A bit of the mantle: Washington Park, Fidalgo Island, Washington

The 'snake-skin' texture of serpentinite lends its name to 'ophiolite'. This is a nice example near the foot of the stairs.

The ‘snake-skin’ texture of serpentinite lends its name to ‘ophiolite’. This is a nice example near the foot of the stairs.

A geology guide to ultramafic rock at Washington Park, near Anacortes, is now available on Northwest Geology Field Trips. Click here to go straight to the field trip! The rock is the metamorphic rock serpentinite, and was originally in the earth’s mantle below oceanic crust. Exposures are found along the dramatic rocky shore and on bare rocky knobs above the coastal cliffs. A single essential outcrop is highlighted in this quick self-guided field trip. The description explains what ‘ophiolite’ is and points out spectacular glacial polish at the same outcrop.

Coming soon:

  • Mount Erie- Fidalgo ophiolite Part 2
  • Finlayson Point, Victoria
  • Rock Trail, Larrabee State Park
  • ?????

Do you have field trips to share? Please do. Let’s branch out to places in southern and eastern Washington. I’m happy to help you write your guide . Contact me via comment or email:

send email here

send email here

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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.

“NAME THE STACK” CONTEST! Scroll down.

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.

Pillows in the Middle Fork Nooksack

Stacked pillows. Stolen from a commercial website. So, sue me.

Stacked pillows. Stolen from a commercial website. So, sue me.

(Click HERE to go straight to the page about the pillows along the road.)

No, pillow lava, silly!

Cross-sectioned basalt lava pillow in the Elbow Lake Formation, Middle Fork Nooksack Road. Click to enlarge.

Cross-sectioned basalt lava pillow in the Elbow Lake Formation, Middle Fork Nooksack Road. Click to enlarge.

I planned a XC ski trip up the Porter Creek Road or elsewhere off the Middle Fork Road today, but it was raining and 41 degrees. Indulging my secret identity as a geologist, I instead poked around in a roadside quarry, looking at pillow lava in the Elbow Lake Formation. A rewarding outing from Bellingham if you have a 2-3 hours on your hands and a deeply felt need to look at some exotic, unusual, and (admittedly) not too beautiful rocks. The saving grace is the lava pillows, and a cup of coffee on the way and on the way back. As an added bonus, there is access to Twin Sisters dunite nearby.

P.S. There ARE some nice roads to ski up and back down off the Middle Fork.