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 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.
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.
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
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.
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.