Geology Reference
In-Depth Information
Coast Range block, an extensive volcanic archipelago
didn't have the extensive glacial system of the Blue
that extended northwest by southeast across eastern
Mountains, Cascade Range, or of the substantially
Washington and into Idaho. Only a very thin veneer of
higher California portion of the Klamaths. Glaciation
Tertiary sediments are found in discrete areas of the
is most evident in the U-shaped valleys, ampitheater-
Klamath Mountains along the coastal margin, from
like cirques, and lakes carved in Chetco Peak near the
Powers to Agness, along the Rogue River above Illahe,
California border at altitudes between 4,000 to 6,000
and at Eden Ridge and Bone Mountain. Micaceous
feet.
sands and silts of the Lookingglass, Flournoy, and Tyee
Present in patches along the western edge of the
formations were initially carried westward by a well-
Klamath Mountains province, high marine terraces at
developed riverine system that drained the Klamaths
different levels are the combined effect of uplift and
and the Idaho batholith to east to be deposited in the
fluctuating global sea levels during glacial and intergla-
forearc basin. Accumulations of plant fragments
cial periods. Although extensive terraces, for the most
intermixed with marine fossils indicate an interfingering
part, are missing between Port Orford and the mouth
of near-shore and nonmarine sediments. Late Eocene
of the Chetco River, south of the Chetco a broad
nearshore environments are represented by conglomer-
marine terrace is present with an occasional raised sea
ates and sands of the Payne Cliffs Formation exposed
stack projecting above the plain.
in a northwest trending belt along the Bear Creek
Valley near Ashland. Grading upward into ash, tuffs,
Structure
and lava flows, the Payne Cliffs also record the earliest
Large-scale lines or lineaments across Oregon
volcanic activity of the Western Cascades, and deposi-
and Washington, best seen on aerial photos, are poorly
tion was primarily by an extensive braided river which
understood. Extending in a northeasterly direction, the
flowed to the north. This formation is overlain by
Klamath-Blue Mountains lineament runs over 400
nonmarine volcanic sediments of the Colestin Forma-
miles from the northern boundary of the Klamath
tion. With the continued uplift, the shoreline moved
Mountains to the northern boundary of the Blue
northward, and intensive erosion and leveling of the
Mountains. The lineament is displayed on gravity maps
Klamath Mountains took place.
of Oregon where it reflects a crustal thickening from
20 miles thick northwest of the line to a crust 30 miles
Pleistocene
thick southeast of the lineament. The meaning of such
Small glaciers formed in the Oregon Klamaths
a zone is unclear, but it may represent a preTertiary
during the Ice Ages, although this range of mountains
continental margin which was oriented north-south
Oregon marine Cretaceous rocks
and basins (after Nilsen, 1984;
McKnight, 1984)
