The San Francisco call. [volume] (San Francisco [Calif.]) 1895-1913, 1895-05-26
At PESCADERO.
GeologiSt Hanks Tells by What
Process They Came
There.
THE ACTION OF THE WAVES.
1t Makes the Pieces of Decompos-
ing Granite Round and
Smooth.

In response to the question, sDo the
pebbles on Pescadero Beach come from the
land, or are they cast up by the sea ?'' pro-
pounded by L. Coburn of San Mateo, Geol-
oglst Henry c.. Hanks has written the fol-
lowing opinion:
SAN FRANCISCO, March %, 1895.
made thorough examination of Pesca-
dero beach, and published the results officially
in the year 1884 in the Fourth Annual Report
OF the State MineralogiSt of California, folio
1856, as follows:
'.The beach at Pescadero, San Mateo County,
has wide celebrity for the beautiful pebbles
found there. These are nearly all quartz
agates, carnelians, jasper and chalcedony, OF
many beautiful varieties. On the shore, under
a low bluff nearly at the sea level, a stratiied
horizontal to the southwest. The strike is
northwest to southeast magnetic. Under this,
uncomformably, lies sedimentary formation,
more recent in horizontal strata, consisting OF
sand, water-worn powders and pebbles This
formation constitutes the bluff, and the peb-
bles On the beach result from its disintegra-
ton. The upper sedimentary seems to be
formed from distntegration of the lower which
extends inland for an unknown distance. In
the lower formations the Sandstones are of
different degrees OF fineness, from the finest
silt to very coarse conglomerate; in the con-
glomerate may be seen small powders of
chalcedony, Jasper, agate and porphyry, which
are are the same as those found on the beach; but
the latter are concentrated by long-continued
actlon OF the waves, which have washed away
the sand, disintegrated the sandstone powders
and gathered the harder pebbles together on
the beach. Some of the sandstoneS are &C.
mented by oxide of iron, and all the loose
sands are highly ferruginous. On the way
from Pescadero to the beach the road is cut
through formation not stratifed, but in
which the powders are imbedded This gen.
eral formation seems to be the same as is OBJ
served in the oil regions OF San Mateo, Santa
Clara and Los Angeles counties"
Although at that time l fully made up my
mind, thought best to again visit the locality.
which did on March %.
l examined the beaches for considerable
distance north of "Pebble Beach," and the
blum or bank and rocks, as well as the surface
OF the ground for a considerable distance from
the sea; and gathered and examined pebbles
which had not been on the beaches, but were
taken from the banks hundreds of feet inland.
Some OF these pebbles l present with this re-
port. I find them to be mineralogically identi-
cal with those on "Pebble Beach,
also obtained specimens of the underlying
bedrock and find it to be the sedimentary
| variety named "arkose," formed apparently
| from decomposed granite. I noticed at several
places in the bluffs along the beaches exam
lned outcropplng strata OF washed pebbles, a
| portion OF which had fallen on the rocks be-
| low. These examinations fully confirm the
| opinion formed eleven years ago.
The sea along the coast OF California, between
| ing upon the land The effect may be seen
along the Pescadero beaches and at the high
sedimentary bluffs between Lobetus and Half
| moon Bay. The waves, which are resisted by
| the hard underlying rocks, erode easily the
softer superimposed sediments which are con-
| tinually falling from the banks. The breakers
| then dashing the detrital matter against the
harder rocks, wholly disintegrate it, the re-
| fiux sweeps away the lighter particles in the
] condition of sand, spreads them out OH the
| beaches and finally washes them beyond the
surf. The heavier portions, including the peb-
! bles, are able in a measure to resist by their
gravity the action of the waves, and remain for
a time exposed and concentrated; but they in
! turn are also swept out to sea, and a new crop
! from the caving bank take their places.
This operation has continued for a long
period and probably viii for many centuries 20
come. The same kind of pebbles exist in the
banks above other beaches, but in less guam
| tity, and owing to the form OF the little bays,
or other causes, the conditions differ, and the
pebbles are sooner carried out to sea or are at
once covered out of sight by the sand.
At several other localities on the Califor-
to those of Pescadero, the most noted OF
which are those near Crescent City, in Del
Norte County, and at Lake Tahoe.