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Section 6, Chapter 20 - Root Hog, Big Butte & Craters of the Moon
Chapter 20:
Geology of the Snake River Plain
Big Southern Butte
Goodale or Jeffrey's Cutoff
Toponce Stage Line and Root Hog
Atomic City
Idaho National Engineering & Environmental Laboratory
Snake River Plain Aquifer
Big Lost River Playas
Lost River Range and the Big Lost River Valley
Craters of the Moon
Morning commuter buses approaching CFA (central Facilities Area) at the Idaho National Engineering Laboratory, 7 a.m., September 15, 1992. East and Middle Buttes rise out of the sagebrush desert in the background. View looks east.
Idaho and Federal Dollars
Map of the east-central Snake River Plain.

Geology of the Snake River Plain
The Snake River Plain contains Pleistocene basalt and interbedded sediment in the upper few hundred feet. The Snake River Plain Aquifer, the key to southern Idaho's agricultural economy, mainly consists of basalts and interbedded sediments, deposited in Pleistocene time (the last 2 million years). The underlying rhyolite does not seem to be a major aquifer because many of the pore spaces are filled with chemical precipitates. Within basalts, permeable zones are mainly the tops and bottoms of lava flows, with columnar jointing providing vertical transmission of water.

Big Southern Butte
The Big Butte, elevation 7,560 feet, is a prominent landmark visible from the entire Pocatello-Fort Hall-Blackfoot-Burley-Arco area. The Butte is a composite rhyolite dome that was intruded about 300,000 years ago and poked through a capping of basalt lava. This uplifted lava is now present on the north and east side of the Butte. For more discussion of the Butte and the surrounding geology see Bonnichsen and Breckenridge (1982), Pierce and Morgan (1992), and Hackett and Smith (1992).