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    Northwest Geology Field Trips, by Dave Tucker, is licensed under a Creative Commons Attribution-Noncommercial- Share Alike 3.0 United States License. You can use what you find here, repost it with attribution to the author, "remix" it for your own purposes, but may not use it with the intent of making money off of it.

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This is a collaborative website. If you come across a good geology trip or a place worth sharing with others that is not already posted, drop me a comment and I’ll get back to you. If you have photos, so much the better!

I will try to post a new field trip for you soon. I am running short of places within a couple hours or so from Bellingham [my home] so I will need to range a bit farther afield. In the meantime, the new folks should browse the field trips I offer and get outside! There are a lot of handy tips and links on the website so be sure to check them out. I hiked the Rock Trail today in the Chuckanuts south of Bellingham and it is a great geology trip, as always.



5 Responses

  1. Lake 22 off the Mountain Loop Highway is an interesting hike, and I’m curious about the geology. I’ve tried to decipher what the heck is going on there, and I can’t figure it out.

    Some previously compiled notes below, which is just me trying to put the scraps of info I have together to make some sort of sense of it all. Not succesfully! I’d love a coherent explanation.

    Bottom part of trail is through what must be old lakebed, I think. There is some clay, and fine-grained sediments, and some weird boulders, like a giant red, smooth specimen, that must’ve floated in on an ice floe. All the way up you walk over this slaty-looking stuff — which is maybe the Western Melange Belt schist??? — but at the back of the lake, up against the wall there, you start seeing the granite of the so-called “Pilchuck Stock.” See Mt Pilchuck geology notes, below. You also see, right in and along the trail, lots of pieces of the actual contact rock, which I find thrilling. Little chips of schist in the granite (or tonalite?), or schist invaded by streaks of granite along fractures.

    Mt Pilchuck geology notes
    This is a weird area. Can’t figure this out. Pilchuck’s top is called, in the Sauk DNR document, the “Pilchuck Stock” and is younger than the ridge southeastward, which is Bald Mountain, and is referred to in the USGS document as the “Bald Mountain Batholith.” Evidently a small, discrete, obscure batholith, and not found by that name anywhere on Google! A couple major faults run along its south edge. The stock is related to the Granite Falls Stock (larger) and may be associated with the Hanson Lake rhyolite. Hanson Lake is 5.5 miles SE of Granite Falls.

    This ridge/batholith and the stock are designated Tei, (USGS), Mid Eocene intrusives, granite/granodiorite — old magma chambers. It might be from the Eocene Extensional Event, but there is an alternative theory: that it is the direct product of the subduction of a spreading zone, at least for this batholith: not sure about the stock. A very difficult image to get my head around, and why would this result in a magma chamber? I suppose if the magma coming up at the spreading center intruded into the continental rock as it was subducted, rather than into the ocean? So it emerged at some depth? But then, that stock, if associated with the rhyolite, DID erupt, and so could not have been a subducted anything. Could it? !!!??? Wonderfully puzzling!

  2. I bought your Western WA geology book, that’s the reason I signed up! Read the whole thing and came to Bellingham from Astoria to walk in the muck and barnacles to professionally photograph some stone lace along the waterfront, while my boyfriend preps his tuna boat for the season. I’m really greatful for your book and hope to see the geology exhibit at the University soon for more inspiration for my photography work.

  3. I wonder if you can explain the rock formations at Beverley Beach on the Oregon coast? I”m hoping you’ve been there – I know there are many fossils, but I’m curious about the geometrically shaped rocks near the fossils. There are a few photos of them in this post I did on my blog about an Oregon coast trip.
    Many thanks!

    • bluebrightly, those are wonderful photos at Beverly Beach. I have never been there. However, I am planning to be in SW Oregon in July so will definitely put this place on my list. I took a look at your photos of the geometric features you refer to, and did some research into the geology in the area. Page 83 of Marli Miller’s excellent Roadside Geology of Oregon helped. The rocks are the Miocene Astoria Formation, which was originally near-shore marine sediments, now lithified into sandstone and shale. The patterned rocks you show in your blog look like they may have originally been mud. Now I’m going to wave my arms and make a hypothesis. The meandering, non-parallel cracks could have formed in the fine-grained sediment as it was uplifted and dried out, like the cracks we can see in dry lakebeds. There are also prominent parallel horizontal fractures, which cut across the ‘mud cracks’. These could be tension cracks that formed as the Astoria Formation rocks were gently folded, after they were hardened to stone. So, how’s that? Maybe I can refine, or refute, my hypothesis after I visit.

  4. Hi Dave, Can’t remember if I told you I moved back to Spokane when Seattle rents & traffic became out of sight 2 years ago.

    Am glad to see the Rock Point Trail in your site & to know that the Park is so well used beyond just the beach. 🙂


    Consuelo Please note my new email address. Consuelo Larrabee larrabee414@icloud.com


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