The weekly Iberian. (New Iberia, La.) 1894-1946, 1915-01-30
The SAN FRANCISCO
FOLIO.
Geological Report Shows That
Golden Gate Was Once
Mouth Of a Great River.

Il
Few people in viewing the Ba
OF San Francisco think of IL q
other than II magnificent lane
locked harbor about which ha
grown the commercial m,tropoll
OF the Pacific coast of the Unite
States. Yet this harb0r did no
always exist, according to n r,
port on the geology of the regio
recently published by the Unite,
States Geological Survey, for a
one time through The depression
now occupied by the bay ran
great river that drained the Sacra
meDto and san Joaquin valleys
Ihis river probably flowed be
tween the libuton Peninsula ant
Ange! Island, and then through
the gorge of the Golden Gate
where at present the greatest
depth Of water IL over 400 feet
After the river valley was formed
the coast slowly sank and th,
ocean entered through the Golden
Gate, flooded the valley, and form
ed the present bay. Thus the
valley occupied by the bay is real.
ly not 80 very different from San.
Ia Clara and Santa Rosa valleys,
and should the Pacific coast sink
u few hundred feet lower, those
fertile valleys would form great
additions to the present bay.
But the change from q river
valley to a bay happened very
slowly, for even great earthquake es
as rule do not move large areas
of the earth over few inches,
and IL ii likely TO be thousands OF
years before the outlines Of the
present bay are greatly changed
by nature.
The region Is particularly inter-
esting, for IL ia one Of the newest
parts Of our continent. Some Of
The rocks belong TO the geologic
period known as the Jnraseic, but
many Of them are much younger
aud were laid down on the floor of
the ocean long after the Appala
chan Mountains we're formed.
these rocks are relative
~ young, yet they are hundreds
of thousands OF years old and
have been raised out OF the sea and
tilted until in places they stand
nearly on end. The forces which
have raised lheBe rocks and have
folded and broken them are still
active, yet their effects are 80
accomplished that for
the most part they are inapprecia
Bile in a generation or even In sew
oral centuries Sometimes, how
sver, their effect ia more apparent,
~ for instance when they cause
earthquakes. Earthquakes are
iue to slight movements along |
What geologists call ''faults,"
Which are really broken places In |
he earth's uri,s. where the rocks
~ one side OF the break ~ crack
move past those on the other .,ide.
some Of these faults may be trac
VI for miles--not, OF course, an
'pen cracks, for to anyone but a
lose observer the rocks seem az
lnbroken In most places along the
ault lines an they do In any other
,art Of the region.
The different faults In the re
ZION about San Francisco are
arefully mapped In this report.
dost Of these breaks art fortunate.
y old, and it is to be hoped that
movement along them has ceased,
'ut two are alive, and along one
{ these occurred the movements
hat resulted In the earthquake of
April, 1906. These active faults
re described in the report, and
he fact Is pointed out that al
hough slight shocks may occur
+, Quently, only once In several
eneratlons are serious earth
uakes to be expected A very
feature Of the report ls
le discussion Of the effect Of
arthquakes upon the construc
on Of buildings, roads, tunnels,
ire lines, and the like. It shows
lat buildings constructed on
Iade land are much more serious
/ affected than those whose
rest upon rock.
The geologic report, which il r
nown as the San Francisco folio, !,
one unit Of q geologic map that {
10 United States Geological Sur !
~ Is preparing. This folio con. !
inns topographic, geologic, and i
.rnctural maps Of Ave rectangu i
II areas known as qnadraDgles, !
hich embrace the San Francisco i
ID Marin peninsulas, San Fran !
SEQ Bay, and the east shore al /,
ost as far as Mount Diablo. La /,
10 test accompanying the maps |,
q character OF the different i
cks is described In detail and
. mineral deposits Of economic
.lue are pointed out The folio
50 q ntl ns a number OF b slf,cne =
ews of feature that are of geo. "
gic interest, including one show !