Forums: Climbing Information: Regional Discussions:
RSS FeedRSS Feeds for Regional Discussions

Premier Sponsor:



Oct 27, 2004, 1:58 PM
Post #1 of 3 (2885 views)

Registered: May 13, 2004
Posts: 422

Geology  (North_America: United_States: Oregon: Willamette_Valley: Flagstone)
Report this Post
Average: avg_1 avg_2 avg_3 avg_4 avg_5 (0 ratings)  
Can't Post

I am taking geology 101 and we just did a few labs on different types of rock. The guidbook says that flagstone is plagioclase andesite, however in my labs the rock at flagstone looks and feels much more like ryolite or a welded ash tuff. Does can anybody confirm what type of rock this is? Andesite and ryolite have the same formation and cooling process, one has just a little more quartz in it than the other. Also, all the ryolite i have seen is greyish and the andesite is often much more black. If anybody knows more about geology than a 101 student please help me out and respond.


Dec 10, 2007, 10:04 PM
Post #2 of 3 (2119 views)

Registered: Feb 13, 2004
Posts: 92

Re: [korntera] Geology [In reply to]
Report this Post
Average: avg_1 avg_2 avg_3 avg_4 avg_5 (0 ratings)  
Can't Post

I thought the same thing when I visually classified the rock at Flagstone.
I called a climber/geologist from the Eugene area and he varified it as and andesite with light feldspar plagioclase.
It would really take a thin section anaylsis. Definitely let me know what you find out.
This would be a great project for a geo-climber. A thin section analysis of climbing areas in western oregon. My hypothesis which is only touch, site, and a drill is that most of the rock climbing in western oregon is primarly on formations that are dacitic.
I'd love to know what others think.


Dec 11, 2007, 9:18 AM
Post #3 of 3 (2099 views)

Registered: Feb 8, 2007
Posts: 403

Re: [korntera] Geology [In reply to]
Report this Post
Average: avg_1 avg_2 avg_3 avg_4 avg_5 (0 ratings)  
Can't Post

I can also verify that Flagstone is andesite. Andesite is usually light grey in the Oregon Cascades, and is the only stuff on the major peaks solid enough to climb.

Andesite, to the layperson (like me) is complex in that it is rhyolite-like.......In GEO101 (U of O) they presented the two rock-types as if they were one in the same, but apparently (of course) it's more complex than that.

The climbing geology of Western Oregon would have been a good research project for a student like 50 years ago, but the outcrops have all been identified by the USGS long ago. The information is all out there already. Greg Orton's books are highly accurate in regard to geology of the crags.

Rhyo-dacite is the very common "Old Cascades" geology. In some areas it is intermingled with Breccia outcrops (very cool, with pockets, but also has rare plants). As you move up the west-cascade slope, Andesites and Basalts outcrops dominate. Of course, Basalts are by far the most common rock-type in Oregon, so you'll find them everywhere. "Flood Basalts" are the only kind worth climbing (Skinners Butte Columns).

Tuff's aren't uncommon (Petrified pyroclastic flow) anywhere from Smith Rock and to the SW, and of course, the Callahans are made of a coarse grained sandstone.

If you were ever to make it into the SW oregon backcountry, you'd have granites, peridotite, and marbles all within walking distance of each other. Cool stuff!

Forums : Climbing Information : Regional Discussions


Search for (options)

Log In:

Password: Remember me:

Go Register
Go Lost Password?
$17.95 (10% off)

Follow us on Twiter Become a Fan on Facebook