Photograph of the tephra layers of Blind Spring Valley (massive white beds in foreground) in Pit 6 of the Cowan Pumice Mine, Blind Spring Valley, Calif. View to southeast, looking across Blind Spring Valley to Blind Spring Hill in the middle distance and the White Mountains in the far distance. USGS photo by A.M. Sarna-Wojcicki.
Numerical ages have been determined for a stratigraphic sequence of silicic tephra layers exposed at the Cowan Pumice Mine in Blind Spring Valley, near Benton Hot Springs, east-central California, as well as at Chalk Cliffs, north of Bishop, Calif. The tephra layers at these sites were deposited after eruptions from nearby sources, most of them from near Glass Mountain, and some from unknown sources. The ages were determined primarily by the laser-fusion 40Ar/39Ar method, mostly on sanidine feldspar; two were determined by conventional K-Ar analysis on obsidian clasts.
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