Geology Reference
In-Depth Information
Blue Mountains
Physiography
The Blue Mountains physiographic province in
northeast Oregon is defined on the east by the Snake
River Canyon, on the south at Ontario in Malheur
County, on the north by the Washington State line, and
to the west by an irregular line running near Pendle-
ton, Prineville, Burns, and back to Ontario. Topogra-
phy of the Blue Mountains intensifies eastward begin-
ning with the low hills of the Ochoco Mountains in
Wheeler County and rising to glaciated summits of the
Wallowa Mountains in Wallowa County. The western
portion of the province is part of a wide uplifted
plateau, while the eastern section contains a striking
array of ice sculpted mountain peaks, deep canyons,
and broad valleys.
The multiple origins of the Blue Mountains
are evident in the topography. It is not a cohesive
mountain range but a cluster of smaller ranges of
various orientations and relief. To the northeast are
the Wallowa Mountains; the Elkhorn and Greenhorn
mountains are centrally located; and the Ochoco,
Aldrich, and Strawberry mountains are in the southern
part of the province. The highest peaks of the province
are the Wallowas, an immense oval-shaped range 60
miles long and 30 miles wide. Within the Eagle Cap
Wilderness, the most rugged portion of the Wallowas,
nine peaks rise over 9,000 feet and seven others over
8,000 feet. A number of broad valleys as Grande
Ronde Valley, Baker Valley, Virtue Flat, Sumpter
Valley, Lost Valley, and Bonita Valley lie between the
mountain ranges.
Several extensive watersheds drain the Blue
Mountains. The Grande Ronde, Imnaha, Wallowa, and
John Day are the longest rivers. Of these, the John Day
River, one of the most lengthy in Oregon, cuts across
the province in a northwesterly direction before making
a sharp turn toward the north near Clarno, entering
the Columbia River close to Rufus in Sherman County.
The John Day runs 280 miles from its headwaters in
the Blue Mountains.
Geologic Overview
The Blue Mountains are geologically one of
the most fascinating areas in Oregon. The unique
aspect of the province is its patchwork origin of
separate massive prefabricated pieces of the earth's
crust. Permian, Triassic, and Jurassic rocks were swept
up and accreted to the late Mesozoic shoreline, which
at that time lay across eastern Washington and Idaho.
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