Geology Reference
In-Depth Information
The Wallowa Mountains have been called the
Oregon Alps because of their glaciated, picturesque
slopes. With the exception of the Ochoco Mountains,
all of the individual ranges in the Blue Mountains
province display the unmistakable marks and deposits
of valley glaciation ranging from glacial striations on
bedrock to thick layers of till and moraines. Most of
the higher valleys are U-shaped in cross-section mark-
ing canyons previously occupied by ice masses. Along
valley floors, many lakes are glacial tarns or low spots
cut by a previous glacier.
Great symmetrical piles of glacial till called
moraines are easily identified by the poorly sorted
mixture of large and small pebbles and clay in the same
exposure. Perhaps the most dramatic glacial terrains of
the range are the lateral and end moraines adjacent to
the small community of Joseph. These moraines have
created a natural dam for Wallowa Lake that occupies
the toe of the valley. Joseph itself is built upon the
ancient outwash plain, characterized by irregular
hummocky ground, that spread out in front of a
melting glacier.
Covering about 280 square miles, nine large
glaciers, each more than 10 miles long, occupied the
Wallowas during the Pleistocene. The Lostine glacier
was 22 miles long, the Minam 21, and the Inmaha
extended 20 miles. Averaging 1,000 feet in thickness,
the ice masses reached down the valleys but failed to
cover the ridges above 8,500 feet so that a true ice cap
did not occur. The longest of the Wallowa glaciers in
the Lostine River basin developed an ice mass up to
2,500 feet thick before melting and retreating.
As late as 1929 the last small glacier existing
in the Wallowa Mountains was recorded on the ridge
of Glacier Lake in Union County. The glacier was 800
feet long, 60 feet wide, and 24 feet thick. Today small
stagnant masses of ice, but no moving glaciers, occupy
a few of the cirque basins.
Thin section of Rattlesnake ignimbrite ash-
flow reveals a complex mass of broken shards
(photo courtesy of Oregon Dept. of Geology
and Mineral Industries)
The last major widespread eruption in eastern
Oregon took place during the late Miocene. Streams
deposited up to 1,000 feet of sands, gravels, and silts of
the Rattlesnake Formation which was capped by an ash
flow that erupted from vents 90 miles to the south in
Harney County. Today the distinctive Rattlesnake
ignimbrite forms the skyline of much of Wheeler and
western Grant counties. Horses, camels, rhinoceroses,
mastodon, antelope, and sloths were supported by the
high plains grasslands of this time. Fossils of these
animals along with those of weasels, cats, coyotes,
bears, and a variety of rodents are preserved within
Rattlesnake sediments.
Pleistocene
Glaciation came to eastern Oregon with the
Pleistocene or Ice Ages beginning a little less than 2
million years ago and winding down about 11,000 years
ago after several ice advances and retreats that lasted
about 150 to 200 thousand years each. With Ice Age
glaciation, moving ice masses took over valleys origi-
nally cut by streams. After sculpting the valley, the
glaciers melted, and the streams reoccupied the can-
yons to resume fluvial erosion. Continental glaciation,
where large ice sheets expanded south across the
landscape during the Pleistocene, was more typical of
the Great Lakes and upper Mississippi Valley. In the
western part of North America continental glaciers
only spread as far south as Washington, Idaho, and
Montana.
Structure
The Blue Mountains are caught between two
massive earth blocks that have been in motion for
millions of years. Stresses built up by the movement of
these huge blocks initially created folds then faults
within the Blue Mountains area. Due to changes in
plate motion, direction, and speed, folding and faulting
has occured in stages approximately every 5 million
years. During Tertiary time, the Blue Mountains block
was rotated 65 degrees in a clockwise direction. This
rotation appears to have resulted from what is called
dextral shear where the block was caught between
large-scale, north-south faults that dragged it clockwise.
