Geology Reference
In-Depth Information
Geo-inSight
Point Fermin—Slip Sliding Away
Dubbed the “sunken city” by resi-
dents of the area, Point Fermin in
Southern California is famous for its
numerous exam-ples of mass wasting.
The area is underlain by fine-grained
sedimentary rocks interbedded with
diatomite layers and volcanic ash. When
these layers get wet, they become slippery
and tend to slide easily. The rocks also
dip slightly toward the ocean and form
steep coastal bluffs that are being
undercut by constant wave action at their
base. This wave action results in
oversteepening of the cliffs, which causes
slumping.
Mass wasting began in 1929 with
minor slumping in the area. In the early
1940s, water mains in the region were
broken and several individual blocks
began slumping. Movement
largely ceased following this main phase of slumping, but it has
continued intermittently until the present, and residents have paid
the price for living in an unstable coastal area.
Los Angeles
Santa
Monica
Palos
Verdes
Penisula
Long Beach
Pacific
Ocean
Point Fermin
2. A view of one por tion of the Point Fermin
slide area, showing the fine-grained sedimen-
tar y rocks dipping slightly toward the ocean
and the oversteepened cliffs resulting from
slumping and sliding in the foreground.
1. A map of Southern California,
showing the location of Point
Fermin and an aerial view at low
tide of the sliding that has taken
place. Note the numerous slump
blocks and oversteepened cliffs.
The continuous pounding of waves
and surf along the base of the
cliffs fur ther erodes and
undercuts them, leading to even
more slumping and sliding.
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