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If Rocks Could Talk: 6 Glacial Erratics to Visit Around Puget Sound

Huge rocks carried by glaciers are fascinating to see and super fun to climb on

Published on: January 02, 2014

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erratics geology for families in the puget sound big rocks

Like an old-growth tree, a magnificent rock has a presence about it — a spirit, if you will. You don’t have to understand geology to commune with a great rock — or scramble up it and stand victorious on top — but knowing a little about the forces that  created and shaped such rocks can turn a big boulder into a real-life natural history experience. 

Wandering rocks

When the glaciers advanced into the Puget Sound region two million years ago, the major features of our Northwest landscape — the volcanoes, mountain ranges, rivers, and basins — already existed. The glaciers, colossal sheets of ice on a continental scale, scraped and sculpted the terrain with cycles of freezing and flooding.

As the ice advanced, sheering cliffs and cutting ravines, the glaciers picked up hunks of rock and carried them along. When the ice melted, these rocks were left miles away from their place of origin.

Rocks that have been transported by glaciers are called erratics because they have landed somewhere far from their natural homes.

The term erratic is typically applied to large, irregularly shaped boulders, which, upon close inspection, are unlike any of the other rocks in the area. They often contain veins of minerals and other unusual surprises such as fossils.

Technically, a pebble transported a great distance by glacial activity can be also be considered an erratic, but these are harder to notice. And people sometimes refer to large landscape rocks as “urban erratics,” which is fun but not really correct as glaciers had nothing to do with their turning up in your garden.

Geologists, natural historians and friends of rocks (and kids who like to climb stuff!) prize naturally placed erratics because these unusual rocks provide important clues to the geology of our region — and fun.

“Yeah, erratics are pretty cool,” agrees David Williams, urban geologist and author of The Seattle Street-Smart Naturalist and Stories in Stone. “They reveal the power of ice, not only to sculpt the land, but also to act as giant trucks moving huge rocks, tens or hundreds of miles. I think it's something every kid can relate to.”

Climbing boulders

Like climbing trees, scrambling atop big rocks is one of those genuine childhood pleasures that these days, in our nature deficiency, we have to schedule into the family program.

Des Moines Highland Rock girls climbingIt’s okay to encourage kids to climb rocks! And it’s cool for grown-ups to climb rocks, too. For a quick afternoon pick-me-up, there’s nothing like scaling a boulder to elevate energy and make you want to woot with joy.

Climbing tips: Use your best judgment. Some erratics are more scramble-friendly than others. Some are just too big to climb safely without proper climbing equipment; others are in balance with a fragile ecology, so be mindful of their lichen, moss and ferns. Some have signs prohibiting climbing.

Even if you can’t climb, whenever possible touch the rock, lean against it, smell it, measure it against your body. Take time to inspect it closely for fossils and minerals, as well as interesting mosses and lichens. Introduce yourself to the rock. Make friends.

Now let’s go climb some rocks!

Here are a few big rocks you can easily make friends with. Stop by for a climb or a pat when you are in the area, and be sure to say hello.


1. Martha Lake Airport Park Boulder, Lynnwood

Martha Lake Rock ErraticsTucked behind the picnic shelter at Martha Lake Airport Park in Lynnwood is our first angular erratic.

Snohomish County Parks gets major props for preserving their erratic at Martha Lake Airport Park by cleaning it of graffiti and honoring it with an informational sign. Covered with spray-paint, erratics can tell us very little. When we can see the stone and examine its natural color and texture, we can guess at its history.

About 18 feet tall, this lovely rock is probably greenschist: a mineral-rich basalt that has been chemically changed by heat and pressure long ago under the sea floor. This type of erratic is common in lowlands of the Puget Sound area.

Getting there: To drive to Martha Lake Airport Park at 200 146th St. W., Lynnwood, WA 98037.

If you go: Bring your wheels! This park has a skateboard ramp and scooter-worthy paths. After your rock appreciation, chow on nachos and quesadillas at Casa Guerrero  at 402 164th St. S.W., Lynnwood, WA 98087.


2. Upper Leschi Park Erratic, Seattle

Leschi Erratic large rocks pacific northwestIn the natural area on the slope above Seattle’s Leschi Park tennis courts, you will find this gorgeous, trailside erratic.

Laden with fossilized shells, this very special erratic was made by the compression of layers of sand or silt. As a result, the ancient submarine rock is a little more brittle than most, so be gentle with your affections.

Please be gentle with this very special erratic. Apparently, a few selfish rockhounds have been trying to collect fossils: The damage they’ve made with their hammers is irreparable. Leave no trace of your visit. Take only pictures.

“This boulder is unique because of the way it traveled to Seattle, as well as the richness of the fossils,“ says Forest Steward Darrell Howe. “It shows how dynamic, complex and ancient the earth under our feet is.”

Getting there: Leschi Park at 201 Lakeside Ave. S., Seattle, WA 98122 is south of Madrona Beach on the shores of Lake Washington. You can find street parking along Lakeside Avenue (Lake Washington Boulevard) and take the old cable car bridge to E. Yesler Way. Follow the trail uphill into upper Leschi Park’s Natural Area.

If you go: Bring a picnic or grab a deli-made sandwich at the family-owned Leschi Market at 103 Lakeside Ave, Seattle, WA 98122.

Or, climb the hill to 34th Avenue in nearby Madrona and find an array of ice cream, cake and espresso opportunities, including the original Cupcake Royale (1101 34th Ave, Seattle, WA 98122) and Molly Moon’s Homemade Ice Cream (1408 34th Ave, Seattle, WA 98122).


3. Big Rock of Duvall

Big Rock of Duvall Erratics in Pacific Northwest Geology trips for kidsNestled between two giant redwoods in a pocket park southwest of Duvall’s Main Street Safeway lies this enormous, historically significant erratic.

Already an ancient wonder of the Snoqualmie River valley long before the town of Duvall was established in 1913, so-called Big Rock has a road, a park and several businesses named after it. The sequoia trees that keep it company are probably more than 100 years old themselves.

Big Rock is a gneiss (pronounced “nice”): it’s a rock sandwich made up of different rocks and minerals all schmooshed together, often in visible bands or stripes. Look for bands of white quartz in Big Rock, and admire its patterns of lichens and moss.

Getting there: Located between Monroe to the north and Carnation to the south, Duvall sits on Highway 203. The Highway 203 becomes Duvall’s Main Street. South of historic Main Street you will find Duvall’s Safeway at 14020 Main St, Duvall, WA 98019. The erratic is southwest of the Safeway parking lot.

If you go: Enjoy the pleasures of a small-town Main Street in Duvall. Have a treat at Grateful Bread (15602 Main St. N.E., Duvall WA 98019) or browse for good reads at Duvall Books (15635 Main St. N.E., Duvall, WA 98019).


4. The Excellent Erratics, Big and Small, of Des Moines

Des Moines Beach Rock Erratics in Puget SoundHere are two erratics to visit in Des Moines: an enormous one at Highline College and a pretty, petite one at Des Moines Beach Park.

Dominating a landscaped bed of rosemary and fountain grass in front of the Higher Education Center, Highline Community College’s monster erratic is so heavy (over two tons) that it held its ground against two Caterpillar dozers during the building’s recent construction!

Predominately salt-and-pepper granite, this easily climbable rock has been polished smooth by grit under the glacial ice. With its southern exposure, this erratic grows warm in the sun, perfect for students to lounge on to catch a few rays.

Crouching like a stone lion watching the tide, the second erratic sits on the Des Moines waterfront. Only four feet high, this erratic looks nothing like the other rocks north of the marina at Des Moines Beach Park.

Look closely: this noble glacial rock is flecked with sparkles of mica and streaked with veins of smoky quartz.

Getting there: Highline College at 2400 S. 240th St., Des Moines, WA 98198 is a few minutes from I-5. The erratic is adjacent to the South parking lot, in front of the Higher Education Center. To avoid dealing with parking permits, plan to visit on a weekend or a holiday.

Des Moines Beach Park at 22030 Cliff Ave. S., Des Moines, WA 98198 is a short drive from Highline College. Park near the historic Des Moines Beach Park Auditorium. You will find the erratic on the beach, north of the Marina’s jetty.

If you go: Bring your camera. Des Moines Beach Park has picturesque historic structures and scenic views of the Sound.

After shutter-bugging on the shore, find some good eats on Marine View Dr. Try Des Moines Dog House (22302 Marine View Dr. S., Des Moines, WA 98198) and Auntie Irene’s for ice cream and other treats (22504 Marine Dr. S., Des Moines, WA 98198).


5. The Wedgwood Rock, Seattle

Wedgewood Rock Erratics Geology Trips for kids in the Pacific NorthwestThis incredible erratic holds court in an informal pocket park at the intersection of N.E. 72nd St. and 28th Ave. N.E. in Seattle’s Wedgwood neighborhood.

To read this erratic’s geological story in epic detail, pick up a copy of Maria Dolan and Kathryn True’s excellent guide Nature in the City: Seattle. According to Dolan and True, the Wedgwood Rock was formed 163 million years ago, but deposited by the Puget Lobe ice somewhere between 15,500 and 17,000 years ago.

According to historian Coll Thrush in his book Native Seattle: Histories from the Crossing-Over Place, the Wedgwood Rock was once a meeting place for the native peoples of the region. For more about the fascinating cultural history of this erratic, browse the Wikipedia entry.

Approximately 20 feet tall and 75 feet around and ringed with conifers, this marvel should not be climbed as there is a $100 city fine for those who dare scale it.

Unlike some other erratics thick with ferns and moss, the Wedgwood Rock’s moss is thin and patchy. The serpentinized greenstone that makes up this rock is low in plant nutrients and high in toxic metals. (So don’t lick this erratic!)

For the most fun, park at Dahl Playfield and walk neighboring streets uphill to the erratic. In addition to parking, the Dahl Playfield has rocks aplenty to climb on in the play area, including a stony art installation by John Hoge called “Boulder Wash.”

Getting there: Dahl Playfield  is at 7700 25th Ave. N.E., Seattle, WA 98115.

To walk to the erratic, exit the playfield at 26th Avenue N.E. and carefully cross N.E. 75th Street. Continue on 26th until you reach N.E. 72nd and follow it uphill to the erratic at 28th Avenue N.E.

If you go: Bring your wheels. Dahl Field has plenty of paths to scoot, plus a skate dot and in the summer months, a fun water feature!

For some of the best bagels in the city, go over the hill to Grateful Bread at 7001 35th Ave. N.E., Seattle, WA 98115.


6. Cougar Mountain’s Fantastic Erratic

Cougar Mountain Fantastic Erratic Geology Big Rocks in the Pacific NorthwestWearing a gorgeous shag of licorice fern, the Fantastic Erratic watches over the Talus Open Space adjacent to Cougar Mountain Regional Wildland Park in Issaquah.

For a quick visit to this majestic wonder, park at the Bear Ridge Trailhead off SR-900 and take the Bear Ridge Trail uphill about a mile. (For a longer hike, you can come down from the Shangra-La trail. Check out the Park’s trail map for alternate routes.

Deposited on this hillside by a northern glacier, this erratic is metamorphic, created under the Earth’s surface by intense pressure and heat. In some of its jagged cracks you might notice orange staining, which is from iron-rich water percolating through the rock. The ferns must lust for rust because they are really digging in to this erratic!

Fantastic Erratic Sign at Cougar MountainGetting there: From I-90, take exit 15 and drive south on 17th Avenue N.W., which becomes Renton–Issaquah Road S.E., also called SR-900. Just past Talus Entrance Road, look for the small parking area at the Bear Ridge Trailhead. Note that Bear Ridge Trail connects with the King County Cougar Mountain Regional Wildland Park trail system.

Follow the trail uphill for about a mile. It may seem longer, but it is not. Bring chocolate.

If you go: Drive back through Issaquah town if you work up an appetite. Try the hand-shaved noodles at Shanghai Garden (80 Front St N, Issaquah, WA 98027). For lighter fare and hot chocolate, try the Issaquah Coffee Company (317 NE Gilman #46, Issaquah, WA 98027).


More information on erratics and Puget Sound geology

For in-depth information on our area’s erratics, check out Dave Tucker’s excellent website Northwest Geology Field Trips.

To gain a broad understanding of our regions rocky history, view the Burke Museum’s online exhibit “Northwest Origins: An Introduction to the Geologic History of Washington State.

Go back in time to when the Sound was under ice at the Department of Ecology’s Puget Sound Shorelines Geology page.

Learn all about glaciers at the National Snow & Ice Data Center.

And, for a really good time, watch this, go to this Burke Museum Waterlines Project site and click on "20,000 Years in Puget Sound" for a nifty glacial animation.

Editor's note: This article was originally published in 2014 and updated for 2016.

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