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Geologic Units

Quaternary alluvial deposits
Quaternary surficial cover, fluveolian cover on Snake River Plain, alluvial fans (Snake River Group)
Quaternary windblown deposits, dunes, loess
Pleistocene silicic volcanic rocks (Yellowstone Group)
Pleistocene basalt lava
Pleistocene and Pliocene basalt lava and associated tuff
Pliocene and Upper Miocene felsic volcanic rocks, rhyolite flows, tuffs, ignimbrites (includes Moonstone rhyolite)
Pliocene and Upper Miocene stream and lake deposits (Salt Lake Formation, Starlight Formation, Idaho Group)
Eocene granite, pink granite, syenite, rhyolite dikes, and rhyolitic shallow intrusive
Pleistocene and Pliocene stream and lake deposits
Miocene intrusive rock (Salt Lake Formation)
Paleocene and Cretaceous sandstone and conglomerate  (Beaverhead Formation)
Cretaceous sedimentary rocks
Triassic sedimentary rocks
Permian and Carboniferous sedimentary rocks (Snaky Canyon Formation)
Mississippian sedimentary rocks 
Ordovician and Cambrian sedimentary rocks
Devonian, Silurian, and Ordovician sedimentary rocks
Neoproterozoic sedimentary rocks
Jurassic sedimentary rocks.  
Permian and Pennsylvanian sedimentary rocks

  Map Key
Geologic units with unit designation.
Normal Fault: certain; dashed where approximately located; dotted where concealed.
Thrust Fault: certain; dashed where approximately located; dotted where concealed.
Detachment Fault: certain; dashed where approximately located; dotted where concealed.
Interstate Route.
U.S. Route.
State Route.
Location of Rockwalk rock from the county.
Feature location.

Clark County

Clark County, one of the least populated of Idaho's 44 counties, lies at the foot of the continental divide, and includes the cold, arid northeastern Snake River Plain. Quaternary basalt covers much of the area from Dubois north to Spencer, and supports sparse sagebrush grazing land. Alluvial areas are farmed with irrigation from the Snake River aquifer.

Miocene and Pliocene felsic volcanic rocks, associated with the Heise and Yellowstone volcanic fields, occupy the area east of Spencer, and host Idaho opals. These formed when hot silica-rich water circulated through the young rhyolite lavas and tuffs.

The Centennial Mountains, in the northeastern corner of the county, along the border with Montana, contain folded and thrust faulted Mesozoic and Paleozoic sediments, which lie above Paleoproterozoic basement rock. They belong to the Laramide province, where Mesoproterozoic and Neoproterozoic strata were never deposited beneath the basal Cambrian sandstone.

West of Spencer are the Beaverhead Mountains, divided by the Medicine Lodge Creek from the Tendoy Range to the east along the Continental Divide with Montana. Limestones and tuffs of the Oligocene and Miocene Medicine Lodge Formation occupy the foothills on both sides of Medicine Lodge Creek. The Beaverhead Range extends north to Salmon and merges with the Bitterroot Range in southwest Montana, north of Lost Trail Pass. The range contains a thick package of folded and thrust faulted sedimentary rocks, including Mesoproterozoic Belt Supergroup (Apple Creek, Gunsight and Swauger formations), Neoproterozoic Wilbert Formation, and Paleozoic limestone and dolomite.

The Tendoy Range is capped by the Cretaceous and Paleocene Beaverhead Conglomerate, which was shed from rising mountains to the north and west. Large rivers carried boulder conglomerate east to the Tendoy Range and much of southwest Montana. These quartzite clasts spread east to Jackson Hole, Wyoming and are now making their way back west again, carried by the Snake River.

On the extreme southwest corner of the county is the dry Birch Creek valley and the southeast corner of the Lemhi Range. An active normal fault, the Beaverhead Range fault, bounds the east side of the Birch Creek Valley against the Beaverhead Range. Paleozoic carbonate rocks are exposed here.

Additional Reading

Rocks Rails and Trails: pages
The Snake River Plain-Yellowstone Hot Spot and its Effect on Drainage Patterns
Challis Volcanic Group & Intrusive Rocks
References on Idaho Geology

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