battering
on them from above. The strata are oftentimes tumbled over upon a cliff
side like a row of books, and rest upon heaps of fragments broken away
by the strain upon the bottom layers, or crushed off from their exposed
surfaces.
Roches moutonnees are
those rounded and swelling prominences often seen in a landscape
which, when examined more closely, show themselves to be truncated
masses of rock whose asperities have been smoothed away by the same
agency which has planed the rocks everywhere. Only the roches moutonnees have
been left furrowed and scratched upon one side, whence the abrading and
engraving tool advanced, but upon the other unscored and hidden beneath
a tail of fragments ground from their opposite slopes.
Thus, imperfectly described, we have reviewed the most prominent features of a comparatively modern period, vis., the
widely grooved and polished condition of northern rocks; especially
hard-grained rocks, which retain these impressions; the occurrence of
wandering boulders, transported longer or shorter distances from their
primitive sites and the detrital matter, from continental abrasion,
deeply burying the rocky face of the country, and in ridges, mounds,
and sheets extend-* ing east and west and along the great
water-courses, stretching itself down southward in irregular tails,
curves, and projections.
Prepared
now to detect the traces and monuments of this stupendous geological
agency, let us briefly look for the evidence that establishes its past
presence in and about New York.
As
regards more in detail the circumstances and features of the Ice Age in
eastern North America, it can be safely concluded that so momentous a
climatic alteration began and progressed over a long period of time.
And further, that its phenomena, the ice, and its action were
progressively intensified; that if the ice sheet formed (as, of
course, it mainly did) in the north, it was a confluence, in part, of
lesser units, that it may have been preceded by glacial accumulations
on the high ranges in New England or in the New York High-
