Part 3

For the next two miles, to the Sunken Mountain viewpoint, the road winds through landslides in the John Day Formation and Picture Gorge Basalt.

P. 22.0

Sunken Mountain. A small landslide in the lower part of the John Day Formation is called Sunken Mountain. When the valley wall was over-steepened by normal stream erosion, the jumbled material in the lower part broke away and slid down from the steep bare slopes above. Absence of tilted trees indicates that the slide is not very active now. The bare “badland” slopes are being eroded by rain wash. The cliffs of the John Day Formation, which the road climbs half a mile farther east, are the result of rapid but normal headward erosion by the creek. Eventually, because of its lower elevation and steeper gradient, this branch of Cottonwood Creek will intercept and behead Deer Creek just east of Hamilton. A photogenic perspective view of the future stream piracy can be seen from the road on the ridge just south of Sunken Mountain, about a mile and a half from the highway.

Q. 26.4

Long Creek Mountain. An uplifted block of Picture Gorge Basalt 1400-1500 feet thick forms Long Creek Mountain; Round Basin is eroded in the John Day Formation on which the basalt rests (Fig. 11). The base of the basalt can be seen in road cuts on either side of Basin Creek. The Hamilton fault follows the gulch to the left just below the parking area, goes up the tree-filled gulch across Basin Creek, along the low ridge at the northeast edge of Round Basin, and then along the northern foot of the mountain. The small slab of basalt south of the fault in Basin Creek has been tilted about 10° north by downward drag along the fault. The Hamilton fault system extends about 15 miles farther east.

(In Long Creek turn right—south—on U. S. Highway 395).

R. 99.5

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.

S. 180.3

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.

Strawberry Lake and Vicinity. At Strawberry Camp, about 12 miles south of Prairie City, the broad floor and steep walls of Strawberry Creek valley indicate that the valley has been glaciated. The precipitous cliffs and rounded valley bottom above Strawberry Lake are characteristic of glaciated mountains (Fig. 15). Strawberry Lake is dammed by landslides which probably came from the west wall of the valley after the glacier melted and left the valley wall over-steepened. The hummocky surface and blocky material in the slide are well shown along the last half mile of the trail to Strawberry Lake. Strawberry Falls mark the front of a glacial step over a massive flow of platy andesite. Little Strawberry Lake is dammed by a low glacial moraine.

Most of the lavas in the Strawberry Mountains were erupted from a central vent about 4000 feet in diameter which is exposed in the cliffs above Little Strawberry Lake. The pinnacles known as “Rabbit Ears,” above the prominent talus in figure 15, are of vent breccias that consist mostly of welded blocks of scoriaceous basalt, but also contain volcanic bombs which were blown out as blobs of fluid lava. Huge blocks of the breccia have fallen onto a gentle bare slope west of Little Strawberry Lake. The massive, vertically-jointed cliffs are formed of basalt which cooled slowly and formed a plug in the throat of the volcano after the eruptions ceased. The thin irregular scoriaceous andesite flows, which are exposed in the cliffs east of Little Strawberry Lake adjoining the plug, contrast strikingly with the massive even flows of the Picture Gorge Basalt.

Tilting of the Strawberry Mountain block is shown by the southward dip of all the flows in the area. The flows in the cliffs west of Strawberry Lake, for example, originally must have sloped northward away from the vent where they erupted. Their present southward dip of about 15° therefore indicates that they have been rotated more than 15° by faulting, partly along the northern edge of the mountain range. (Fig. 14 ).

Selected References To Detailed Reports

U. S. Geological Survey Maps: No. 1-447 Geologic Map of the Canyon City quadrangle, northeastern Oregon, by C. Ervin Brown and T. P. Thayer, 1966. The map covers the entire region at a scale of one inch equals 4 miles. GQ-438 Geologic Map of the Aldrich Mountain quadrangle, Oreg., (with text) by T. P. Thayer and C. Ervin Brown, 1966. GQ-548 Geologic Map of the Mount Vernon quadrangle, Oreg., (with text) by C. Ervin Brown and T. P. Thayer, 1966. MF-51 Preliminary Geologic Map of the John Day quadrangle, Oreg., by T. P. Thayer, 1956. U. S. Geological Survey Professional Papers: No. 550-C Local thickening of basalts and silicic volcanism in the Canyon City quadrangle, Oreg., by T. P. Thayer and C. E. Brown, pages C73-C78, 1966.

(From material provided by Thomas P. Thayer)

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Transcriber’s Notes

--Silently corrected several typos.

--Provided all images resized and oriented for use on a portable eBook reader.

--Provided clickable links to higher-resolution versions of detailed images.