_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.
[Illustration: Fig. 13.—Panorama of the Strawberry Range and the John
Day River valley from the north.]
[Illustration: Fig. 14.—Section through the Strawberry Mountain, along
Strawberry Creek.]
[This image in higher resolution]
_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.
[Illustration: Fig. 15.—Strawberry Lake, the glaciated valley of
Strawberry Creek, and cirque walls formed by the Strawberry volcanic
plug.]
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
).