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Fig. 2.—View of Mount Vernon Butte and diagram of faulting along its south slope.

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.

Fig. 4.—View northwest across the John Day River valley toward Picture Gorge.

E. 126.0

Picture Gorge. Visible to the left of Picture Gorge, from north around to west, in order of their deposition and geologic age from oldest to youngest, are the Picture Gorge Basalt, Mascall Formation, and Rattlesnake Formation (Fig. 5). The Picture Gorge Basalt flows and ashy beds of the Mascall Formation were tilted southward together and eroded before the Rattlesnake Formation was laid down horizontally across them. Picture Gorge and the present valley were then cut by the John Day River after the Rattlesnake Formation had been tilted in its turn. The five benches or terraces on the basalt just east of Picture Gorge mark temporary halts in downcutting of the John Day River.

F. 122.0

Thomas Condon Viewpoint, John Day Fossil Beds State Park. From Picture Gorge to the cliffs opposite, the Picture Gorge Basalts and varicolored ash beds of the John Day Formation rise together nearly 2000 feet. The basalt that caps Sheep Rock is an erosional remnant. The lower beds in the John Day Formation are colored red by clay eroded from thick soil on the Clarno Formation, which is exposed in the farthest red hill. The soil was formed by tropical weathering some 30-35 million years ago, before the John Day Formation was laid down. Faulting on a small scale is illustrated in Sheep Rock, where the thick olive-drab ash flow in the middle of the John Day Formation is offset 75-100 feet (Fig. 6). The fault slopes about 45° eastward. About two miles downstream, large-scale movement on two faults has dropped the basalt flows in Middle Mountain 2000-2500 feet. One of the faults follows along the upstream base of Middle Mountain (Fig. 6).

Fig. 5—North-south section along the John Day River through Picture Gorge and Middle Mountain.

Fig. 1.—Geologic map of the John Day Country showing log route.

[Higher-Resolution Map]

G. 118.7

Munro Area, John Day Fossil Beds State Park. The valley of the John Day River has been widened to nearly five miles by erosion in the John Day Formation. Large tilted slide blocks of the John Day Formation and basalts jumbled together show how important landsliding of soft beds under hard rocks can be in widening valleys.

Fig. 6.—Sheep Rock from Thomas Condon viewpoint.

Here one can appreciate the regularity and extent of basalt flows of the flood or plateau type, which form the Columbia Plateau. Individual flows have been traced 100 miles. Travelers will see few other rocks between here and The Dalles, Wenatchee, Pendleton, or Spokane as they cross parts of the Columbia Plateau.

H. 116.2

Cathedral Rock. The bluff called Cathedral Rock is the front face of a large block of the John Day Formation that has slid from the west (Fig. 7). Inside the next horseshoe bend downstream a large mass