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 change.

Hells 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 AGO

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 mountains.

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 YEARS AGO

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 PRESENT

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.