OBSIDIAN CLIFF
The most famous obsidian
exposure in North America is Obsidian Cliff on the northern side of
Yellowstone. The cliff itself is rhyolitic obsidian with obvious columnar
jointing (referred to by geologists as a vertical colonnade). Large boulders
of obsidian litter the base of the cliff. Park administrators don't want
people walking among the boulders anymore, but they can still be closely
examined alongside the road. The rocks here are part of the
Pleistocene-aged Roaring Mountain Member (Plateau Rhyolite).
The pic above shows the
typical character of the boulders at the base of Obsidian Cliff. The
obsidian has nice flow banding, with whitish spotting (“snowflake obsidian”),
the result of devitrification. A little south of here, the obsidian
eroding from the cliff above lacks the devitrification spots. Yellowstone
obsidian is of such high quality that early American Indians used it for making
various implements. Archaeological artifacts demonstrably composed of
Yellowstone obsidian have been found throughout America.