The Strawberry Mountain Range and the John Day River Valley
Posted by: Volcanoguy
N 44° 29.899 W 118° 38.404
11T E 369614 N 4928530
One of a series of geologic history signs placed by Grant County in the 1970’s.
Waymark Code: WM2Q2N
Location: Oregon, United States
Date Posted: 12/04/2007
Views: 102
Sign about the Strawberry Mountain Range and the John Day River Valley along U.S. Hwy 26 at east of Prairie City. Sign has been relocated here from it’s orginal location. Sign is very faded with bullet scars.
Marker Name: The Strawberry Mountain Range and the John Day River Valley
Marker Text: Strawberry Mountain, 9038 feet above sea level, is the highest peak in the Strawberry Range. For about 50 miles, from here to Picture Gorge, the Strawberry and Aldrich Mountains stand about 5000 feet above the John Day River valley. The Strawberry Range was uplifted as a tilted block by vertical movements mostly on the John Day fault, which follows the base of the mountains. The rocks in Strawberry Mountain and to the east are volcanic; the layers are ancient lava flows. The rocks to the west consist of much older peridotite and gabbro which were intruded from great depth, like granite. A fault across the mountain range separates the two kinds of rocks. The John Day River valley is a downwarp between the Strawberry Range to the south and the hills to the north.
The valleys in the higher parts of the range, above about 5000 feet, were widened from the narrow V's to their broad U profiles by glaciers during the Great Ice Age, which ended about 12,000 years ago. The alluvial fans or pediments in front of the mountains are composed 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.
The Strawberry Volcanics were erupted from several volcanoes south and east of Strawberry Mountain. When the eruptions ended about 10 million years ago, this region probably resembled the eastern side of the Cascade Mountains between Mount Jefferson and Crater Lake. The folding and faulting which raised the Strawberry Range and the erosion which scuplured it destroyed the old volcanic cones, but a large volcanic plug is exposed above Strawberry Lake.
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