The Geology of Sand Mountain
"The modern appearance of Sand Mountain Cones is one of surprising freshness and perfection"
-- Dr. Edward M. Taylor, Geology Professor Emeritus,
Oregon State University, as written in The Ore Bin,
Volume 27, No. 7, July 1965.
The Sand Mountain Volcanic Alignment erupted between 3,000 and 3,750 years ago atop the ancient headwaters of the McKenzie River, representing some of Oregon's most recent geologic activity (Benson 1965). The volcanic soils of Sand Mountain serve as a purifying filter for rain and snow melt, which passes directly into the soil, keeping it cold. This water percolates down to the now buried trubutaries of the McKenzie River and re-emerges cold and pure (about 38o on a year round basis) at nearby Clear Lake.
Clear Lake itself was itself formed by a lava flow from Sand Mountain that dammed the McKenzie River. Some of the ancient forest that was drowned in that event 3,000 years ago can still be seen beneath the surface of the deep lake. Preseved by the frigid waters, these ancient trees provide geologists with some of the best known samples ever discovered for carbon-dating: serving as the basis for dating other volcani c activity in Central Oregon.
Formed over a period of several hundred years, the Alignment consists of 23 cinder cones, 42 distinct volcanic vents, and over 3/4 of a cubic mile of lava located in the western half of Santiam Pass near the crest of the Cascade Range in Central Oregon (Taylor 1965). Volcanic ash from Sand Mountain blanketed the terrain toward the northeast, forming numerous deposits of gray-blue "sand."
Great mounds of volcanic ash deposits (like the one in the photo at right) dot Santiam Pass to the east of the Volcanic Alignment. Placed by eddies in wind currents on the leeward side of hi gh points in the Pass, the largest of these ash deposits is 5 air miles away from its source on the neighboring Deschutes National Forest.
It was this volcanic ash for which "Sand Mountain" was named by pioneer travelers along the Historic Santiam Wagon Road (an important pioneer route that passes directly through the Alignment). The term "sand" describes these mounds of volcanic ash adjacent to the Wagon Road, but is really a misnomer for the mountain itself, which is composed of all sorts of volcanic tephra, primarily cinders.
Many names in the area, like "Sand Mountain," were similarly descriptive (i.e. Big Lake and Clear Lake). Interestingly, the names of two nearby features were mistakenly swapped by a cartographer: Hoodo Butte (known for its winter recreation facilities) was supposed to be named "Hayrick Butte" because it resembles a haystack in shape. The ski area's neighboring mountain to the southeast -- a black-cliffed, flat-topped peak -- was actually supposed to be named "Hoodoo Butte." Hoodoo is a Native American term used to describe interesting or imposing rock formations (McArthur 2003).
Whereas Sand Mountain is literally a textbook example of cinder cones forming along a fissure, the mis-named Hayrick Butte is one the best known examples of a "tuya" formation: an eruption that took place during a period of glacial ice. The lava pooled up to the depth of the ice and then spread laterally. This means Hayrick Butte represents the depth of glacial ice in Santiam Pass during an ice age.
Hayrick Butte looms like a table above a nameless lake in Santiam Pass. Its summit blanketed by Sand Mountain ash, wind currents also placed enormous deposits of "sand" on the lee side of this imposing geologic landmark.
The mis-labeled flat-topped peak, like all of Santiam Pass, is blanketed by Sand Mountain ash that has been redistributed by centuries of precipitation, leaving a mixture of soft pockets of ash with occasionally protruding lave outcrops.
Where undisturbed by human activity, the "perfection" of the craters that Dr. Taylor described can still be enjoyed by visitors to the area. Not unlike the Painted Hills -- once trammeled, these beautiful gradations in color in the crater at left can be lost forever. Recent administrative prohibitions on cross country travel are aimed at preserving these treasures. Other parts of the Volcabic Alignment have been hard hit by off-road travel by dirt bikes, quad cycles, and other off-heighway vehicles (OHVs). Thanks to recent protective measures taken by the Forest Service (with your encouragement), cross-country motorized travel is prohibited on the craters of the Aligment during the summer months.
Ground disturbance by snowmobiles during winter months is still serious a problem, however. Sand Mountain is a popular desitnation for "high-marking," a sort of "sport" wherein snowmobiles charge up the outside face of a crater and normally arc to one side or the other as they begin to run out of momentum. This leaves an arch-shaped track (or high-mark) in the snow on the side of the mountain. Unfortunately, not all snowmobiles make the arc , and they often charge headlong off the snow into the super-saturated, bare summit ridge of the crater. This severely impact soils and leads to severe erosion. It can even lead to mass-wasting of soils when spring rains are heavy with associated thunderstorms.
At right the impacts of snowmobile use are clealy visible. According to Special Forest Order #98, snowmobile use is to be prohibited at Sand Mountain where snow depth is less than two feet. This prohibition is routinely ignored, and there is no money to enforce compliance.
While our work preserving historic structures gets a lot more attention, SMS volunteers spend hundreds of hours each summer reclaming soils disturbed by snowmobiles during the winter months. Unfortunately, in spite of our best efforts, we can never return the delicately layered soils of Sand Mountain to their original state. Some of the beauty -- like the soil -- is washed away each year.
If you would like to help us put an end to this annual damage, please contact us. You can make a donation or help on the ground... we need you.