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Geologic History of the Columbia River
Gorge
From: U.S. Army Corps of Engineers, Portland
District, and the U.S. Department of the Interior, U.S. Geological
Survey, The Geologic History of the Columbia River Gorge:
Information Brochure.
KA-BOOM!
40-20 million years ago ( Eocene to Miocene)
Thousands of volcanic eruptions piled layers
of volcanic ash, lava, and mudflows over the region, creating
the Ohanapecosh Formation. These rocks weathered into slippery
red clay and greenish rocks visible near Stevenson, Washington.
Millions of years later, mudflows poured
off volcanoes, covering the land with hundreds of feet of
ash, boulders, and cobbles, creating the Eagle Creek Formation.
You can see this beige formation on cliffs north of Bonneville
Dam and along I-84 near exit 41. Beacon Rock, just downstream
from the dam, is the ancient core of one of these volcanoes.
Trees buried in the Eagle Creek Formation
petrified and their leaves fossilized. If you have a sharp
eye, you might spot an ancient trunk in a rock outcropping
along a trail. A petrified log from this formation lies in
from of the Bradford Island Visitor Center.
Lava, lava everywhere!
17-12 million years ago (Miocene)
During this period, unusual volcanoes,
called basalt floods, erupted in eastern Washington and Oregon.
These volcanoes were cracks in the earth's crust, several
miles long, which poured out floods of liquid molten rock.
41,000 cubic miles (170,000 cubic kilometers) of this lava
spread to cover large parts of Oregon and Washington. Out
of 270 lava flows that spread across the region, 21 poured
through the Gorge forming layers of rock up to 2,000 feet
(600 meters) deep. Look at the cliffs in the Gorge. Can you
see these layers?
As the lava cooled it formed a dark gray
rock called basalt. Many of these lava flows cooled into columnar
basalt; the lava cracks, forming six-sided columns. As you
look for lava layers, notice that some contain columnar basalt.
If you look closely at a columnar layer,
you might notice it is divided into two parts. At the bottom,
the lava cooled slowly forming regular, widely spaced columns.
Higher up, it cooled rapidly creating a jumbled looking mass
of irregular, closely spaced columns.
Columbia
River Flood Basalts Map
The Birth of the Gorge
2 million to 700,000 years ago (Pleistocene)
Hundreds of volcanoes erupted in the Cascade
mountain range. You can still see the 14 major peaks and hundreds
of smaller peaks and cinder cones that form the range. Near
Hood River, Oregon, you see dramatic views of Mount Adams
and Mount Hood. Both are dormant volcanoes that could erupt
within the next 50 years.
During this period, the Cascades began
to uplift. As the mountains rose, the Columbia River carved
out a deep gorge. This is the only near sea-level passage
through the Cascades.
Cascade
Range Volcanoes Map and Links
The Missoula Floods
16,000-14,000 years ago (Pleistocene)
Did you know that the largest floods to
occur on the planet happened here? During the last ice age,
ice sheets covered much of Canada. One lobe of ice grew southward,
blocking the Clark Fork Valley in Idaho. This 2,000 foot (600
meters) high ice dam blocked the river, creating a lake that
stretched for hundreds of miles. When the lake was full, it
contained 600 cubic miles (2,500 cubic kilometers) of water.
How much is that? Imagine a block of water a mile high (as
high as the mountains around Bonneville Dam), a miles wide,
and stretching from Bonneville Dam to San Francisco!
Eventually, water traveled under the ice
dam. The water drained out of the lake in two or three days,
flooding eastern Washington. The flood, moving up to sixty
miles per hour, scoured out hundreds of miles of canyons called
coulees, created the largest waterfall to ever exist, and
left 300 foot (90 meter) high gravel bars. At Bonneville,
the water crested at 650 feet (200 meters). If you look on
the cliffs southeast of the dam, you will see a transmission
tower (the one with three poles) that is 200 feet (60 meters)
above the high water mark.
During a period of 2,500 years as many
as 100 of these floods scoured the Gorge.
Missoula
Flood Map
Sliding into History
500 years ago
Near Bonneville, the lava layers making
up Table Mountain slid into the Gorge. This series of four
landslides, covering five square miles, blocked the Columbia
River. The Second Powerhouse butts against this landslide.
If you look north of the dam, you can see cliffs exposed after
the mountain gave way.
Original inhabitants of the area may have
marveled at the 200 foot (60 meters) high landslide blocking
the Columbia. They could have crossed on foot, possibly giving
rise to a story about "The Bridge of the Gods".
This natural dam created a lake that stretched almost seventy
miles (up to the present day John Day Dam). After a few months,
the Columbia rose high enough to wash through the southern
side of the landslide creating a flood of water that was 100
feet (30 meters) deep at Troutdale.
Things returned to normal, except the river
was displaced a mile to the south and a set of rapids, the
Cascades, had formed. In 1938, the rapids disappeared under
water rising behind Bonneville Dam. The only hints of their
existence are the remnants of a navigation lock at Cascades
Locks built in 1896 to allow boats around the rapids.
The Gorge is still changing. In the winter
of 1996, landslides similar to the Bridge of the Gods landslide
destroyed homes in Warrendale. At milepost 35 on I-84 you
can see this damage.
Useful Links:
USGS
Columbia River Gorge Web Page
Geology
of the Columbia River Gorge - Columbia River Gorge NSA
The
Geologic Story of the Columbia Basin - BPA
Books:
Geologic
History of the Columbia River Gorge, As Interpreted from the
Historic Columbia River Scenic Highway
The
Magnificent Gateway: A Layman's Guide to the Geology of the
Columbia River Gorge
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