Friday, January 28, 2011

Twin Sisters Range and Dunite

Twin Sisters Range, Northwest Cascades, Whatcom County as viewed from Upper Samish River Valley
Summit of Mount Baker, not part of the Twin Sisters is on left.

The Twin Sisters are a spectacularly jagged set of peaks rising above the Northwest Cascades approximately eight miles southwest of Mount Baker. The highest peak of the Sisters is South Twin at 7,000 feet. There are several small glaciers on the peaks and they remain snow covered until very late in the summer. Their western position means they get lots of very heavy snowfalls. It should be noted that Mount Baker ski area located northeast of Mount Baker holds the world record one year snowfall record. I suspect that places on the Twin Sisters would beat that record if someone wanted to spend the winter making the official measurements required. 

I once did a presentation on local geology to a group of young girl scouts including my daughter. One of the lessons was "What are the Twin Sisters made of?" Answer "Dunite", "Where did the Dunite come from?" Answer "The mantle of the earth". The range is essentially a huge block of the earth's mantle somehow exhumed and thrust up into the Northwest Cascade Range. Not a common thing. My daughter recently drove a friend from out of state from Seattle to Bellingham and pointed out this tidbit of information when they saw the jagged twin Sisters above the Skagit Flats south of Mount Vernon. Good girl learned at least one thing from her geologist father.
The underlying dunite adds to the sense of height to the range. Note that trees do not extend very far up the lower slopes. Dunite poses two problems for plants, it has almost no phosphorus essential for plant life and a metals ratio that is toxic to most plants.

I took the above picture while heading towards a geology project site in the Samish River Valley. Before investigating the area in question I had a couple of theories regarding a certain land form I had seen in the topography of the area I was working. The presence of dunite boulders in that land form helped in interpreting how the land form came to be. My field work involved lots of steep slope traverses and ultimately a wade up a stream. It would have been a lot more pleasant in the summer.

Taking a break from wading by standing on a boulder of dunite

Typical orange dunite boulder
I broke this dunite cobble in half to reveal the green olivine and black chromite

No comments: