Page 12
Geologic Provinces along the Oregon Trail
The route of the Oregon Trail was generally followed by the transcontinental Oregon Short Line railroad. This Overland Route traverses several geologic provinces from the Great Plains to the Pacific Ocean.

The Wyoming Rockies and Basins
The country of central Wyoming belongs to the stable North American craton, whose metamorphic and igneous rock foundations crystallized in Archean time more than 2500 million years ago. The topography of this area is controlled by mountain structures made in the Laramide orogeny, 70 to 50 million years ago. Generally, the Laramide orogeny consisted of large uplifts of Archean rocks of the Wyoming province, which punched upward through their thin Paleozoic and Mesozoic sedimentary cover. Examples include the Wind River Range, Owl Creek Mountains, Ancestral Teton Range and Gros Ventre Range. In between the Laramide uplifts, basins subsided, including the Green River and Wind River Basins, which now produce petroleum and natural gas.

Folds in Jurassic rocks (mostly Twin Creek Formation) in the Crawford thrust sheet just west of Geneva, Idaho. Aerial view looks north, U.S. Highway 89 follows the transverse valley in middle distance, (august, 1984).

Idaho-Wyoming Thrust Belt
Near Fort Bridger in southwestern Wyoming the Oregon Trail entered the Idaho-Wyoming thrust belt. Today the thrust belt contains vast, largely unpopulated, forested mountains. The area is underlain by a thick succession of sedimentary rocks that was deposited off the subsiding edge of North America in Proterozoic, Paleozoic, and Mesozoic time and then compressed, folded, and thrust as much as 100 miles eastward toward the continent during the Sevier orogeny (about 130 to 55 million years ago in late Mesozoic and early Tertiary time). The Idaho-Wyoming thrust belt is just one segment of the Cordilleran thrust belt that contains folded and thrusted sedimentary rocks along the North American Cordillera from Alaska south to Mexico.

Basin and Range
At Bear Lake Valley the Oregon Trail entered the Basin and Range Province, an area affected during the last ten million years by extensional forces, creating uplifted ranges and downdropped valleys bounded by normal faults. The Basin and Range Province overlaps the Idaho-Wyoming thrust belt in the area from Montpelier to Pocatello. It extends northeast to Jackson Hole, west across southern Idaho to the Albion Range and north across the Snake River Plain to east-central Idaho east of Mackay.

North of the Snake River Plain in east-central Idaho is the northern part of the Basin and Range Province, with the huge Lost River, Lemhi, and Beaverhead Ranges separated by the sparsely settled and arid Lost River, Little Lost River-Pahsimeroi, and Birch Creek Valleys.

Gateway to the Snake River Plain
Along Interstate 15 and U.S. Highway 30, and the Union Pacific Railroad line, Portneuf Narrows serves today as the gateway from the Basin and Range to the Snake River Plain. Pocatello, at the mouth of Portneuf Narrows, is thus located at the junction between the Basin and Range, Thrust Belt, and Snake River Plain.  This change in geologic structure is a fundamental cause for the major topographic, climatic, and cultural change at Pocatello, from the agricultural valleys, flanked by tree-covered mountains, of southeastern Idaho, to the open, windswept, irrigated desert of the Snake River Plain.

Snake River Plain Columbia Plateau Province
The Snake River Plain-Columbia Plateau Province is a vast, relatively flat, but dry and rough area of basalt and rhyolite lava erupted in the last 17 million years and today cut by deep canyons.

Formation of the Snake River Plain began with inception of volcanic activity of the Snake River Plain-Yellowstone Hot Spot in southwestern Idaho about 17 million years ago. The origin of this volcanic activity is not well understood, but is probably related to a plume of heat from the Earth's mantle (a "Hot Spot"), which melted the lower crust and produced the lava which now covers much of southern Idaho.

South-Central Idaho and the Idaho Batholith Country
West of the Lost River Valley is the complex area of the Pioneer, Boulder, Smoky, and Salmon River Mountains. Much of this country has potential for mineral resources contained in Paleozoic black shales, and much of it is largely untrammeled wilderness. The Copper Basin and Wildhorse Creek areas on the east side of the Pioneer Mountains contain some of the most beautiful mountain country in Idaho. In the high Pioneers, east of Ketchum, Early Proterozoic metamorphic rock is exposed, in a "metamorphic core complex", which contains an uplifted part of the middle continental crust. West of the Big Wood River and north of the Salmon River, much of the country is underlain by granitic rocks of the Idaho batholith, which were intruded in Cretaceous time, and have since been uplifted by several miles. Eocene volcanic rocks of the Challis Volcanic Group cover much of the eastern and southern sides of the batholith.