GEOGRAPHY OF SAN DIEGO:

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GEOGRAPHY OF SAN DIEGO Fieldtrip to the Anza
Borrego Desert
W. H. Berger, PhD NSTA meeting, March 2002
Split Mountain near Ocotillo Wells
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The Anza-Borrego State Park takes up a
substantial portion of San Diego County.
The trip from San Diego, at the coast, to
Ocotillo Wells, in the desert, takes about 3
hours by car. It gives us a view of the many
different landscapes making up San Diego County,
from the semi-arid coastal mesas to the
foothills, to the brush-covered lower mountains
surrounding large basins and valleys (ranch
lands), to the forested high mountains and down
into the rain shadow desert.
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For a self-guided tour follow the blue line to
Split Mtn. Places illustrated in narrative are
marked. Black arrows show the short way home.
Map courtesy Anza-Borrego Desert State Park
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The mountains of San Diego are part of the
Southern California batholith system, which forms
the spine of Baja California. Uplifted coastal
plains make the mesas in the west (Linda Vista
Terrace). The great rift to the east is part of
the Gulf of California. Salton Sea is below sea
level. The Colorado would fill up the basin but
is prevented from doing so - this would destroy
prime agricultural land.
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Coastal zone and ice age heritage The people of
San Diego have settled mainly in the coastal
zone. It is dominated by the raised coastal plain
that is the Linda Vista Terrace and the valleys
that have been carved into it.The valleys are
filled and they have lagoons at their seaward
end. In the past, they were deep canyons, and
fish-rich embayments. The sea level falls and
rises in response to buildup and melting of large
continental ice sheets. The biggest ice caps
formed in the region of Canada. During glacial
time, the sea level was lower than now by 400
feet, and deep canyons formed where now there are
lagoons. When sea level rose (16,000 to 8,000 y
ago), the sea
invaded the canyons, making bays. In the
following thousands of years, the bays filled
with sediment, changing into lagoons and wetlands.
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Origin of the coastal cliffs The cliffs that
line the shores of San Diego County are the
product of uplift (which makes the mesas and the
canyons separating them) and the attack by ocean
waves, especially during heavy storms. As the
base of the cliff is attacked by waves, it
recedes. Eventually the over-steepened cliff
collapses, and the face shears off, making a
fresh exposure. Wave action removes the material
that falls on the beach.
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The rise of San Diego County The uplift of the
coastal plains in the last 2 to 3 million years
is only a modest manifestation of the major
uplift experienced by the county as whole. This
uplift brought rocks from 10 km down into plain
view in the mountains. As a rule of thumb the
higher the countryside, the deeper we look inside
the Earth, and the more rapid the uplift. The
uplift of the mountains is responsible for the
origin of the ultra-dry rainshadow desert in the
rift valley. The process started some 10 million
years ago, as seen in the sediments in the rift
valley (which we shall inspect).
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The geologic map shows that the bulk of the rocks
in the mountains are granitic rocks (intrusions
into the crust, at depth) and metamorphic rocks
(gneiss, schist). A major fault system separates
the land west of the great rift from the land to
the east. San Diego Co. is part of the Pacific
Plate.
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Large faults run in parallel to the rift zone.
They are active and produce large earthquakes on
occasion.
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Climate and rainfall Coastal San Diego has a
Mediterranean-type climate, with only 10-12
inches of precipitation per year. Summers are
cool because of the cold current offshore. Much
of the rain may come in 2 or 3 storms in a year,
and there may be a series of drought years
interspersed with El Niño years with greatly
increased flooding and erosion. During long
drought spells the creeks and rivers are commonly
dry. Most of the water used in San Diego is
imported from elsewhere. The Colorado River is an
important source for all of Southern California.
The south-facing hillsides in Clevenger Canyon
are bare except where irrigated. Any rain runs
off quickly and evaporation is strong in this
sunny climate. Fire is common whenever brush has
grown sufficiently to provide fuel.
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The updraft of air results in precipitation, so
there is a zonation of vegetation from
drought-adapted brush to oak forest to pine
forest. There is an analogy with the upwelling
off the coast of San Diego, which brings
nutrient-rich waters to the surface, stimulating
the growth of algae which serve as food for
fish. On land, water is limiting for growth of
plants. In the ocean, nutrients are limiting.
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After travelling on S2 for about 10 miles or so,
we turn east toward the State Park.
Road cuts reveal how granite intruded older
metamorphic rocks, deep in the crust. All rocks
are shot through by quartz dikes, marking the
late stage of the magmatic activity.
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Details of the interaction between the intruding
granite and the gneissic host rock are well
exposed in the sections along the road.
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Burrows, tracks and trails are evident in thinly
layered sand/silt stones covering the hillsides
as a lag deposit.