Fig. 3.—View of the John Day fault in Mascall Formation at the mouth of Fields Creek, and its relation to the structure of the John Day River valley.
B. 144.0
Fields Creek Road. In the road cuts just south of the highway, the John Day fault ([Fig. 3]) passes through the Mascall Formation where the slope or dip of the beds changes abruptly from gently southward to steeply northward. The fault continues westward under the floor of the valley. Fossil leaves and snails can be found in the beds south of the fault, and 1,000 feet farther south vertical Picture Gorge Basalt flows are exposed.
To the north across the valley at the White Hills, beds of the Mascall Formation have been dropped down against the Picture Gorge Basalt along the Belshaw fault. The White Hills are a well-known locality for collecting fossil leaves.
C. 138.5 to 135.0
Vertical Ribs. The prominent vertical ribs, visible south of the river in steep slopes below the high bench (pediment), are flows of Picture Gorge Basalt tilted vertically in the north limb of the Aldrich Mountain anticline. The John Day fault follows the base of the steep front in which the ribs are exposed.
D. 132.6
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.
