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Conglomerate interbedded with rhyolitic ash, Starlight Formation, The Cove, south of the Blackfoot River, Fort Hall Indian Reservation. This conglomerate was deposited in Neogene braided stream and alluvial fan systems that were periodically choked by great volumes of white ash produced by eruptions on the Snake River Plain, (October, 1987). Basaltic cinder cone south of Grace, formed within the last 100,000 years. Coarse, welded scoria is visible at lower left. This is overlain by channel-fill deposits of lapilli to fine ash, and then by laterally continuous planar beds of lapilli to fine ash. Colluvium and loess form the top of the exposure. Arrow points to 20 cm volcanic block in fine ash bed, (September, 1984).
Spheroidal weathering of the Cub River diabase sill along Cub River Canal, east of Preston. The Cub River diabase is of Miocene age and was intruded less than 10 million years ago. It intrudes conglomerate of the Mink Creek Member, Salt Lake Formation. Spheroidal weathering is typical of igneous rocks that have regularly spaced orthogonal (right-angle) joints, (September, 1984). Snake River Plain, east of Shoshone, (July, 1991). View looks south at a basalt cinder cone built on a shield volcano, less than 1 million years old. A summer thunderstorm passes in the background.