CAPE BLANCO, OREGON
Cape Blanco, the westernmost point of the conterminous United States,
protrudes 0.5 mile (0.8 km) into the sea as a windswept, nearly treeless
plateau, some 200 feet (60 m) above the waves. It ends abruptly as a cliff,
flanked by long empty beaches that stretch in either direction. The cape
hosts Oregon’s oldest and highest continuously operating lighthouse,
built in 1870 and now open to the public during spring and summer
Perhaps the most striking thing about Cape Blanco is its apparent
flatness, formed from an uplifted marine terrace, called the Cape Blanco
terrace. Coastal sand and gravel deposits, which compose the terrace, are
well exposed on the beach trail north of the lighthouse parking lot. These
deposits are estimated to be 80,000 years old, so the Cape Blanco terrace
is probably equivalent to the Whisky Run terrace, which is the lowest
terrace at Cape Arago, north of Bandon.
Just inland of the Cape Blanco terrace and at a slightly higher elevation
lies the older and much more extensive Pioneer terrace, estimated to be
about 105,000 years old. It is also the second-youngest terrace at Cape
Arago. Because of its great extent, we can see that it is gently folded into
an anticline. At Cape Blanco, the terrace slopes ever so slightly toward
the south, joining sea level some 5 miles ( 8 km) away; to the north, the
terrace also reaches sea level in about 5 miles ( 8 km). Deformation at the
subduction zone, less than 50 miles (80 km) to the west, has formed this
Beneath the terrace lies bedrock, much of which is beautifully exposed
in the sea cliffs. You can see these cliffs from the beach along the southwest
Making road trips more
interesting for over 40 years!
MARLI B. MILLER
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$26.00, paper • Item #246
side of the cape, reached by a road at the far end of the campground. Most
of the rocks dip toward the southeast, so they get younger in that direction.
The oldest rock, the Jurassic-age Otter Point Formation, consists of dark-colored mudstone and sandstone with scattered blocks of greenstone and
even blueschist. The Otter Point is a mélange that formed elsewhere but
was accreted to North America sometime at the end of the Mesozoic Era.
Rocks of the Eocene-age Umpqua Group, part of a fault slice here, consist
of gray shale that was deposited in deep water during and following the
accretion. The Miocene-age Sandstone of Floras Lake, named for outcrops
near Floras Lake north of Cape Blanco, consists mostly of sandstone and
conglomerate, whereas the Miocene-age Empire Formation is largely
tan-colored sandstone and siltstone. Needle Rock, the prominent sea
stack below the parking lot on the south side of the cape consists of the
lower part of the Sandstone of Floras Lake. The Pleistocene-age Port
Orford Formation consists mostly of marine-deposited, darker colored
sandstone and thin beds of conglomerate that overlies a 30-foot-thick ( 9
m) base of river-deposited conglomerate and sandstone.
Numerous landslides are eroding the narrow peninsula between the
mainland and the lighthouse area. Much of the trail to the beach on the
north side winds down through one of these slides. Immediately south
of the parking lot, you can see small faults forming in the gravel deposits
as they pull away from the terrace edge at the top of one of these slides.
Numerous other slides affect the southeast edge of the cape.
North of Cape Blanco, the Sixes River meanders over its floodplain.
An example of how its channel changes position through time is the large
cutoff meander that occupies the floodplain about 2 miles ( 3. 2 km) from
the river’s mouth. The road to the cape crosses this area where it descends
almost to sea level before rising up to the terrace, about 1 mile ( 1. 6 km)
east of the parking lot.
OtterPointFm UmpquaGroup landslides lighthouse
sloping gently southward
upper Pioneer terrace
Aerial view of Cape Blanco toward the north.