Geologic Units

Quaternary alluvial deposits
Pleistocene silicic volcanic rocks (Yellowstone Group)
Pleistocene basalt lava
Pleistocene and Pliocene stream and lake deposits
Pliocene and Upper Miocene felsic volcanic rocks, rhyolite flows, tuffs, ignimbrites (includes Moonstone rhyolite)
Cretaceous sedimentary rocks
Jurassic sedimentary rocks.  
Triassic sedimentary rocks
Permian and Pennsylvanian sedimentary rocks
Mississippian sedimentary rocks 
Devonian sedimentary rocks
Ordovician and Cambrian 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.

Teton County

Much of Teton County occupies Pierre's Hole, or the Teton Valley, which lies west of the Teton Mountains and northeast of the Big Hole Mountains. The bulk of the county is underlain by Quaternary sediment, and is thus eminently suited for agriculture, which has dominated the economy until the last 20 years when vacation homes and tourism have become increasingly important.

The Big Hole Mountains, in the southwestern part of the county, contain folded and thrust faulted sedimentary rocks, which in Teton County are mostly of Mesozoic age. Coal is found in Cretaceous strata in the Horseshoe basin west of Driggs. In the 1980s wells were drilled for oil in these mountains.

Teton Pass, at the southeast corner of the county, exposes very complex geology where the north dipping Cache Creek reverse fault, that formed in the Paleocene Laramide orogeny, impinged on rocks carried northeastward by the south-dipping Jackson thrust fault of the Idaho-Wyoming thrust belt

Miocene rhyolite of the Heise and Yellowstone volcanic fields occupies the north end of the Big Hole Mountains, and is incised deeply by the Teton River as it tracks westward toward the Snake.

The eastern border of the county, and the state of Idaho, is in the agricultural area west of the Teton Mountains. The Tetons rise dramatically on dip slopes formed from west-dipping Paleozoic carbonate strata. At the top of the Targhee ski area, the rocks are Mississippian. On Table Mountain they are the Cambrian-Archean unconformity, and on the Grand Teton the rocks are Archean gneiss.

P.K. Link, 9/02

Additional Reading

Rocks Rails and Trails: pages
Introduction to the Geology of South & East Idaho
Introduction to Geologic Diagrams
Geologic cross sections
Relations between Geology, Topography and Drainage Systems in Idaho
Drainage and mountain range map for eastern Idaho
Table of Geologic Events in Southern Idaho
The Thrust Belt of Southeastern Idaho
Views of Thrust Faults
Challis Volcanic Group & Intrusive Rocks
References on Idaho Geology
Section 3, History of Eastern Idaho
Section 3, Chapter 8 -"Famous Potatoes" Agriculture & Irrigation
The Teton Dam Disaster, June 5, 1976

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