Wy’east Engineering

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Why Wy’east?

The Geology

 

The Cascade Mountain Range is, by geologic standards, quite young.  As a result, much of the vulcanism that shaped the present configuration has occurred during the time of human habitation, which is estimated to be the last 10,000 to 50,000-years.  Between what are now the states of Oregon and Washington flows the Columbia River, one of the four largest rivers in the United States.  Approximately twenty miles south of the River is Mt. Hood, at 11,245-feet the highest point in Oregon.  Thirty miles north of the River, set among rugged foothills, is Mt. Adams, at 12,286-feet the second highest point in Washington.  Also in Washington, but thirty miles west of the main range, is Mt. St. Helens.  Mt. St. Helens was, for most of the last 6,000 to 8,000-years, a perfectly symmetrical cone rising 9,700-feet above sea level.  At the north foot of Mt. St. Helens may be found Spirit Lake.  Formed by mud flows that blocked the Toutle River near its source, Spirit Lake is extremely deep, plumbing several hundred feet only a few hundred feet from shore.  In May, 1980, Mt. St. Helens erupted in a pyroclastic, or combination volcanic ash, mud and gas flow that entirely destroyed the top 1,300-feet of the mountain.  New mud flows blocked the outlet of Spirit Lake to the Toutle River and raised the lake level by approximately 200-feet.

 

This type of eruption was typical of the Cascade stratovolcanoes, of which Mt. Hood, Mt. Adams and Mt. St. Helens are excellent examples.  Evidence of an eruption similar to that of Mt. St. Helens may also be seen on Mt. Hood’s south side where the entire south caldera wall has been destroyed leaving Steels Cliff on the east, the Summit Ridge on the north and Illumination Rock and Cathedral Ridge on the west.  Although Mt. Adams does not presently show such dramatic evidence of its violent history, there is enough evidence to show a similar formative process.  Since none of the volcanoes are extinct, eruptions such as that of Mt. St. Helens cannot be ruled out at some future date.  Even now there are periodic bursts of gas and steam, sometimes accompanied by mud flows, on Mt. Hood.

 

At one time, where the Columbia River cuts through the Cascades, there was what could be termed a bridge although its exact configuration is unknown.  Geologic evidence does show, however, that a landslide occurred dropping portions of Table Mountain into the Gorge and blocking the river to a height of approximately 200-feet.  This landslide was subsequently breeched by the river. This landslide occurred in approximately 1100-AD followed by massive landslides that occurred in 1700-AD in response to a Magnitude 8.5-9.2 earthquake.  This structure formed a massive dam above the location of the present Bonneville Dam and, when breached by the River, formed a series of small falls and large rapids, called cascades, that eventually became the name of the mountain range as well.

 

On either side of the River, massive slide scars may be seen on the mountains near Cascade Locks, Oregon and Stevenson, Washington.  Until inundated by the pool behind Bonneville Dam, the cascades themselves could be seen along with a completely submerged forest that was flooded by the damming of the River when the bridge collapsed.  As can be seen by comparing the legend and the geology, it is apparent that great geologic events have occurred in this region during the time that man has resided here.  Since most of us can remember the eruption of Mt. St. Helens in May, 1980, which was relatively minor in geologic terms, one can only imagine the excitement and terror that must have accompanied the events surrounding the “Bridge of the Gods.”

 

Author’s Note:

 

The legend presented here is the version told to the author as a boy in much the same manner it was passed down through generations of Indians.  There are, in fact, a number of variations of this legend.  Some of these may be found in such works as “Wy’East - The Mountain” by Fred McNeil, “Mt. Hood” by Don and Roberta Lowe and “Columbia River” by William Lyman.  One of the best accounts of the geology of the region may be found in “Fire and Ice” by Steven Harris, as well as numerous news articles in the Portland Oregonian following the eruption of Mt. St. Helens in 1980.

 

Douglas R. Allen, PE

 

Wy’east Engineering

Salinas, California

©January 2012

©2012 - Wy’east Engineering

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