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Tuesday, April 3, 2007

TABLE MOUNTAIN: Columbia River Gorge Geology

In a previous blog entry, Bridge of the Gods (http://thenarrativeimage.blogspot.com/2007/03/bridge-of-gods.html), I noted that some geologists have posited a link between a landslide that likely occurred in the 1200s and local myths/legends describing a land-bridge across the Columbia.

Current, apparently misnamed, Bridge of the Gods.

John Eliot Allen, author of The Magnificent Gateway, writes, “The lobe of the latest (“Cascade”) slide covers about 5 ½ square miles. It diverted the river a mile to the south, and contained a dam long enough, in all probability, to give rise to the Indian legend of the “Bridge of the Gods”

Mr. Allen describes the unstable geologic situation like this. “Heavy Grande Rhone Basalt-flows cap Greenleaf Peak and Table Mountain, resting upon 1000 feet of weak, clay bearing Eagle Creek sediments.”

The trail I took to Table Mountain starts at the Bonneville trail head. It’s on the Washington side of the river across the highway from the Bonneville Dam’s visitor center parking lot. You can pick up a parking/forest pass at a Chevron station a couple miles west of the trail head (Five dollars for a day pass).

At the east end of the trail-head parking lot, find the Tamanous trail sign and proceed about a half a mile until you merge with the Pacific Crest Trail. A sign at the intersection almost counter intuitively points North and South. Go north, ultimately, along the Pacific Crest trail, though it will feel an awful lot like you are really going west. Whenever you come to an intersection, be sure you see the little metal Pacific Crest Trail diamonds posted in the trees when you resume your walk or you may be off track.



Heading down the trail toward Gillette Lake, I noted that were it not for the moss, you would think the landscape was still rolling. While some carbon dating results put the landslide event at about 1260 A.D., other geologists have attempted to date the slide by measuring lichen growth.


Gillette Lake has some nice camping sites and appears to periodically host beavers. There are remnants of lodge and dam building activities. I’m inclined to describe the color of Gillette lake as a green not found in nature, but that apparently can’t be accurate.


A little bit past Gillette Lake, the trail opens up onto what may be an old logging road and a clear cut area that affords a good view of the sheared away face of the mountain. This image shows Table Mountain as it appeared this year on March 3rd (left) and again on April 1st (right).



A former trail to the top of Table Mountain has been closed due to erosion and other environmental concerns. A posted sign encourages you to continue another half mile to this newer access point. If the trail so far has been a medium ‘5’, it is about to become a “9” and for old out of shape fat men, maybe an “11”.


Later on, there will be the additional psychological challenge of exposure to sheer drop-offs. I had to shield my eyes with my hands – like blinders – to keep focused on where I was placing my feet.



From this vantage point, you can see how the river bends to the south behind Bonneville Dam.


From the summit, looking east up the Columbia River gorge.


This ridgeline leads to the top (The point) of the sheer face of Table Mountain



In retrospect, I’m not sure it is a good idea to get this close to the edge of an unstable mountain with a demonstrated propensity to slide.



I thought it was hard going up the mountain, but my knees had a surprise for me when I started back down. Fortunately, I got off the rocky dangerous part well before dark. Limping past Gillette Lake on the way out, I was thankful I had my flashlight and my bag of survival gear – just in case (about 15 1/2 miles round trip).

2 comments:

  1. Breathtaking, Scott.

    And I love the way you showed the difference a month makes on Table Mountain.

    ReplyDelete

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