Geology of the Salmon River Region
The Lower Salmon is separated from what is commonly called the Main Salmon by Carey Falls, about twenty miles upstream from Riggins. Roads parallel the river throughout the stretch from Carey Falls to White Bird, so this stretch doesn’t lend itself to wilderness river trips. Our trips begin at either Hammer Creek or Pine Bar, a short distance downstream from White Bird.
From White Bird, the Salmon River winds northwest to Horseshoe Bend, where it makes a 180 degree turn before winding in a generally southwesterly direction to its confluence with the Snake River. The Snake flows north-northwest to slack water backed up by the Lower Granite Dam near Lewiston, Idaho. Our trip ends at Heller Bar, about 25 miles south of Lewiston.
The oldest rocks here, called "basement" rocks by geologists, date from the early Mesozoic period, and are more than 200 million years old. Indications are that these rocks have been transported from the southwest Pacific area. Rocks include an array of ocean floor sediments and volcanics. Most have been subject to a low degree of heat and pressure, and as a result, display distinct signs of alteration, or metamorphism.
The widely accepted theory of plate tectonics suggests that these rock structures collided with North America during the middle Mesozoic period, after being carried across the Pacific Basin by a process related to continental drift. Fossils here are altogether different from those of the same age found nearby in Nevada and Utah. Rock masses of this type, which have been transported by the movement of the Earth’s crustal plates, are referred to as "exotic terranes." Typically, they are completely different from the local or "country" rocks and are confined to separate areas of exposure. There is growing evidence that the foundation or "basement" bedrock over much of eastern Oregon and western Idaho may have been formed elsewhere, then was transported to North America.
The metamorphic process of heat and pressure has distorted and altered this rock, which originally formed as sedimentary deposits, eradicating many of its original structures; but some features, including fossils,
are occasionally visible. Most of the fossils are of oceanic animals from ancient continental shelf environments. These are mainly of mollusks, but recently fossils of large marine reptiles have been discovered.
The overlying rock series in the geological "layer cake" of the Salmon River Canyon are lava flows, which were erupted during the Miocene period, 15 million years ago. These lava flows are part of the Columbia River Basalt flows that are visible over much of eastern Oregon and Washington as well as in central and southern Idaho. During these many volcanic episodes, very fluid lava flowed like syrup over wide areas of the landscape from low volcanic cones and fissures or cracks. Remarkably, although these flows cover hundreds of square miles to depths of thousands of feet, no exact source has been located.
In the period just prior to these lava eruptions, a spectacular, rugged topography had been eroded into the existing older terrain. This erosional surface is estimated to have measured as much as 4,500 feet from the canyon bottoms to the crests of the divides. Careful mapping of this ancient surface shows that streams in the Salmon area roughly paralleled their present patterns. The flood-like lava flows inundated the valley system, covering all but the highest peaks with layer upon layer of hard black columnar basalt. After the eruptions, the Clearwater embayment, including the Salmon, was a flat expanse of cooling lava with island-like buttes projecting here and there above the lava plain.
Lava eruptions were not continuous. Between major eruptions and flows, vast inland lakes developed and slowly filled with volcanic mud, ash and dust. These lake sediments, referred to as the Latah Formation, are situated at several levels within the lava flows and bear the distinctive fossilized leaves of Miocene flora. Near White Bird, Idaho, exposures of Latah sediments are famous for their beautifully preserved deciduous (hardwood) leaves, quite different from the local coniferous forests of today.
The geologic map of the Salmon region shows alternating exposures of basement rock along the river. This patchiness is due to the roughness of the underlying topography. As the river erodes through the lavas, it first exposes the old basement "highs" that were divides before the lava covered them. Two such divides parallel the present Salmon in its last thirty-nine miles before meeting the Snake River.
Very young, unconsolidated sands and gravels of the river system, as well as a layer of very fine dust known as the Palouse Formation overlie the lake sediments (Latah Formations) and basalt layers in the Salmon River drainage. The Palouse is a powdery, buff-colored, wind blown dune deposit (loess). This partially consolidated deposit is very thick in the vicinity of southeastern Washington. As it decomposes, it forms the foundation for the rich Palouse soil that is the basis for eastern Washington’s soft wheat industry.
Along the river between White Bird and Heller Bar, gold has turned up several times, but no large strikes occurred. This gold occurs in trace amounts along the entire length of the Salmon, with very scattered richer pockets. As a rule, the gold is very fine, almost like flour.
Salmon Geology at River Level
The Salmon from White Bird to the Snake does not directly cut across the Idaho Batholith, however, exposures of granites are only a few miles away to the east. Consequently, much of the river gravel is composed of quartz-rich, gray to white crystalline granites.
Many north-south faults occur in the canyons north of White Bird. In places, the river has followed these breaks in the rock in straight lines for many miles before meandering away. The faults are visible in the canyon walls as smooth, striated surfaces that reflect the sun like polished metal. Along the old faulted surfaces, mineralization is evident where stains discolor the rocks.
Just north of White Bird, the river cuts into the top of an old divide, below the skyline lavas. 200 million year-old (Triassic) rocks in this twelve mile stretch bear a distinctive, dark greenish hue, due to characteristic heat treated minerals (metamorphic rock).
Metamorphic rocks are typically very hard, and one effect of this is the formation of narrow canyons. Constriction of the river channel causes the stream's velocity and erosional power to increase substantially.
Erosion of these rocks causes curious textures. One of the more striking of these is a fluting or sand-blasted effect. Sand, borne in the water, cuts long, finger-size grooves or flutes parallel to the direction of stream flow. The rocks are often incredibly smooth to the touch where the water and sand have polished the surface. Near the water line, the rocks receive a final patina of "desert varnish" or a jet black color due to exposure alternating from wet to very dry and hot.
A short section of basalt just before the confluence with the Snake River is an example of an old stream channel now filled with lava.
In the stretch of lower Hell’s Canyon between the mouth of the Salmon River and Heller Bar, several good examples of basement rock can be seen. Just a few miles south of Heller Bar, limestone appears on both sides of the river. These limestone deposits are cooked in places to form the metamorphic rock, marble. The limestone is folded up and lying nearly on its edge, which creates a very sharp series of ridges.