_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.