Geologic Structure of the John Day River Valley - Oregon Historical Markers on Waymarking.com

Sign about the Geologic Structure of the John Day River Valley along U.S. Hwy 26 between Dayville and Mount Vernon.

Marker Name: Geologic Structure of the John Day River Valley
Marker Text: The large cross section shows the geology of the John Day River valley here. The small diagrams show how the rocks got into their present positions, and the geologic relation of the valley to the mountains.
The light-colored beds in the road cut are waterlaid volcanic ash of the Mascall Formation; some beds are darkened by plant material from ancient swamps. Lava flows, collectively called the Picture Gorge Basalt, form the walls of the valley. Note that the basalt layers north of the valley dip gently southward about like the ash beds in the northern part of the road cut. South of the gap in the middle of the road cut, the Mascall Formation dips steeply northward; the gap marks the position of the John Day fault. Road cuts across the end of the ridge to the south expose more Mascall Formation and the Picture Gorge Basalt, both standing nearly vertically. Fossil leaves, seed pods, and freshwater snails occur in the beds south of the John Day fault.
The Picture Gorge Basalt poured out of fissures as a series of horizontal lava flows about 12-20 million years ago, and the basalt then was buried about a mile deep under ash of the Mascall Formation which was erupted from volcanoes to the south and east. Soon after the eruptions ceased, about 10 million years ago, pressures in a north-south direction compressed the Earth’s crust into east-west folds. When the pressure became too great, the rocks broke along the axis of the sharp fold, and the John Day and Belshaw faults formed.
Erosion stripped the soft Mascall Formation form the higher parts of the rising folds and cut the present flat-floored John Day Valley. South of here, in the Aldrich Mountains, erosion by Fields Creek has exposed older rocks that once were buried many hundreds of feet under the Picture Gorge Basalt.

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