remained, to the formation of the peculiar kettle holes now seen in the glaciated areas.
Physical
features developed in the whole process of denudation with its
accompanying floods, to which distinct descriptive terms have been
applied. It must be remembered that the inorganic burden of the glacier
was in and on it, that much that was on top of an ice mass sank within it and became englacial, until it actually descended to the bottom and contributed to the formation of the ground moraine.
This ground moraine was now mingled stones, boulders, gravels, clays,
over which the ice slid, partially dragging them on or leaving them
behind. There were surface streams on the ice, that made their way
quickly into crevasse openings, and according to the accidents of the
ground on which the glacier rested emerged, or, chilled anew, were
reformed into ice.
