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EVIDENCES OF GLACIATION 197
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solid rocks of the hills and highlands, hiding disfigured surfaces beneath a covering of ruin.
Over
New England the same deposit is widespread; it lies up and down the
valleys, it forms the terraces of its rivers, the shores of its lakes,
and, spread over the face of the land, is frequently the immediate soil
beneath the feet. This member of the geological series, exhibiting
various phases in its deposition, from the boulder clay to the lake
ridges, is widely distributed, indeed, is widely universal over the
Northern States, and as far south as 400 north latitude
extends its sheets and centers of pebbly and sandy deposits in mounds
and ridges, themselves capped with accidental boulders, and resting
upon the furrowed and seamed surfaces of the rock beneath. Sometimes
they may be found collected in heaps and walls at the foot of the
polished rocks, as if silent and incontrovertible witnesses of their
severe and prolonged erosion.
In Scotland it is the till, a
stiff clay, interspersed with polished stones, crowding down the
valleys and prevalent over the lower slopes, varying in its
lithological character with the character of the surrounding rocks.
Gravel and sand beds are intercalated with it and superimposed upon it.
In England, Ireland, Scandinavia, and Switzerland, we discover
identical strata; strata which, while yielding different subdivisions,
in their entire extent are the same thing, and only varied according to
the local force and extent of the wearing agent, the local
peculiarities of the country over which it operated, and the effect
which submergence beneath the sea had in redistributing and rearranging
the beds of detritus already laid down. Associated with these phenomena
are the appearances known as crushed ledges and roches moutonnees, both
of which testify to the exertion of enormous pressure— the one of
pressure continuous and progressive, the other, perhaps, of percussive
and intermittent attacks.
Crushed
ledges designate those plicated, overthrown exposures where parallel
laminae of rocks, as talcose schist, usually vertical, are bent and
fractured as if by a maul-like force
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