Mountains and Rivers: In Exotic Terrane
June 14, 2009
by Norm Klobetanz
Some morning soon, I’m telling myself, I should drive to Cold Springs. As a follow up to shooting video on our Hell’s Canyon raft trip over Memorial Day weekend, I want to get “overview” video clips from high-up looking into the canyon and the Snake River. Cold Springs is on the back side of the narrow mountainous ridge behind Riggins that separates the Salmon River and the Snake River and offers views into Hell’s Canyon. In the future I am planning to produce a video guide of Hell’s Canyon for boaters and I am collecting video for the project.
The canyons are still relatively green and the snow drifts on the roads should be melted out of the norths by now; so driving to an overlook to video before it gets too hazy or smokey as summer days tend to be, or before I get too busy photographing rafters, or before the grasses and shrubs get brown, makes sense. The two deepest river gorges in North America coming together here in the rugged mountain country of west central Idaho makes for a unique topographical mix of mountains, rivers, and canyons. Idaho is blessed with a wonderful variety of topography, climates and habitats. It is well known for its many outdoor wonders and activities. But no features are more impressive than the mountain and river country surrounding Riggins.
The Salmon River after flowing west through the rugged granite mountains of central Idaho suddenly at Riggins turns away to the north cutting through softer basalt rocks making a long fish hook run north and then west to then loose its identity to the Snake River in lower Hells Canyon. Much of this eighty mile fish-hook run is through terraced topography of columnar basalt that rises into palisades, buttes, bluffs, and steep grass covered slopes that include clusters and strips of hackberry, mountain mahogany, and ponderosa pine on riverside benches and shoulders or in stream carved draws. Tight along well watered side streams are deciduous Rocky Mountain maple, cottonwoods, birches, alders, and the like. Bushes or shrubs like serviceberry and syringa are evident on slopes that are not too southerly and don’t get too much of the summer sun. But mostly grasses with some prickly pear cactus, goat heads, and sand burrs grow on the hot low elevation exposures. The lower elevations in our canyons are in essence below tree line. It is too dry and hot for trees on much of the lower slopes. Riggins is only 1800 feet in elevation. The boat launch site just below Hell’s Canyon Dam is 1480 feet.
Hells Canyon on the Oregon Idaho border is considered the deepest river gorge in North America, and the Salmon River gorge as it flows through central Idaho to Riggins is considered the second deepest. The Seven Devils Mountains, surmounting the dividing country between Riggins and Hells Canyon, peaks out on He Devil at 9,383 feet. This is almost a mile and a half vertical drop to Riggins. On a map it is but perhaps 10 miles as the crow flies. It is about 7 miles form the highest peak to the Snake River in Hells Canyon and a whopping drop of 8,043 feet. Yes, this is very steep mountainous country and it progresses from below tree line up through the different habitat and plant zones including tall coniferous forests of ponderosa, Douglas-fir, and tamarack, until going into the subalpine zone of lodgepole, spruce and fir, and finally but barely into the edge of the alpine zone at timberline among the highest Seven Devils Peaks--all in this short distance.
When I first came to the area in 1982 I started hearing and reading things about the geology that sort of explained this unique geography. Many of the words and ideas seemed strange and were new to me. Words like oceanic and island arc terrane, accreted terrane, Idaho Batholith, and exotic terrane where only a small part of the explanations given by geologists that confounded this confused but curious river guide. I pieced it together enough so I was able to explain, at least to myself, what I was seeing when I floated the Main and Lower Salmon Rivers or the Snake in Hells Canyon. I since put it to rest, but now years later as I collect more footage for future video guides that will feature the Salmon and Snake Rivers I have reprised my interest. At the Riggins library I just took out Exploring Idaho Geology by Terry Maley and Islands & Rapids by, A Geology of Hells Canyon by Tracy Vallier to thumb through and refresh my memory if not my vocabulary. Vallier and the Forest Service have also produced a video called Exotic Terrane. I stopped out at the Hell’s Canyon Rec. office the other day and they didn’t have the video in stock at the time. I am chagrined to admit I have never seen it. It gives a geologist’s explanation of how our area formed. Exotic Terrane can be difined as a bunch of rocks that where formed far from where they are currently found.
In any case, here goes my interpretation of this area’s geology as I understand it and in no way should it be considered an expert or perhaps in some cases even an accurate explanation:
It is important to remember there is a vast expanse of time involved, and this explanation is a simplification of a very complex geology. A very long time ago the west coast of what we now call the North American continent was literally a few miles east of Riggins running basically north and south as the Pacific Ocean does now. That’s right the Pacific Ocean literally lapped on the western shore near what is now Island Bar. Around Lucile, down river 10 miles, can be found a lot of exposed limestone. Limestone if pressed under great pressure can form marble. The limestone rock bluff rafters float past just below the mouth of Cow Creek looks like it was getting close to becoming marble. Limestone is formed by the vast accumulation of calcium such as what can be formed from the skeletons of reef creatures from ancient ocean coastal waters. Limestone is porous and the spring of water coming out of the mountain just behind the Lucile boat ramp has dissolved a cavern perhaps 30 feet by 80 feet and is known as one of the Lucille Caves. A little rock climbing and a rope is needed to get into it. When my kids were young I got some pretty cool video footage of an excursion there. Just enough sun light came in through several portals and no artificial light was needed.
Today the Pacific tectonic plate collides with the North American plate near the current western coast line. Many millions of years ago a similar thing was going on here as the Pacific plate gradually subducted, or as the oceanic crust moved east it was forced under the continental plate. The friction and dewatering of the oceanic plate as it dove under the edge of the continental plate caused massive amounts of molten magma to be formed. This magma rose lifting the rocks and ground above to form the mountains of central Idaho. This rising bulging magma cooled and formed Granite rocks. This complex upheaval of granite and preexisting rocks formed such a rugged mountainous area that it resisted any extensive development by man. It now comprises what is considered the largest contiguous wilderness in the lower 48 states. This collection of granite extrusions in central Idaho is known as the Idaho Batholith. One of the most western exposures of the Batholith is cut by the Salmon River 14 miles up river from Riggins and is known as The Crevice and is spanned by the Manning Bridge.
The Crevice is cut so narrow and steep walled because the granite is harder than nearby rocks. The rocks just up river and down river of the Crevice eroded back into a softer canyon structure while the harder Crevice rocks eroded much less allowing only a narrow cut by the river. From there the Salmon River continues west until Riggins and encounters rocks that are called the Seven Devils group. If this was ancient ocean, where did these mountains and rocks come from? According geologists as the pacific ocean floor moved east subducting under our continent just east if Riggins, it dragged with it island arc groups like the Pacific islands of today formed by volcanoes. When this thicker island arc group hit the continental plate it did not subduct, but the slow inexorable collision caused the rocks to be tortured and squished up. The oceanic crust broke and a new subduction zone and new coast line formed in what is now central Oregon. This process more or less happened again over time and thus we have the current plate contact area along what is now our west coast. The rocks around Riggins including the Seven Devils are a tortured mix of continental rocks, oceanic plate, and island volcanic rocks smushed together. It might be noted that high up in the Devils can be found limestone and fossils of shallow Pacific Ocean life that most likely once existed far out in the Pacific Ocean as part of a shallow reef of a warm island paradise.
After all this, huge shield volcanoes opened in what is now west central Washington and a succession of massive flows of lava flowed over much of Washington, northern Oregon and into parts of western Idaho. This molten material had a different chemical make up than the rocks of central Idaho and when cooled did not form granite but instead formed relatively softer basalt. These massive molten flows filled the valleys and created great level expanses except where mountain peaks poked up like islands in a sea. Over time the rivers down cut through the basalt exposing fascinating and variable formations of columnar basalt. However, the relatively level ground left between the drainages formed the prairies and plains to our north including the Camas, Palouse, Doumecq (due-mack), and Joseph.
The smushed up rocks of the island arcs that created the Seven Devils and other mountainous terrain to the west of Riggins apparently were harder than the basalt rocks and other metamorphosed coastal rocks to the north of Riggins; so the Salmon River carved off through the softer rocks to the north on its 80 mile fish-hook journey through primarily basalt to join the Snake River. Thus we have the Lower Gorge of the Salmon. River travelers on the Lower Salmon see what is described as four distinct inner canyons separated and over-topped by basalt formations. These inner canyons, Green, Cougar, Snow Hole, and Blue, though not granite like the Crevice described earlier, are made of ancient, hard metamorphic rock that are part of the “basement” of the Seven Devil’s group. These rocks were part of the undulating terrain that was covered by the huge molten basalt flows. As the river cut down through the solidified basalt it also encountered knobs of the harder older rock. The basalt eroded back into a more reclining posture while the older rock resisted easy laid-back erosion and much like the Crevice formed smooth and steep sided canyon walls. Examples of this older hard rock include the rocks on which the north end of the Time Zone Bridge rests. Another nearby example is the large bluff at rivers edge, carved and scalloped with many nooks, just downstream from the bridge, across the river, and just above Tight Squeeze rapid.
They say the Snake River carved through Hells Canyon as part of ancient mega-floods including the Lake Bonneville flood that drained a gigantic lake into the relatively small Great Salt Lake we see in Utah today. Ancient Lake Idaho in southern Idaho broke out and flooded before this. I picture the island arc groups pushing up against the continent and the floods finding a seam or wrinkle between the Wallowa Mountains in Oregon and the Seven Devils in Idaho and voila! We have Hells Canyon right beside and for almost eighty miles more or less parallel with the Salmon River carved gorge.
Riggins sits on a narrow high narrow bench right above where the river makes the big sweeping turn to go north around the Seven Devils. While flipping through the Vallier book cited above, I came across his explanations for the big high benches like what is found at Pittsburg Landing in Hells Canyon or perhaps a bench like the one Riggins is on. He claims such benches are the result of floods; some were mega-floods from ancient lake discharges, some were from high snow pack run-off, and some were from huge canyon landslides temporarily daming the rivers and then flooding— food for thought in such “exotic terrane.”
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