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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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Book Presentation in Edmonds September 14

I will be giving a talk about my book, Geology Underfoot in Western Washington on September 14 in Edmonds.

7 PM

Edmonds Senior Center

220 Railroad Ave.

 On the waterfront in Edmonds, directly across the street from the AmTrak station. There is some off-street parking at the Center, and there is elevator access. This is the montly meeting of the Ice Age floods Institute. Free and open to the public.

I will have copies for sale, $24. Also copies of the cover poster, $15. See you there!



Giant Cairn in the Wharf Roundabout, Bellingham Washington

Some guy in a funny hat looks at the giant rock cairn in the roundabout. Hey! Wait I think I know that guy! Where're the cats?

Some guy in a funny hat looks at the giant rock cairn in the roundabout. Hey! Wait I think I know that guy! Where are the cats?

A number of Bellingham people have written me for a geological description of the 12′ rock cairn in the middle of the new roundabout at Wharf, State, Forest, and Boulevard. An article appeared in Dean Kahn’s Dec. 22 column in the Bellingham Herald; prior to that, I sent a photo and a brief notice to David B. Williams’ GeologyWriter.com blog. The 13-foot-tall stack of 4 boulders sits in the new roundabout at the south edge of downtown Bellingham. Shrubs have been planted around the cairn, but they are still spindly and short so it is still approachable on foot.


The lowest two, and the uppermost, boulders are dunite from the Olivine Corporation’s Sven Larsen Quarry on the north flank of the Twin Sisters Range west of Mount Baker. (48° 44.640′ N , 122° 0.345′ W.) The stone second from the top is serpentinite. It also came from the quarry. The stones were supplied by Princess Jade, a stone-supply company in Everson Washington. Get up close enough to see the smooth highlights on the serpentinite boulder, polished by Princess Jade prior to installation. The 8-mile-long Twin Sisters Range is a narrow slab of mantle dunite, among the largest in the world, faulted upward into the crust during plate collisions, and exposed by erosion.

The dunite boulders feature tiny black chromite crystals surrounded by olivine.

The dunite boulders feature tiny black chromite crystals surrounded by olivine.

Dunite is a dense crystalline rock. By definition, it consists of around 90% olivine crystals; the remainderis pyroxene and chromite. Dunite is believed to be the residue left behind when basaltic magma forms in the mantle, typically deep in subduction zones. Water carried into the mantle via the subducted ocean plate (the ‘slab’) lowers the melting point of the mantle, generating basaltic melt. The lower density basalt magma rises upward through the mantle taking lighter minerals such as silica along for the ride, leaving behind the denser minerals – olivine and pyroxene. Serpentinite is a low-temperature metamorphic rock formed when water enters the molecular structure of ultramafic rocks such as dunite. Hydrated olivine becomes a different mineral, serpentine.

Polished highlights on the softer serpentinite boulder.

Polished highlights on the softer serpentinite boulder.

The City of Bellingham had a choice of two designs incorporating the large boulders. The stack won out over the other possibility, scattering the rocks on the ground. Bellingham’s mayor, Kelli Linville, said “The landscaping we chose reflects the natural beauty of our area. Since cairns are traditionally used to help people find their way, a cairn is an appropriate part of the landscape at this important crossroads in our community.” (Bellingham Herald, Dec 22, 2013). According to Sam Shipp, the project engineer at Bellingham’s Public Works Department, the cairn option helps protect drivers from the glare from oncoming headlights and directs drivers’ vision toward other vehicles approaching from the left inside the roundabout, as well as providing an aesthetic component.

Dunite is very dense, about 3.3 grams/cubic cm. Serpentinite is around 2.7 g/cc. Contrast that with basalt (2.8-3 g/cc), andesite (2.5-2.8) and granite 2.6-2.7). The largest stone, the one on the bottom, weighs around 15.5 tons. The two in the middle are 8.5 tons each, and the little one on top is about 3.5 tons. The four stones are held upright by a central steel rod sunk into a concrete base. The project cost $75,000.

OK. “NAME THE STACK” CONTEST! Let’s name that stack o’ rocks! Submit your suggestions via comment and I’ll forward them to the City via my very special confidential connection.

Seattle street mnemonic contest: and the winner is…

I received a number of submissions for a geologically themed mnemonic to help remember downtown Seattle streets. I’d use the new version in my guide to Seattle building stones that will appear in my book, Geology Underfoot in Western Washington (Mountain Press Publishers, Missoula). Thanks to everyone who sent in a suggestion.

Seattle’s downtown center uphill-downhill street names (from south to north, the direction of the field trip) are Jefferson, James, Cherry, Columbia, Marion, Madison, Spring, Seneca, University, Union, Pike, Pine. The time-tested mnemonic is “Jesus Christ Made Seattle Under Protest (alternate: “Pressure”)” (JCMSUP; note this doesn’t help with the order of the streets in each pair. Which is further south, Jefferson or James? Seneca or Spring?)

The decision was a tough one. After consulting BOTH my cats, and settling a heated argument between them, the winner is:

Juan de Fuca Crashing Mighty Slowly Under People” by Peg Ferm.

Thank you, Peg. You will receive a free book when it comes out.