Close up outcrop view of Mesa Falls ash-flow tuff and underlying fa...

In this photograph, the Mesa Falls ash-flow tuff is easily distinguished from the lighter-colored, underlying, fallout ash. This area was near the depositional margin of the ash flows and consequently the ash-flow tuff is nonwelded and contained abundant large pumice blocks. The tuff was deposited during a large eruption that produced the Henrys Fork caldera southwest of Yellowstone National Park 1.2 million years ago. Photograph by Dan Dzurisin.