_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.
[Illustration: 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.
[Illustration: Fig. 10.—Ancient landscape on John Day Formation buried
under Picture Gorge Basalt.]