The Potter journal and news item. [volume] (Coudersport, Pa.) 1872-1874, 1873-04-25

3. Continuous excavation of river.
valleys, and formation Of fats or
6ood-piains.
4. Elevation Of those plains above
the overflow of the river, forming
terraces.
The process Of formation We have
already described; nor does it APP
pear that any dynamic agent was
then in operation which is not in
operation now. The work has been
continuous.
The superpoeition Of terraces ear
" suggested the idea that their ori-
gin was due to a succession of sud
den elevations Of the land rather than
to a continuous movement and such,
indeed, may have been the case in
some instances. But their usual
want of uniformity through long dis
tanccs and Of correspondence on OPP
posite sides Of the valley induces the
conclusion that their immediate dis
tribution is controlled by local cir
cumStances, while the general cause
has been a continuous and gradual
elevation Of the land and the equally
continuous action Of running water.
Currents of rivers are thrown from
side to side by ice-bornc powders
and accumulationS Of debri&, and pcb-
hles will become so adjusted in the
rivers bed rs TO resist erosion.
This is one of the causes Of the
sinuosities of rivers. The water, as
Sir Charles Lyell observes, is thus
frequently forced to cut new chan
ncls, by which means new terrace
flats may be formed by, rc-distribu-
tiou Of materials.
The transporting power of running
water depends on its velocity. Hop
kins, cited by Prof. Dana, says that
its force varies as the sixth power Of
its vclocity-that is, doubling the
rate increases the force siNty-four
times. If a stream running 10
miles m hour would just move Il
block of ) tons weight. one Of ~
niles would move one of 32O tons. ''
When shallow streams are suddenly
swollen to torrents, powders of con
siderable size are borne along with
heavy roar-are broken by collision
and ground to pebbles and sand.
Hence We lind that heavy rock-mass-
es are abundant near the sources Of
river, finer materials along its val
ley, fine sand and silt at its mouth.
The material thus comminutcd Is
prepared for distribution over the
tcrrace-hats and plains along the
rivers bed.
The following table is from Math-
er'v Report on the Geology Of the
State Of New York ":
4 stream having a velocity Of
in per see wears away fuc tough clay.
removes fine sand.
L removes gravel
q removes pebbles n in diameter.
g. moves angular fragments having q
diameter " , OF inS.
The latter velocity is, however
very moderate, being little over 2
miles an hour. In estimating the
transporting power of rivers we must
consider the important fact that rocks
lose nearly oue-thjrd Of their weight
Ia water.
The formation of terraces ON the
borders Of lakes has, equally viii
those along the bankS Of rivers, aris
en- from elevations of the land. By
this means the drainage of Lake Su
pcrior became possible, and its terra
COS represent former levels of its wa
ters. Nor less interesting is the fac
that the great plains of Northern
Africa, including the Sahara and the
valley Of Egypt, were emerging from
tie sea while the Nile was excavat
ing its valley and carving into terra
ers the sands Of the Nuban Descr
more than POO feet above the prcscn
bed of the river The wave-eatcl
shores of Patagonia have been eleva
ted above the ocean. and its terrace
plains, equally with those of lake an,
river valleys, constitute an epoch o
geological history and record tn
most profound of the earth's sccula
changes.
The Potter Journal
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