The Snake River Plain west of Twin Falls, Idaho, contains a section of upper Cenozoic volcanic and detrital rocks several thousand feet thick. The rocks are entirely continental but extend below sea level, thereby demonstrating regional subsidence. Based partly on terminology of earlier classifications, these rocks are divided in this paper into four broad units: an unnamed sequence of Miocene age, the Idavada Volcanics, the Idaho Group, and the Snake River Group.

The Miocene rocks include thick lava flows of basalt and layers of rhyolite, interbedded with sandstone, shale, and water-laid silicic pyroclastic material generally altered to yellow, gray, and brown clay. The rhyolite contains conspicuous phenocrysts of quartz, sanidine, and oligoclase, and locally hornblende and biotite. Several thousand feet of this unit is exposed in highlands north and south of the central lowland, where it is cut by gold-bearing veins and considerably disarranged by north-trending faults and folds.

The Idavada Volcanics, locally more than 3000 feet thick, unconformably overlie the Miocene rocks and consist dominantly of nonmineralized silicic flows of welded ash and beds of vitric tuff. They rarely contain quartz or sanidine and contain no hornblende or biotite. The Idavada Volcanics form large parts of the uplands and are broken by northwest-trending faults that define the central lowland. For the most part, this unit is of early Pliocene age in the western Snake River Plain but contains fossils of middle Pliocene age in the eastern Snake River Plain.

The Idaho Group, at least 3000 feet thick, occupies the central lowland, where the successively younger rocks usually lie in troughs bounded in part by the next older rocks. The group is divided, in ascending order, into the Poison Creek Formation (lower Pliocene), the Banbury Basalt (middle Pliocene), the Chalk Hills Formation (middle Pliocene), the Glenns Ferry Formation (upper Pliocene and lower Pleistocene), the Tuana Gravel (lower Pleistocene), the Bruneau Formation (middle Pleistocene), and the Black Mesa Gravel (middle Pleistocene).

The Snake River Group, which crops out mostly in the eastern Snake River Plain, was formed during entrenchment of the present Snake River canyon. The group is subdivided into formations of comparatively local extent, which are from oldest to youngest: the Madson Basalt, the Sugar Bowl Gravel, the Thousand Springs Basalt, the Crowsnest Gravel, the Sand Springs Basalt, the Bancroft Springs Basalt, the Melon Gravel, and Recent basaltic lava flows.

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