Geology Reference
In-Depth Information
lake. Even with much of the damming material re-
moved, larger blocks of rock form rapids in these
streams as evidence of the existence of former landslide
dams.
the same as that carried south, so the supply remains
approximately the same. This reuse of sand back and
forth, year after year, has important implications for
beach mining. As the sand is recycled, most minerals,
except quartz, are removed to deeper water so that the
process argues against economic sand mining ventures.
Today the Oregon coast is retreating by as
much as 2 feet per year. The construction of riprap,
piers, and breakwaters often accelerates this coastal
erosion. With poorly designed jettys, sand piles up
unevenly against man-made obstructions or is flushed
out to sea. The jetty on the north side of Tillamook
Bay has drastically retarded the flow of sand transport-
ed along the coast here so that the deficiency of sand
south of the jetty has caused an increase in erosion of
Tillamook Spit, which retreated by 50 feet from 1936
to 1960. At Cape Meares directly to the south the
coastline was cut back 320 feet during the same period,
and the winter storms of 1960 to 1961 moved the cape
back by 75 feet. Along the southern coast, the south
jetty at Coos Head has in turn altered Bastendorff
Beach. Prior to construction, beach sands were limited
to the area between Yoakam Point and Tunnel Point.
With the jetty in place, sand filled in behind the new
Beach Processes
With at least 140 miles or almost one-half of
the 310 mile long Oregon coast bordered by sand
dunes, the state boasts much more of this fragile
resource than either Washington or California. The
main source of beach sands is erosion of sea cliffs
supplemented by material transported and deposited by
rivers in coastal estuaries. Except for a small percent of
sand carried to the Clatsop Plains, Columbia River
sediments move north along the Washington coast.
Oregon beaches go through a cycle that is
dependent on longshore currents and shifting, wind-
blown beach sands. During the winter, storm waves
strip sand from many beaches, depositing it offshore in
submerged bars. During the summer months the sand
is redeposited on the beaches. Dunes build up when
moving sands, carried northward with the winter storms
and south during the summer, are trapped behind
headlands. The amount of sand moved north is about
Dominated by high cliffs, the small cove at Cape Meares is viewed with the wide Tillamook Bay in the
background (photo courtesy Oregon State Highway Department).
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