RESTING SPRING RANGE
A traditional field trip stop in southeastern California's Resting Spring Range is a nice roadcut of obsidian. The jet black band of rock shown in the pic above is the obsidian. It is highly fractured, and only relatively small pieces can be collected. The rocks above & below the obsidian are rhyolitic welded tuffs. These volcanics are Tertiary in age.
Locality: northern side of Rt. 178, just west of the crest of the Resting Spring Range, east of Shoshone, southeastern California, USA. GPS of cut: 35º 59.830' N, 116º 13.187' W. See map. Note: this black band of obsidian even shows up well on Google Earth.
The photo above shows a nice subvertical fault in a Tertiary-aged succession of volcanics.