Region 4: The Columbia Plateau

Rock formations from the late Proterozoic are the oldest fossil-bearing formations in the Columbia Plateau region. The earliest fossils found here are stromatolites, similar to those seen in Montana’s Glacier National Park (see Figure 3.51), which have been reported in the Gospel Peak area of northwestern Idaho.

Paleozoic rocks are not present in the Columbia Plateau, as the land has been covered with igneous rock related to eruptions of the Yellowstone hot spot as it moved along the track of the Snake River Plain.

Triassic rocks occur in two areas of Idaho. Along the state’s border with Oregon and southern Washington lie deposits of metamorphosed volcanic and sedimentary rocks that contain a variety of marine fossils, primarily from adjacent areas of Oregon. These include corals, sponges, ammonoids, clams, gastropods, echinoids, and bryozoans. In southeastern Idaho, Triassic deposits are largely composed of marine sedimentary rocks with sparse fossils, except for the Thaynes Formation, which contains fossils of fish, crinoids, bivalves, gastropods, ammonoids (see Figure 3.52), crustaceans, and shark teeth. The Columbia Plateau has Triassic red beds and thin deposits of coal, both of which indicate some terrestrial deposition.

Outcrops of Jurassic-aged rocks occur in western Idaho, along the border with Oregon and southern Washington. These rocks, formed mostly in deep water marine environments, have been slightly metamorphosed. Fossils from these rocks include ammonoids and oysters. Jurassic rocks in the southeastern part of the state are mostly shallow marine yielding mostly poorly preserved fossils, including crinoids, oysters, sea urchin spines, ammonoids, and corals (see Figure 3.26).

The Neogene river and lake sediments of westernmost central and southern Idaho contain abundant and beautifully preserved fossils of fish, rhinos, rodents, rabbits, horses, camels, and many other species. The Hagerman Fossil Beds National Monument on the Snake River just northwest of Twin Falls, in south-central Idaho is the most famous of these deposits, and includes Horse Quarry. This particular outcrop has yielded hundreds of fossils of zebra-like horses, Equus simplicidens, that are about 3.5 million years old (Figure 3.60). Tolo Lake in western Idaho near the Washington-Oregon border is known for its Quaternary-aged mammoth fossils.