Geology Reference
In-Depth Information
Terrace levels in the vicinity of Cape
Blanco (after Peterson, Gleeson and
Wetzel, 1987; Mclnelly and Kelsey,
1990)
More recently these erosional surfaces have
been identified by determining the age of invertebrate
fossils from within terrace sediments. These fossils are
dated by measuring uranium /thorium radioactive decay
from the calcite in corals, bryozoa, and sea urchin
plates and spines. A second strategy for dating fossils
uses the alteration of amino acids preserved in mollusc
shells. After an animal dies, organic chemicals in its
shell change at a regular rate. Popular molluscs used
for dating are the thick-shelled clams, Saxidomus and
Mya.
Coastal Terraces
Varying from 20 to over 50 feet in thickness,
gravel and sand of ancient terraces as high as 1,600 feet
have been mapped along the southern Oregon coast
between Coos Bay and Port Orford. In the vicinity of
Cape Arago and Bandon recognized terraces are
Whisky Run, Pioneer, Seven Devils, Metcalf, and the
extremely limited, poorly preserved Arago Peak. At
Cape Blanco a small terrace covers much of the point,
while the extensive Pioneer terrace continues on the
landward side toward Port Orford. Silver Butte, Indian
Creek and Poverty Ridge terraces rise eastward of
Pioneer terrace. The Cape Blanco terrace is dated at
80,000 years, the Whisky Run terrace at 83,000 years,
the Pioneer at 103,000 years, the Seven Devils at
124,000 years, and the Metcalf terrace at 230,000 years.
The oldest terraces are inland at the highest
levels, whereas the younger recently emergent terraces
are on the coast at lower elevations. Terraces cut late
in the Pleistocene are ordinarily elevated less than 100
feet. The present elevation of the youngest most promi-
nent terrace ranges from a point below sea level to 225
feet above. It is difficult to correlate or determine the
association between the terraces by comparing their
elevations because the rates of uplift vary considerably
from place to place along the coast, while deformation,
faulting, and erosion have obscured the surfaces. A
single, continuous terrace may then have separate
sections at elevations which differ by hundreds of feet.
The thick-shelled molluscs
Saxidomus
(left) and
Mya
(right) are used for dating terraces.
