Glacial erratic geology field trips

A glacial ‘erratic’  is a rock fragment carried by glacial ice and transported some distance from the outcrop from which it was derived, and generally resting on bedrock of a different type* [although glacial or alluvial sediment may directly underlie the erratic. Although technically the dimensions of erratics include pebbles, these trips visit BIG erratics in northwest Washington and southwest British Columbia.

Ways that erratics may be incorporated into glacial ice. Click to enlarge.

How do really big rocks, such as the Waterman and Lake Stevens erratics, get carried by glaciers? The conventional wisdom is that glaciers do not pick up really huge rocks they override. They may push big monsters along, but it is likely that the ice mostly flows around these big ones by way of “pressure melting”- the ice simply melts as it is forced against these big obstacles, and reforms downstream. More likely, these big blocks fall onto the ice surface in the mountains, and are carried on the surface, or near the surface fi buried by seasonal snow, of the ice. Or perhaps large blocks are plucked by glacial pressure from high points the ice overrides, and then carried within the glacier. Once the climate is too warm to sustain further glacier growth, the glacier ice begins to melt until eventually the erratic rocks are left sitting on the surface where the ice once stood. Much smaller boulders are easily enough shoved along by the base of the glacier, as they don’t have enough mass to interfere with the forward pressure of glacier movement.

Trips to these erratics are categorized geographically. Use the ‘categories’ menu to find the area and then the erratic you’d like to visit.

*Definition is from Dictionary of Geological Terms, American Geological Institute (Doubleday).

Field trips here visit

Lower Mainland of BC erratics

Shasta Court, Coquitlam

The Aldergrove erratic

Vancouver Island erratics

Harpoon erratic, Harling Point, Oak Bay

Bellingham erratics

Whatcom County, including these excavated ones in Ferndale , this one in Blaine, and the Saar Creek erratic east of Sumas.

Cascade Range erratics

Erratics in the San Juan Islands

Jackass conglomerate erratics on Crescent Beach, (near Eastsound) Orcas Island.

Everett-Seattle-Tacoma area erratics:

Lake Stevens Erraticthe largest in the USA?

True Granite Erratic– Lake Stevens

Wedgwood Erratic , Seattle

Beach erratics at Discovery Park, Seattle

Ravenna Park erratic, Seattle

Leschi Park fossiliferous erratic , Seattle

Big Rock , Duvall

Edmonds Way erratic, Edmonds

Fantastic Erratic, (hike in Cougar Mountain Park, Issaquah)

Martha Lake erratic, Martha Lake County Park

Des Moines Beach Park erratic, Des Moines

This en echelon dikes in the Highline Community College erratic, Des Moines

Whidbey Island erratics

Big Rock erratic, Coupeville

Waterman Erratic, (hike near Langley)

Olympic Peninsula Erratics:

White Rock (Hood Head)

13 Responses

  1. Your blog and geology-bits were a big, pleasant surprise to me. It all sounds great!

  2. Thanks again for another great resource to aid me in my quest to awaken young minds to the wonders of the world. Much appreciated! Kudos!

  3. Wow, what a fantastic amount of information! Thanks for all your work here. While the massive glacial erratics are impressive, I’ve also become very interested in the pebbles since I first came across some sites in the San Juans with beaches full of them. Two questions: can you recommend any good sites with lots of pebble till and how can I learn more about some of these beautiful rocks that I find?

  4. My 800 square foot house three blocks north of Green Lake sits on a six foot tall erratic (the highest point measured from the basement floor). The basement was dug out a decade or two after the house was built in 1908. It is a metamorphic rock. I have no idea how deep or how wide it is below the basement floor. About one eighth of the rock was chipped away by previous owners in a fruitless effort to get rid of it.

    • Diane,
      That is one inconvenient erratic! A tremendous amount fo effort to chip away at it as much as the previous owners did. Would be cool to see a photo of the rock as it sits in your basement.
      Wish I had a big rock like that- but in my yard, no in my basement. But, I don’t know, might be cool to have it in the living room!

  5. There’s also what appears to be a good – sized erratic on the Salvation Army’s property in downtown Everett, just east of Rucker Av and north of 26th St., between the building and the alley.

    • Yes, I have heard of this rock. Is it accessible to the public? I mean, can you walk up and touch it? Photos? description?

    • Dave,

      It is very accessible. Just park your car in the street and walk up a set of steps and you’re there.

      The Salvation Army building consists of some offices and a chapel. The erratic is surrounded by lawn on 3 sides and landscaping bark & a few shrubs on the fourth side, facing the building. (Almost as if they though it was unattractive.) It is approximately 15′ to 18′ wide / long and about 8′ or 9′ high. Judging by its shape there could be anywhere from 4′ to 7′ (or more) of it below grade.

      I am not very well – versed in rock identification and am mostly unfamiliar with geology’s descriptive lexicon. I don’t have any pictures,but would be happy to take some. How can I post them here? Or can I email them to you?

      The surface of it is mostly pretty smoothe, with light and medium gray patches and small pock-like marks in many areas. At night it is dimly lit by nearby sodium (orange) streetlamps. We’re supposed to get rain all week; next sunlight is forecast for Saturday. I’ll try to get by there with a camera & iphone then.

      Hope this reply lands in the right place — there’s no ‘reply’ button on your reply.

    • yes I happened to be parked less than a block away, the other day and passed this likely erratic on my walk East into downtown. You should be able to see it on this link to Google Street View,-122.2108607,3a,75y,32.14h,80.83t/data=!3m6!1e1!3m4!1sFofIAeuJQZ_XI9eatrBH2w!2e0!7i13312!8i6656!6m1!1e1

    • Hey Paul, that’s a good one!!

  6. Hi I live on orcas island in Wa st and have found a rock I’m curious about . Can I send a picture ?

    • Of course. Include something for scale, and take a photo of a clean rock surface. If you need to, use a stiff brush to clean off dirt and any lichen or other vegetables growing on the surface. Email to me at dtchico at gmail dot com

    • Sure, send it along and tell me where you found it…..

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