Studies in the Sierra, No. V [pp. 393-402]

THE
OVERLAND MONTHLY
DEVOTED TO
THE DEVELOPzMENT OF THE COUNTRY.
VOL. I3. -NOVEMBER, I874.-NO. 5.
STUDIES IN THE SIERRA.
NO. V.-POST- GLACIAL DENUDATION.
HEN Nature lifted the ice-sheet
from the mountains, she may well
be said, not to have turned a new leaf,
but to have made a new one of the old.
Throughout the unnumbered seasons of
the glacial epoch the range lay crushed
and sunless. In the stupendous denudation to which it was then subjected,
all its pre-glacial features disappearedplants, animals, and landscapes were
wiped from its flanks like drawings from
a blackboard, and the vast page left
smooth and clean, to be repictured with
young life and the varied and beautiful
inscriptions of water, snow, and the atmosphere.
The variability of hardness, structure,
and mineralogical composition of the
rocks forming the present surface of the
range has given rise to irregularities in
the amount of post-glacial denudation
effected in different portions, and these
irregularities have been greatly multil)ied and augmented by differences in
the kind and intensity of the denuding
forces, and in the length of time that
different portions of the range have
been exposed to their action. The summits have received more snow, the foothills more rain, while the middle region
has been variably acted upon by both
of these agents. Again, different portions are denuded in a greater or less
degree according to their relations to
level. The bottoms of trunk valleys are
swept by powerful rivers, the branches
by creeks and rills, while the intervening plateaus and ridges are acted upon
only by thin, feeble currents, perfectly
silent and nearly invisible. In like manner some portions of the range are
subjected every winter to the scouring
action of avalanches, while others are
entirely beyond the range of such action.
But the most influential of the general
causes that have conspired to produce
irregularity in the quantity of post-glacial denudation is the difference in the
length of time during which different
portions of the range have been subject
Entered according to Act of Congress, in the year -874, by JoHN H. CARMANY, in the Office of the
Librarian of Congress, at Washington.
VOL. 23.-26.