Pretty much what it says on the tin
Fumaroles cut through the ash fall of the Mt. Mazamas eruption
According to the sign, the streambed was buried in several hundred feet of ash and pumice during the Mt. Mazama eruption about 7,700 years ago. Water trapped at the bottom was boiled to steam and forced its way up through the ashfall, leaving behind fumaroles.
So, yeah. Fossilized steam.
Photos © 2013, Joseph E. Lake, Jr.
This work by Joseph E. Lake, Jr. is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.