Geology Reference
In-Depth Information
beach south of the entrance to Coos Bay. Deeply
eroded sandstones of the Coaledo Formation at
Yoakam Point and Gregory Point have been honey-
combed with erosional chasms. The impressive formal
gardens at Shore Acres State Park are underlain by
steeply-dipping Coaledo Formation studded with
cannonball concretions. Offshore here, Simpson reef is
a sandstone remnant of the Coaledo Formation that
was once continuous with the headland. Squaw "Island"
at the entrance to Sunset Bay is connected to the
mainland by a ridge of rock. During low tide the
exposed bedrock creates a causeway to the island. Sand
carried by longshore currents is accumulating along the
bedrock exposures to build up the causeway. North of
Florence, Eocene volcanic rocks of Yachats Basalt
form resistant promontories at Sea Lion Point, Heceta
Head, and Cape Perpetua. Narrow ledges are carved
along the base where the fractured basalts are being cut
away. At Heceta Head and Devils Elbow State Park
several large rock masses appear offshore as isolated
stacks that were once part of the mainland. A series of
flows in the basalt and the nearly horizontal layers
suggest the lavas issued from low profile shield volca-
noes.
point as well as being the most southern projection of
the Coast Range province. The cape was named "White
Cape" by 16th century Spanish explorers who noted the
white fossil mollusc shells that wash out of buried
Pleistocene terraces high on the bluffs. Rocks of the
promontory are a complex of Jurassic Otter Point
conglomerates covered in turn by Eocene mudstones
capped by the the highly fossiliferous late Miocene
Empire Formation. North of Cape Blanco a number of
stacks and large islands are found from Bandon State
Park to Cape Arago. A field of durable Jurassic rocks
of the Otter Point Formation are common here
creating such landmarks as Face Rock and Table Rock.
Blacklock Point, immediately north of the cape, is also
formed of resistant Jurassic rocks.
The most dominant promontory north of Cape
Blanco is found at Coos Bay, where a complex of
headlands at Cape Arago, Gregory Point, Yoakam
Point, and Coos Head are braced by massive sand-
stones. Coos Head is supported by the Empire Forma-
tion, while fine-grained sandstones of the Tunnel Point
Formation compose the point that projects onto the
Basalt eroded into a variety of rocks shapes
and sizes makes Seal Rocks among the most pictur-
esque along the coast. Here the southern coastal extent
of Columbia River basalt flows, originating in eastern
Oregon, have been eroded to expose dikes and sills. Of
these, Elephant Rock is the most conspicuous. Now
part of the mainland, this rounded knob was an isolat-
ed sea stack during Pleistocene interglacials when sea
level was higher. A distance offshore, steep-sided Otter
Rock is an isolated block of the same invasive basalt.
Northward from Seal Rocks, headlands as
