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The Lake Stevens monster- largest erratic in Washington. Largest in the US?

The back side of the Lake Stevens erratic towers out of the woods. Bob Mooers photo. Click to enlarge any image.

By Dave Tucker

July 18, 2011

What may be the largest glacial erratic in the US has been spotted in Lake Stevens Washington, which is southeast of Marysville and east of Everett. First reported in a post on this website, this hulking brute of a monolith lies hidden among the bushes in a tranquil-appearing subdivision. It was brought to our attention by two Lake Stevens policemen, Craig Valvick and Matt McCourt. They carefully measured the erratic. It is 34 feet (10.36 m) tall and 78 feet (23.77 m) in length. The circumference is 210 feet (64 m). This appears to be the largest known measured erratic in the state of Washington. The previous reigning ‘biggest mutha’ erratic, as reported on this website, was the Waterman erratic. Craig and Matt measured the Waterman, too, and found that it is ‘only’ 38 feet (11.6 m) high, and 60 feet on each side (18 m). Circumference is 155 feet (47 m). The volume, and therefore the weight is more difficult to estimate, as the Lake Stevens rock is very irregularly shaped. A sample was taken to calculate the density, which is necessary to then calculate total weight. Stay tuned for a report with that information. Unfortunately, because of the trees, brush, and slope of the ravine behind the high side of Lake Stevens erratic, it is difficult to get a good perspective of the size of this rock. See below for more examples of big erratics.

The rock is greenstone. The rust-colored areas are just that- iron from within the rock,  oxidized by water, deposited in hairline fractures, and then exposed as the rock flakes away. Friendly cat for scale.

The Lake Stevens Erratic, like the Waterman, is composed of somewhat serpentinized greenstone. Greenstone is a rock that results when basalt lava is subjected to low-grade metamorphism. The lava was at one time erupted onto the seafloor of the Pacific Ocean, then carried eastward and subducted beneath North America. Pressure and temperatures metamorphosed the original basalt minerals to green chlorite and related minerals. The serpentine mineralizaton occurs when the rock is subjected to shearing forces, not a surprise when you consider that the ancient seafloor lava was caught between two tectonic plates.

Thin veneer of the mineral serpentine is found on the surface of sheer planes within the erratic.

It is not possible to say where the erratic began its journey on or within the great Vashon glacier that flowed out of British Columbia and into the Puget Basin. There is similar-looking greenstone at the north tip of Whidbey Island [37 miles NW of Lake Stevens, and a possible source of the Waterman Erratic]. You can see an animation of the advance and retreat of the Vashon ice sheet into the lowlands here. You’ll have to click on the animation to make it start; you can also click to make it stop at any time. Watch it over and over and see the evolution of the glaciated landscape in various places. If you mentally note where Lake Stevens is, and also the Deception Pass area, you’ll see that the ice could have carried this rock from there. However, this animation must be somewhat speculative, and the rock could have originally come from a location further away. There is similar looking rock in the North Cascades, especially around Mount Shuksan, as well as up the Fraser River.

Bob Mooers shows us how large even the low, street side of the erratic is. Thanks, Bob!

Getting there: The Lake Stevens erratic is in a tiny park, maintained by the people of the neighborhood. From I-5 in Everett, head east on US 2 for 2 miles. Take the 20th Street exit straight ahead (east) and go about a mile to  the stoplight at 83rd Ave, turn left (north) and take it to the end. You’re there. There is an informal trail around to the dark, looming east face, rising vertically for 34 feet.  The lat/long is 47° 59.816’N 122° 6.954’ W.

There are rock fragments lying on the ground on the east side, if you wish to examine a fresh rock surface. Please, NO HAMMERS!

Read more about erratics in general on this website, and find out where some other big ones lurk in the region.

Large, measured erratics

An 8th grade class standing under the “largest known glacial erratic” in the U.S.—the Madison boulder in New Hampshire ( Moultonborough School District). Lake Stevens erratic is 11 feet taller.

The Madison Boulder (43°55′52″N 71°10′04″W) is the largest known glacial erratic in New England, and among the largest in the world. Madison Boulder is a granite monolith measuring 83 feet (25 m) in length, 23 feet (7.0 m) in height above the ground, and 37 feet (11 m) in width. It weighs upwards of 5,000 tons. Compare this with the Lake Stevens erratic: 78 feet (23.77 m) in length, 34 feet (10.36 m) in height, circumference 210 feet (64 m). Although I don’t have the measurements, the Lake Stevens erratic may be wider than the 37 foot width of the Madison Boulder.

The Okotoks erratic, largest in the world. Look closely to see the people for scale.

Okotoks Erratic lies 18 km south of Calgary, Alberta. It is widely held to be largest erratic in the world. The large quartize rock is now broken into several pieces. It measures 9 meters (29.7 feet) high, 41 meters (135 feet) long and 18 meters (59 feet) wide. It weighs 16,500 short tons. Though lower, it is much longer than Lake Stevens erratic, and surely has a larger volume.

14 Responses

  1. Is there an age estimate from radiometric dating yet?

    • Age estimate of the rock itself? No. I doubt that there are datable minerals in this rock, as metamorphism would have reset the cooling history.We know it was dropped off at this location when the Vashon ice melted, around 15,000 years ago.

  2. Magmatist has become erratic. Wow! That is one big rock a long ways from any outcrops.

    • To tell you the truth, Dan, I’m getting a little bit tired of erratics. But there sure is a lot of interest in the dang things!

  3. Visited today. Must go to the back side to truly appreciate the size. Looking for erratics in Puget Sound should include those that lay on the bottom of the sound too. Any Scuba rock hounds? Maybe the largest one is submerged?

    • Eric,
      There is a report of a very large boulder [an erratic, presumably] in 220′ of water in Admiralty inlet near the Keystone Ferry, found with sonar. Measures roughly 47 feet high, 66 feet across. I won’t publish a field trip to these treacherous waters. I’ve never seen it, and don’t have reliable coordinates.

  4. […] they get swept up into glaciers, and the extent of the last glacial maximum. The sign mentions the Lake Steven’s Erratic, and states that it may be the largest in the US. Special thanks to David McConnell of the Parks […]

  5. Hi all,
    I grew up in Lake Stevens, mountain biking, climbing, and skating prior to the house explosion…
    I would hardly call it new or undiscoverd. In the early 80’s it was first bolted on the face (ring) and top Leeper and then SS. It was climbed heavily in the 90’s through ’95-’96 until the home owner association closed it. At one point there was not a speck of moss on the face, and we had to clear wind blown many trees from the base,mind you this was 20+ years ago.

    The rock does not apear to be a greenstone. It is largely a fine grained diorite with remnants of contact metamorphism etc. The geo chem from the early 90’s indicated diorite migrated from BC.

    As far as largest in washington… I seriously doubt it. There is the bigger boulder by Coupeville and all the haystack rocks on the Withrow Moraine which are larger than this one. The largest maybe an andesite split outcrop farther north in Skagit county.

    • DT replies:
      Thanks for your comments. Interesting to hear about the climbing history. Looks like the bolts you and your friends installed have been chopped- I didn’t see any on the Lake Stevens Monster. The Coupeville erratic is smaller- I have measured both. The big basalt boulders on the Withrow Moraine in eastern WA aren’t technically erratics, since the moraine also lies on Columbia River basalt. So, I haven’t coundted them. Nor have I measured them.

  6. There’s a bigger one in Anacortes. Between the refineries.

  7. We have a glacier erratic in our backyard. 232 feet around and 27 feet high, but I think it actually another foot taller if you don’t stand in the middle. Everett Rock club just came out and measured it a few weeks ago. We are in Snohomish. One of our neighbors who is I think in his 80’s used to run the deer trails when he was little to climb on our rock. It’s a great place for a picnic.

    • Tamra,
      Very cool, that’s a big one! I don’t publish info about erratics unless there is public access. Is the rock greenish, like the one in Lake Stevens? Dave

  8. I enjoy seeing piles of erratics that accumulated on the down flow side of islands in the San Juans. Especially whitish granitics that contrast well with the island’s mostly dark rocks.

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