Basalt Intrusion. The road is cut through 300 feet of basalt intruded into the John Day Formation. Both contacts are well exposed. The basalt—when molten—baked and reddened 3 to 6 inches of the adjacent beds.
(At 13.9 the highway crosses the North Fork of the John Day River and follows the Cottonwood Creek Valley.)
N. 18.0
Irregular Dikes. Above the highway several small irregular basalt dikes cut the white beds of the John Day Formation. Parts of the largest dike are 10 to 15 feet high.
O. 20.1
Cottonwood Creek Valley. The view northwestward down Cottonwood Creek toward Monument ([Fig. 9]) exemplifies the development of broad valleys by erosion in soft beds of the John Day Formation under the gently warped Picture Gorge Basalt. To the south, the valley ends against massive rocks of the Clarno Formation which were raised as a block by movement along the Hamilton fault. The red beds are in the lower part of the John Day Formation. Note the contrast between the irregular massive intrusion in the valley bottom west of Monument and the thin regular basalt flows.
Fig. 9.—Northwest view down Cottonwood Creek toward Monument.
On your right, the irregular contact between the John Day Formation and Picture Gorge Basalt reveals an ancient landscape buried under lava flows ([Fig. 10]). Some of the flows wedge out against former hillsides and one, marked by the dry falls, fills an old valley.
For the next two miles, to the Sunken Mountain viewpoint, the road winds through landslides in the John Day Formation and Picture Gorge Basalt.