Fig. 10.—Ancient landscape on John Day Formation buried under Picture Gorge Basalt.
P. 22.0
Sunken Mountain. A small landslide in the lower part of the John Day Formation is called Sunken Mountain. When the valley wall was over-steepened by normal stream erosion, the jumbled material in the lower part broke away and slid down from the steep bare slopes above. Absence of tilted trees indicates that the slide is not very active now. The bare “badland” slopes are being eroded by rain wash. The cliffs of the John Day Formation, which the road climbs half a mile farther east, are the result of rapid but normal headward erosion by the creek. Eventually, because of its lower elevation and steeper gradient, this branch of Cottonwood Creek will intercept and behead Deer Creek just east of Hamilton. A photogenic perspective view of the future stream piracy can be seen from the road on the ridge just south of Sunken Mountain, about a mile and a half from the highway.
Q. 26.4
Long Creek Mountain. An uplifted block of Picture Gorge Basalt 1400-1500 feet thick forms Long Creek Mountain; Round Basin is eroded in the John Day Formation on which the basalt rests ([Fig. 11]). The base of the basalt can be seen in road cuts on either side of Basin Creek. The Hamilton fault follows the gulch to the left just below the parking area, goes up the tree-filled gulch across Basin Creek, along the low ridge at the northeast edge of Round Basin, and then along the northern foot of the mountain. The small slab of basalt south of the fault in Basin Creek has been tilted about 10° north by downward drag along the fault. The Hamilton fault system extends about 15 miles farther east.
(In Long Creek turn right—south—on U. S. Highway 395).
Fig. 11.—View of Long Creek Mountain and Round Basin, and diagram of the geologic structure.
