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.
ROAD LOG OF THE JOHN DAY “LOOP”
NOTE: Mileages at lettered stops on the map ([Fig. 1]) refer to nearby mile posts. These are not in numerical order because the route covers parts of four different numbered highways. By taking the tour clockwise, the traveler will be at higher altitudes later in the day.
A. 155.4
Holliday Rest Area. The main John Day fault, which here is buried under river gravel, is believed to be just south of the rest area. A parallel step fault ([Fig. 2]) cuts from left to right across the southern slope of Mt. Vernon Butte about where the juniper trees thin out. Flows of Picture Gorge Basalt in the upper part of the butte slope down to valley level on your left. Rocks in the foreground and to the right are rudely bedded volcanic breccias of the Clarno Formation. The Clarno Formation has been raised as much as 250 feet by movement along the fault to position it against the basalt as indicated in the diagram. The fault can be seen best in afternoon light in the ravine to the right, just south of some brick-red layers in the basalt.
Movement on the main John Day fault to the south appears to have raised the rocks at least 1,000 feet, so the two faults have stairstepped the rock layers. The main John Day fault has been traced about 80 miles.
Fig. 2.—View of Mount Vernon Butte and diagram of faulting along its south slope.
