Geology of Hells Canyon
To understand the geology of Hells Canyon,
the visitor has to understand that the solid ground we stand on
is really a thin crust of earth undergoing constant movement and
Canyon, the deepest river gorge in North America, is the result
of many geologic movements and changes which occurred over millions
of years. The canyon is deep, exposing rocks which tell its geologic
story to anyone with the knowledge to read them. This brochure will
help you 'read the rocks' by dividing their story into four stages:
STAGE 1: VOLCANIC ARC, 300-130 MILLION YEARS
Most of the dark-colored and massive rocks exposed along the lower
walls of Hells Canyon came from volcanoes that erupted on islands
in the Pacific Ocean.
These volcanoes grew where two or more plates of the earth's crust
met, forming a long chain of islands known as a volcanic arc. These
islands were only the tops of a gigantic, submerged mountain range.
In places, limestone rock is found within lava flows in Hells Canyon.
This occurs because after volcanic activity stopped, the region
lowered into the ocean and sediment was deposited on the submerged
platforms. Examples of this limestone occurs south of the mouth
of the Grande Ronde River and along the Oregon side of Hells Canyon
Reservoir near Kinney Creek.
After the limestone settled, the volcanic arc broke apart, forming
basins which received sediment from the erosion of surrounding areas.
Examples of this sedimentary rock occur just south of the Oregon-Washington
border and in the Pittsburg Landing area.
Also during this period of erosion, bodies of molten rock, called
plutons, were thrust upward from deep in the earth's crust into
the older sediments and lavas where they slowly crystallized.
Granitic rock exposed near the mouth of the Imnaha River, Deep
Creek, Getta Creek, and from Corral Creek up to Kirkwood Creek are
representative of these plutons. More extensive granite rock exposures
may be seen in the upper elevations of the Wallowa and Seven Devils
Both during and after these geologic processes, the rocks folded
and faulted. For example, a major fault pushed older granitic rock
over much younger sedimentary rock at the southern end of the broad
canyon at Pittsburg Landing.
STAGE 2: COLLISION AND EROSION, 130-17 MILLION
During the early part of this stage, continued movement of the
earth's crust caused the volcanic arc to collide with and become
a part of the North American continent. The area lifted, moving
the ocean to the west. The newy-exposed land eroded extensively,
with the sediment settling in deep ocean basins. Sediments which
eroded from this new part of North America can be seen at one of
these ancient basins near Mitchell, Oregon.
STAGE3: PLATEAU BASALT, 174 MILLION YEARS AGO
Extensive lava or basalt flows (part of the Columbia River basalt
unit) then covered much of the Pacific Northwest. Repeated flows
smoothed the topography and created a vast and nearly-level plateau.
Most of this activity took place 17-15 million years ago but some
flows, such as those near Asotin, Washington, are as young as 6
million years. An interesting feature of the basalt flows is columnar
joining, which is responsible for forming bold cliffs or palisades.
These are generally located high above the Snake River. Sometimes,
however, the flows did reach the canyon floor, as occurred north
of the mouth of the Grande Ronde River and also along stretches
of river between the mouth of the Imnaha River and Getta Creek.
STAGE 4: HELLS CANYON, 6 MILLION YEARS AGO TO
Hells Canyon was formed by normal stream erosion as the Snake River
cut its way through rocks of a rising mountain range. It is still
being cut and is probably deeper and more rugged today than at any
other time in its history.
Most of the extensive cutting and widening probably took place
during the last two million years when the cutting power of the
river was increased by melting glaciers, large amounts of rainfall,
temporary blockages by natural dams, and spillovers of large lakes.
The spillover of water from Lake Bonneville at American Falls,
Idaho, 15,000 years ago, had an enormous effect on the canyon as
we see it today. The volume of water during this event was at least
1,000 times greater than during present spring run-off levels. This
tremendous.flow did not deepen the canyon, but it did increase the
width of the canyon and formed the large terraces seen near the
mouth of Temperance Creek, at Johnson Bar, and at Pittsburg Landing.
These terraces formed on the upstream side of temporary natural
dams. Water velocity decreased behind the dams, allowing sediment
to fall to the lake floor. Continued down-cutting by the river formed
a channel through the sediment, leaving behind the flat terraces.
OTHER GEOLOGICAL FEATURES OF HELLS CANYON
MINERALS. Mine portals and
prospect pits dot the canyon walls. Mineralizaton occurred when
hot mineral-bearing fluids penetrated the rocks either during or
directly after major volcanic activities. The most common mineral
in Hells Canyon is copper, although several thousand ounces of gold
were recovered from the Iron Dyke mine near Homestead, Oregon. The
metallic-yellow, black, and brilliant blue and green minerals that
can be seen at the Mountain Chief Mine near the mouth of the Imnaha
River are mostly copper and iron ores.
RAPIDS. The rapids in Hells
Canyon were formed when tributary streams, rock falls, or landslides
deposited coarse debris in the river channel. Rapids can change
from year to year, as well as from season to season, depending on
the amount of debris in the river and the water level.
VOLCANIC ASH. Near the top
of some terraces and along river deposit areas (alluvial fans) near
the bottom of draws, white and light-brown layers of volcanic ash
can be seen. A good example occurs on the Oregon side of the river
just north of Pittsburg Landing. These ash deposits are the result
of the violent eruption of Mt. Mazama about 6,600 years ago which
created the crater filled by Crater Lake.
POLISHED AND SHINY ROCKS.
Some rocks near river level are strikingly shiny. Sand and silt
grains carried by the river both erode and polish the rocks. In
places, currents carrying this sediment have cut grooves into the
boulders along the river.
The round, shiny rocks with a black, metallic luster were shaped
by the river. Later, dissolved manganese and iron-rich minerals
were deposited on the rocks' surface through evaporation.
We hope you enjoyed your visit to the Hells Canyon National Recreation
Area where eons of geological time are recorded in the canyon walls.
Hells Canyon, with its unique formations and geologic stories,
is a national treasure.