_Volcanic Ash Flow._ The rimrock north of the John Day River between
here and Dayville is a volcanic ash flow that erupted about five million
years ago as red hot pumice highly charged with gas. A rapidly moving
incandescent cloud probably filled the ancient John Day River valley and
deposited ash to a depth of more than 100 feet over a distance of 60 to
70 miles. As shown in Figure 4, the ash flow blankets about 200 feet of
gravels that had been deposited in the valley. The rimrock and the
gravels above and below it constitute the Rattlesnake Formation.
[Illustration: Fig. 4.—View northwest across the John Day River valley
toward Picture Gorge.]