Geology Reference
In-Depth Information
The dramatic promontory of Cape Lookout with Netarts Bay to the left (photo courtesy of the Oregon
State Highway Department).
tunnel through rock, has been eroded to the degree
that it now resembles an arch.
The largest of the caverns just south of Heceta
Head are Sea Lion Caves, home to a large population
of these marine mammals. The caves have been cut in
bedrock at the intersection of a system of east-west and
north-south fractures and faults through Eocene
Yachats basalts. The largest west facing opening is used
by the sea lions to move in and out of the cave. The
southern part of the tunnel is below sea level, while the
northern section is the location of the public viewpoint
into the cave. Fractures along which the cave devel-
oped can be seen in the ceiling.
Among the best-known of the coastal erosional
features are Devils Punchbowl at Otter Rock and the
spout at Depoe Bay. At high tide and on windy days,
waves, forcing their way through a tube cut into
fractured Columbia River lavas at Depoe Bay, erupt
upward in an opening along Highway 101 sending a
column of water high into the air. These same invasive
Miocene basalts form the wall that separates the
picturesque inner and outer harbors here. At Devils
Punchbowl, the rock where two sea caves met collapsed
to form a pit. During high tides or stormy conditions,
sea water foams and froths as in a boiling pot.
Sand Dunes
Of the several areas of extensive sands along
the Oregon shoreline, the belt of dunes between Coos
Bay and Sea Lion Point north of Florence is the
longest, extending for a distance of 55 miles. Averaging
2 miles in width, the strip is 3 miles at the widest point
near Florence, where it was declared the Oregon
Dunes National Recreation Area in 1972. From
Tillamook Head northward, another strip of sand
dunes that makes up Clatsop Plains is a mile wide at
the southern end and about 3 miles wide in the north
where it extends across the mouth of the Columbia
River to Cape Disappointment in Washington. Rising
from 10 to 100 feet above sea level, long ridges of
these dunes run parallel to the shore for miles.
The Clatsop Plains developed during the late
Quaternary when sand began filling in the ancient
shoreline that lay east of where it is now. Spits between
Cullaby Lake and the mouth of the Columbia increased
the deposition of sand between 1400 to 400 years ago
when major accumulations took place as the coast
migrated westward. At the northern end of Clatsop
Plains, Clatsop Spit is the result of recent in-filling
behind the jetty built on the south side of the Colum-
bia River in 1885. The spit has been stabilized by
