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

by Thomas P. Thayer

Fig. 12.—North-south section across Fox Valley, showing the faulted basin structure.

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Fox Valley. Down-warped flows of Picture Gorge Basalt dip toward Fox Valley from all sides to form a basin ([Fig. 12]). The valley is eroded out of ashy beds and gravels of the Mascall Formation which fill the center of the basin to an estimated depth of 1000-1200 feet. Faults form parts of the northern and southern borders of the basin. The straight, timbered, northward-facing steep slope less than a mile southeast of the viewpoint marks a fault.

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Strawberry Range. This range and the Aldrich Mountains form a mountain range 50 miles long; Strawberry Mountain, altitude 9038 feet above sea level, is its highest peak. The eastern two-thirds of the Strawberry Range ([Fig. 13]) was raised as a great block by uplift on the John Day fault, which follows the northern base of the mountains. The rocks in Strawberry Mountain and to the east are mostly lavas which poured out over the land, whereas the Canyon Mountain part of the range consists of gabbro and peridotite which were intruded at great depth, like granite.

The valleys in the higher parts of the range, above about 5000 feet, were widened from narrow V’s to their broad U profiles by glaciers during the Pleistocene Epoch, or Great Ice Age. The alluvial fans (Rattlesnake Formation) in front of the mountains were built up of bouldery gravels and finer sediments. These materials were eroded from the mountains, carried by streams down the steep narrow canyons, and spread out on the valley floor. Because much more material came into the John Day River from the Strawberry Mountains than from the lower mountains to the north, the river was pushed to the north side of its wide valley. Faulting and erosion have completely destroyed the cones of the volcanoes from which the volcanic rocks were erupted in Miocene and Pliocene time.

Fig. 13.—Panorama of the Strawberry Range and the John Day River valley from the north.

Fig. 14.—Section through the Strawberry Mountain, along Strawberry Creek.