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]]