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Geologic Setting of the John Day Country, Grant County, Oregon


Differences in the rates of erosion of various rocks were as important as folding and faulting in forming the topographic features in the John Day Country. The chief agents of erosion are chemical weathering processes that eventually break down or decompose all rocks, and running water which carries away the weathered or partly weathered and broken material. Picture Gorge is narrow because the basalts in the walls are very resistant to weathering and break along vertical joints into large blocks that do not move easily. In contrast, the John Day River has cut a broad, flat-floored valley in the Mascall Formation at the south end of Picture Gorge because the ashy beds weather to fine clay which is easily washed away.

Landslides have marked the face of the John Day Country. Large rock masses commonly slide where steep cliffs form in soft rocks that are capped by hard resistant rocks. When the cliff face becomes too high and steep, the soft beds give way and large blocks or masses slide downward, usually tilting backward as they move, as shown in the diagram of Cathedral Rock on page 15. Cathedral Rock and two small landslides north of the river, about two miles east of John Day, show this classic form. Many landslides, however, are just jumbled, hummocky masses of slumped material. Large-scale landsliding in the John Day Formation under the Picture Gorge Basalt is colorfully displayed along the river for 8 miles north of Picture Gorge. Some landslides occur suddenly, but many move forward only as fast as the toe or lower end is eroded away, as at Sunken Mountain.

Glaciers have sculptured the principal valleys that lie above an altitude of about 5,000 feet. Moving ice, hundreds of feet thick, plucked out semi-circular amphitheaters called cirques at the heads of the valleys. The 2,000-foot cliffs above Little Strawberry Lake were formed this way. Rocks held in the ice, like the teeth of a giant rasp, ground off irregularities and widened the valley bottoms to a broad U shape as the glaciers moved down the valleys. The effectiveness of ice scour can be seen by comparing glaciated Strawberry Creek above Strawberry Lake (as shown in figure 15 on page 22) with unglaciated Picture Gorge or with Canyon Creek just above Canyon City.

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Last Updated: 28-Mar-2006