Evolution of the Puget Lowland, last 20,000 years

Animation of the advance and retreat of the last great glaciation in the Puget Lowland is available on the Burke Museum’s YouTube channel. Thanks to reader ‘Deb’ who notified me that the link in the ‘Lake Stevens Erratic- the largest in the US?’ page was broken. It’s been repaired. The animation is based on a model by Ralph Haugerud of the USGS- see the very end of the animation for full credits. Unfortunately, the model is only for Puget Sound, and does not include the San Juan Islands or points north. The source of the glacial lobe is not shown- the interior of British Columbia, the BC Coast Mountains, and the Canadian Rockies. The diagram shows the bigger picture, which has a chapter on glacial erratics, and another on the history of glaciations in the lowlands of western Washington.


Diagram from my book,Geology Underfoot in Western Washington showing sources and paths of ice related to the Vashon Lobe. The glacial tongue reached its maximum extent south of Olympia around 16,000 years ago.

This animation is a bit different from the original. This one shows events in the Lowlands following retreat of the Vashon glacial lobe, such as the Osceola Lahar from Mount Rainier 5600 years ago that reached the salt water Duwamish Embayment near Auburn, and local rebound of the crust from the weight of the ice.

If anyone finds broken links anywhere on this website. please notify me by way of a comment on the page. If you know if a replacement link, please include that.

Winchester Mountain trail side geology

I hiked up to the restored fire lookout on Winchester Mountain on Labor Day, 2022. Winchester is above Twin Lakes off the Mount Baker Highway. I wrote up what I could figure out about the trail side geology and posted it in the ‘field trips’ section of this website. Thanks to the Mount Baker Club for maintaining the lookout building, and boo to the anonymous idiots who broke into it and damaged the door last winter.

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The geologic highlight of the Winchester Mountain trail, other than the stupendous views into the complex geology of the North Cascades, is the trail climbing along a yellow dike cutting across the basement rocks.

The approach is on a challenging but very fun (if you like that sort of thing- I do!) rough road. I’m glad I had my Crosstrek with its ‘X-mode’ for traction. The views into the Cascades are the main attraction for most who visit, but I got a big kick examining a 2+ meter wide dike that intersects the trail 300 feet below the summit. The hike is moderate, gaining 1300 feet in just over 2 miles on a good trail. But if you are fazed by the road, you will need to add a lot of road walking- up to 2.5 miles each way if you park near the Yellow Aster Butte trailhead. If you are lucky some nice person may offer you a lift.

Mount Pilchuck geology guide is published.

I hiked the Mount Pilchuck trail a week ago and just published a field guide on the Northwest Geology Field Trip website and you’ll find it here. I was motivated by an email from long-time reader Wendy B. She hiked Pilchuck last fall and asked me about this ‘standing stone’ near the summit.

Wendys standing stone

Wendy asked: “One quandary that mesmerizes hikers and is the subject of much debate is the two-story tall “shark fin” rock about ten minutes from the lookout. Curious minds want to know how it came to have this shape and rest in this striking position”

So I finally hiked Pilchuck to see if I could deduce an answer for Wendy. I think I did, its in hike description. Also of geologic interest is the rock itself, the fine-grained granite of the Pilchuck Stock and its sets of parallel joints. The hike is famous for its fabulous views from the summit lookout, as well as the atrocious road to the parking lot.

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The summit block of Mount Pilchuck. The lookout is on the left high point. The trail ascends through really well exposed granite of the Pilchuck Stock. What’s a ‘stock’ you might ask? Go here to find out.

Rock Trail in the Chuckanuts updated

I updated my geo guide to the Rock Trail in Larrabee State Park; I hiked it last Friday on a gorgeous warm spring day. This is one of my favorite geology hikes in the park; I wrote the original description on this website shortly after the trail was finished in 2014. So, what’s new? A ways beyond the last high sandstone wall I came across a new [to me] exposure of sandstone, scoured and cleaned off. Looks like the overlying soil and vegetation slid off and the beautiful rock has since been been flushed clean by heavy rains. The slab reveals some very fine cross beds so I added this near the end of the guide. Go now- before the trees leaf out there are views east to snowy Mount Baker and the Sisters Range.

Cross-bedded stream sand in the ‘pale slab’.

About these field trips- they are not actually ‘virtual’.

Dear friends and subscribers,

There seems to be considerable misunderstanding of my use of the term ‘virtual’ field trip. I do not actually lead the field trips i publish on this website. I write them up so YOU can go do them yourself; they are intended to be self guided. My apologies for misunderstandings and apparent misuse of the term. I will no longer these field trips as ‘virtual’, as that apparently implies you can ‘join me’ on line somehow.

It is my hope you will find field trips to places near you, or that pique your interest, and go there. You can take along the website on a smart phone, or use the tips here to show you how to efficiently print out the trip on actual real paper.

Dave Tucker

Point Whitehorn: updated field trip

I updated my 11-year-old virtual field trip to Point Whitehorn, up the coast from Bellingham and just south of Birch Bay. I have been there many times in the intervening years. It’s always a fine jaunt, especially in winter and most especially on the day after Thanksgiving! There is plenty of changing geo scenery due to wave erosion of the beach bluff. The updated page has a lot of new pictures. Like this one.

Whoo hoo! If any visitors have pictures documenting further erosion around this big erratic, send them to me. How much longer until it is on the beach?

Damfino lakes geology hike

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Mazama O and Baker BA ash on the Damfino Trail.

I just posted my geology field trip guide to the Damfino Lakes Trail, reaching Excelsior Pass from the Canyon Creek road to the north- and a much shorter hike than the Excelsior Trail.

The rock exposures aren’t all that great, but there is a small and lovely exposure of Mazama ‘Layer O’ volcanic ash and Mount Baker BA ash sitting right on top. And of course the flower meadows and the views.

Excelsior Trail geology guide is up!

As promised a week ago, I completed the geology guide to the Excelsior Trail, rising from the Nooksack River east of Glacier to Excelsior Pass and ‘Excelsior Peak’. I will post a separate description for the ‘back door’ trail via the Damfino Lakes trail.

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Steeply dipping Jurassic Nooksack Formation marine sedimentary rocks just east of Excelsior Pass.

Mount Baker Geology Report is published!

Dear friends,

coverMany of you will know that for years I have been among a group of geologists putting together a USGS report on the volcanic history of Mount Baker since the end of the last glacial maximum {that’s geologese for ‘Ice Age’). That paper has just gone up on line and you can read it here: https://doi.org/10.3133/pp1865

The authors are Kevin Scott (USGS, retired); myself; Jon Reidel (North Cascades National Park); Cynthia Gardner (Cascades Volcano Observatory, USGS); and Jack McGeehin (USGS, who did the C14 analyses)

The brief citation reads:

Latest Pleistocene to Present Geology of Mount Baker Volcano, Northern Cascade Range, Washington: This report summarizes the multifaceted and complex surficial geologic history of Mount Baker just before, during, and after withdrawal of the latest Pleistocene Cordilleran ice sheet from the Mount Baker area. Nearly 80 new radiocarbon ages are reported that constrain the timing of glacial and volcanic events and help define four post-glacial eruptive periods: Carmelo Crater (ca. 14–11.6 ka), Shriebers Meadow (ca. 9.8–9.1 ka), Mazama Park (ca. 6.7 ka), and Sherman Crater  (1842–1880 C.E.).  The end of the Carmelo Crater eruptive period marks the end of Mount Baker edifice construction and the beginning of mainly destructional processes (flank failures and resultant lahars) during the Holocene.  It also marks a shift of vent location from the summit to off-summit locations. Findings from this paper dovetail with the bedrock history of Hildreth and others (2003) to give a comprehensive view of the United States’ most northern Cascade volcano.


Scott, K.M., Tucker, D.S., Riedel, J.L., Gardner, C.A., and McGeehin, J.P., 2020, Latest Pleistocene to present geology of Mount Baker Volcano, northern Cascade Range, Washington: U.S. Geological Survey Professional Paper 1865, 170 p.  https://doi.org/10.3133/pp1865

Coming up: Excelsior Trail geology guide

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Looking west along Excelsior Ridge from just below the pass.

In the next couple of days I plan to post a geology guide to the Excelsior Pass Trail. The trailhead is near Nooksack Falls; the trail climbs out of the North Fork Nooksack valley to Excelsior Pass. The trail ascends through the Jurassic Wells Creek Volcanic Member of the Nooksack Formation; I wrote of them waaaaay back in 2010, here. Find the glory of flower meadows and views and, at the pass, the reach small exposures of conglomerate and breccia in the slightly younger Nooksack Formation.

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A fragment of greenstone from the Wells Creek volcanics. Exposure to the atmosphere and water has produced the 1-cm-thick weathering rind on two sides of this bit of rock, but the greenish rock in the center is fresh greenstone- at one time a submarine volcanic deposit.