Geology Reference
In-Depth Information
The long glaciated face of Steens Mountain
stretches northward across the southeastern mar-
gin of the Basin and Range in Oregon.
On the east side of the Steens, smaller glaciers
extended only part of the way down the stream valleys.
As the ice mass eroded the rim of the mountain, the
crest moved westward. Glaciers working on both sides
of the mountain rim cut the rock away leaving only a
thin wall separating Little Blitzen and Big Indian
i
canyons from the glacially cut valleys to the east. At
Kiger Gorge, the east and west glacier eroded through
the mountain crest to produce a gap called The Big
Nick.
Ice masses advanced down Steens Mountain in
two separate stages. The first formed a broad ice cap
over approximately 115 square miles. On the western
slope, the retreating ice left extensive debris, while on
the east slope it reached about 2,500 feet above the
Alvord Desert and cut large, bowl-shaped cirque valleys
as the one at the head of Alvord Creek. The second
glacial advance was much smaller, covering less than 50
square miles down in the canyons.
Structure
The nature of faulting in this province is still
not well understood. Two phases of faulting are
recognized. In an early phase horsts, or uplifted fault
blocks, and grabens, or basins, developed from a series
of tensional faults or tilted fault-blocks. In the Nevada
area of the Basin and Range, the block faulting phase
was succeeded by what is called "listric" faulting where
the normal faults curve at depth to a nearly horizontal
orientation as the crust is pulled and stretched to the
extreme. Listric faulting typically yields a topographic
pattern where all of the fault block surfaces slope away
from a central axis. On the other hand, randomly tilted
blocks suggest horst and graben topography. To
understand the mode of faulting, the surface orienta-
tion of the fault-blocks is carefully mapped.
The age of faulting is particularly important to
the geologic extensional history of the Great Basin,
and, since stretching of the Great Basin was not
uniform, tectonic activity shifted from area to area.
During the later Teritary, faults became progressively
younger northward so the Oregon Basin and Range
province has many Pliocene events. Within the Quater-
nary, however, seismic activity shifted back to the south
into Nevada where several large earthquakes have been
recorded in historic time in the central Nevada seismic
belt.
Of the uplifted blocks in this province, the
Hart Mountain and Steens Mountain blocks are the
most prominent. Hart Mountain and Poker Jim Ridge
are part of a large complex of an uplifted block with a
steep western face and a more gentle east-facing slope.
The sheer scarp of Hart Mountain is more than 3,000
feet above Warner Valley, while the asymmetrical
