How the Farallon Plate helped to Create Mission Ridge

The next time you ski or snowboard at Mission Ridge, you can thank plate tectonics and an oceanic plate called the Farallon Plate for creating this winter playground. Take a look out the window from the Hampton Lodge and you’ll see what could be the oldest rocks of Squilchuck Valley: a wide swath of massive rock, a  sedimentary arkose structure possibly 144 million years old. This remnant could be from an earlier mountain that formed before our modern Cascade ranges, quite possibly due to activity from the subduction of the Farallon Plate beneath the North American Continent toward the end of the Cretaceous time. As you leave Mission Ridge, you’ll see evidence of other geological time periods besides the Cretaceous-Paleocene including Miocene and Pleistocene-Holocene.
 
To show the age spread of the valley, I visited three sites, each representing a different time period, where I took photos and collected rock samples. 1. Holocene Site near Squilchuck State Park, 2. Cretaceous-Paleocene rock outcrop seen from Mission Ridge base, 3. Miocene Rock outcrop at corner of Pitcher Canyon and Squilchuck Road.

Background
The Squilchuck Valley is situated in a canyon south of Wenatchee up Squilchuck Road to Mission Ridge and the visible geology ranges in age from Cretaceous to Holocene. The area is dominated by a narrow valley created by a radial river system that is now a creek, draining from the higher elevations. The low elevations are sparsely vegetated hills of mostly sagebrush and yellow Balsamroot, which blooms in spring, covering the hills in yellow. The hillsides also are dotted with grids of orchards. Low hills undulate up the canyon, until high mountains are covered with Ponderosa Pines as you reach the base of Mission Ridge at an elevation of around 4,800 feet and they continue up to 6,600.
 
The valley is interesting because a whole different world exists at the top, especially in winter where it can be snowing at the top while just a few miles down the road below it can be dry. At a point commonly referred to as the "sand pit" there is a definite demarcation where the terrain and vegetation changes: as you look south up the valley, suddenly the hills are steeper, sharper and covered in the Ponderosa's whereas below, the hills are more rounded and shorter, and are sparsely covered. Apparently the climate toward the top is wetter than below due to the terrain and geology.
 
The valley has many interesting geological formations, including rock outcroppings and several huge columns of rock high up on the hills. Several high hillsides show layers that look like reddish brown stripes alternating with grey and tan stripes. Near Squilchuck State Park, at my primary field site, there is an exposed hillside near the road having a stratigraphic sequence of clastic sedimentary rocks consistent with a large streambed having once filled the valley. Today a much narrower Squilchuck Creek meanders down the valley from Mission Ridge, where it empties into the Columbia River.
 
 
Hypothesis

My theory is that the valley’s present shape was influenced early in the Cretaceous period when the Farallon plate was subducting beneath the North American Plate at an angle that is believed to have caused the Laramide and Sevier Orogenies, which lie (quite a bit) farther East of Mission Ridge. While to us these geological formations seem too far away to have any relationship to our area, in geologic time and space, it’s possible the rumbling of the Farallon plate rolling under the area from west to east, where Mission Ridge currently exists, pushed up rocks there too, before it headed toward the Laramide and Sevier area.
 
Based on information provided by the USGS geologic map from a US Geological Survey, the older rocks at Mission Ridge are of the paleocene-cretaceous age (65 to 144 million years old), in  the Mesozoic era. I believe the large outcrop visible from the lodge at Mission Ridge is from this paleocene-cretaceous age, an older remnant of land that existed prior to the Cascade range, and prior to the Columbia River flood basalts, which formed during the Miocene.

According to the US Geological survey map, the primary rock type is arkose with a secondary type of shale. The tertiary rock is congolomerate, coal, siltstone and tuff. Arkose is a sedimentary rock primarily of sandstone with at least 25% feldspar, and it usually forms from "the weathering of feldspar-rich igneous or metamorphic, most commonly granitic rocks, which are primarily composed of quartz and feldspar.... The sediments were deposited rapidly either in a cold or arid environment such that the feldspar did not undergo significant chemical weathering and decomposition." 

According to R. Dawes, lecture 8. "The Sevier orogeny involved layers of sedimentary rock in the upper crust." Thus this older rock at the peak of Mission Ridge could have been uplifted like the Sevier orogeny was.

Paleocene-Cretaceous nonmarine rocks at top of Mission Ridge

The boulders seen at the top of Mission Ridge, (above) looking southeast, are of the Paleocene-Cretaceous nonmarine group. According to the Geological Map of Washington and the USGS Mineral Resources On-Line Spatial Data, this area has been identified to exist in the aqua color and would be from 65 million to 144 million years old. The primary rock type is arkose. The formations are possibly Chuckanut, Guye, Pipestone Canyon, Swauk and Chumstick. During this period, the Laramide and Sevier Orogenies were being formed, due to the angle of the Farallon plate subducting beneath the North American Continent. It’s possible that the top of Mission Ridge was formed during this time by similar forces.
 
And today, thousands of snowboarders and skiiers can enjoy the thrill of flying down the many hills at Mission
Ridge.
 

Miocene volcanic rocks