Geology Reference
In-Depth Information
Four million year old
Pliocene sediments near
Arlington yielded the
carapace of a 3-foot
long land tortoise.
Butte, large shield volcanoes on either side of Green
from large areas along the river and covered existing
Ridge, probably erupted 3 to 1/2 million years ago. The
gravel bars with coarse flood-borne material and
6,436 foot andesite cone of Black Butte is the highest
erratics. On both sides of the gorge from Wallula Gap
in the area.
to The Dalles, flowing water backed up and spread out
to create Lake Condon. The lake waters reached the
Pleistocene
John Day River valley to the south as well as Maupin
The plateau experienced the last phase of
on the Deschutes where large boulders carried in by
volcanism 1.6 million years ago when early Pleistocene
icebergs were dropped as the ice melted. Such boulders
eruptions from local vents plugged the Metolius,
are common southwest of Arlington, many marking the
Deschutes, and Crooked River canyons with basalt. The
shoreline of the ancient lake,
most extensive of these numerous flows originated near
Frequent successive flooding left gravel, sand,
Newberry crater, filling the Crooked River canyon to
and silt in the valleys and basins along the Columbia
depths of 800 feet in places, while an intracanyon basalt
gorge. A thick, structureless, buff-colored Wasco silt, in
pouring into the Metolius River valley formed a
the vicinity of Moro and Grass Valley, is similar to the
pronounced bench 600 feet above the present river
Portland Hills Silt. Near Ordinance and Boardman, a
level. Within the last million years, however, river
large area of unconsolidated sand has been reworked by
waters have been able to reestablish their previous
winds to form dunes. Pleistocene vertebrate fossils are
channels completely by cutting around the succession
present in the blowouts which reach an elevation of
of intracanyon flows. The basalts have been reduced to
about 600 feet and probably represent fill in the
islands midstream or isolated steep cliffs plastered to
Columbia River valley during high waters. Terraces
the older sedimentary and volcanic layers that make up
along the river at The Dalles are composed of gravels
the main canyon walls.
that also coincide with flooding stages.
The Ice Ages of northern Oregon were a time
of ice build up and repeated catastrophic flooding.
Structure
Pleistocene continental glaciers advanced as far south
The tectonics and geologic history of the
as British Columbia and northern Washington infring-
Deschutes-Columbia Plateau relate closely to the
ing on the Columbia River drainage. Glacial ice
movement of the earth's crustal plates, which brought
temporarily plugged the Clark Fork River, a branch of
about structural changes throughout the province. The
the Columbia River in Montana, resulting in a glacial
evolution of the plateau has been divided into three
lake which drained as numerous separate floods. When
phases that took place 17 to 10 million years ago, a
the dammed up water was suddenly released, it spread
second interval between 10 and 4 million years ago, and
through the Idaho panhandle and southeast Washing-
occurrences over the past 4 million years. During the
ton to reenter the Columbia at Wallula Gateway. After
first stage, tensional stresses from apparent backarc
scouring eastern Washington, millions of gallons of
spreading on the North American plate produced
flood water continued down river to complete the
final
extensive north-south fractures in the southeast corner
sculpting of the Columbia Plateau. Reaching elevations
of the plateau and resultant massive lava flows of the
of about 1,000 feet, the floods removed talus and soil
Imnaha Basalt. As the eruption continued, new vents
