Geology of Southern California.pdf - Grossmont College

40 Geology of Southern California
� Figure SC.78 The bedding in this sedimentary sequence is
oriented parallel to the slope. Note the block of rock near the road
that has moved down along the inclined bedding plane.
� Figure SC.79 Repeated mass wasting near Wrightwood in the
San Gabriel Mountains has left this scar of bare rock and soil on the
steep slope above the town.
are places such as near Santa Monica where uplift is less
vigorous, allowing wide sandy beaches to develop. Elevated
marine terraces representing older wave-cut platforms are
commonly observed above the bluffs near southern
California’s emergent coasts (Figure SC.80). In many
places, multiple marine terraces can be observed from a few
meters above modern sea level to elevations of 400 meters
(1,300 feet) or more. Radiometric ages for fossil material
from the terraces suggest that the most prominent surfaces
have risen to their present elevation over the past several
hundred thousand years.
As the coast of southern California continues to emerge,
relatively soft sedimentary rocks are exposed to wave erosion.
These coastal bluffs are easily eroded, particularly in
the winter when large storm waves are generated in the
Pacific Ocean. Southern California’s sea cliffs retreat landward
as portions of the coastal bluffs are undermined by
vigorous wave erosion and eventually fall, slump, or slide
onto the beach. On average, the rate of cliff retreat along
Frank DeCourten
Frank DeCourten
� Figure SC.80 A marine terrace near Morro Bay, San Luis Obsipo
County.
the southern California coast is currently about 0.2 meters
(8 inches) per year, but in some places can be much higher.
In the Palos Verdes peninsula area, where coastal erosion
has accelerated mass wasting, the top of the sea cliff has
retreated 64 meters (210 feet) in the past 70 years. The
retreat of the sea cliffs sometimes places coastal developments
at risk, and necessitates the use of seawalls, riprap
(large boulders), or other means of armoring the shoreline.
Once the soft bedrock blocks are in the zone of wave
erosion, they rapidly disintegrate to produce some of the
sand that is added to the beach facing the ocean.
Additional sand is carried to the coast by rivers, and transported
along the shoreline by longshore currents that
generally flow southeast in southern California. Beach sand
is thus a combination of grains derived from interior
watersheds and sand produced by local coastal erosion.
Beach sediment is eventually carried to the southeast by
the process of longshore drift until it arrives at a submarine
canyon where it transported into deeper water or is
blown inland to form coastal dunes. If the rate of sand
replenishment and sand transport are balanced, over time
the beach will remain stable as sand arrives at the same
rate that it is carried away.
Human Activities and Beaches: Southern California’s
beaches are world renowned for their beauty and are
important to the recreational and tourist economies of
many coastal communities. In their natural condition, the
beaches were generally stable over time as the rates of sediment
supply, wave erosion, and longshore drift were more
or less in balance. However, as the population of southern
California has grown dramatically over the past century,
the nearshore sediment budget has been significantly
altered by human activity. The intense urbanization of
southern California has necessitated the diversion and
impounding of rivers and streams for flood control and
water supply. This, in turn, has led to a reduction in the
amount of run-off, and therefore sand, that reaches the
coast. At the same time, coastal developments crept ever
Frank DeCourten