By Dave Tucker, April 12, 2011.
“Big Rock” is an erratic in Duvall, which lies in the Snoqualmie River valley. Loren White, a WWU geology student, and John Scurlock, aerial photographer par excellence (fabulous website), originally told me about the erratic, and John sent me the photo. (Please note that John’s photo was NOT taken from his airplane- not even he flies that low! Bob and Adena Mooers provided directions, Bud Hardwick refined them. Bob collected a small rock sample lying on the ground. Thanks to all for photos. Please don’t bang on the big rock!
The erratic is about 11 miles south of Monroe along WA 203 (about 1 mile south of Duvall). WA 203 connects Duvall with Carnation to the south, and Monroe to the north. If approaching on highway 203 from the south; the erratic can clearly be seen between the two huge evergreen sequoias directly behind the black and white sign “Duvall 1913.” The easiest approach to the erratic is by turning east off of WA 203 onto Big Rock Road and almost immediately turning south into the Safeway parking lot. Drive to the far southwest corner of the parking lot (near the highway on the south end) and park. Easy walk to the left either on pavement or down a gentle slope to the rock. The erratic is in the middle of what is now a blocked off segment of the original Big Rock Road (closed when Safeway was constructed).
The erratic is medium-sized, 8 feet high or so. I can’t tell from the photo what kind of rock it is, so can’t take a stab at where it may have come from; John said there are some loose chunks at the base that might have a fresh surface, which would help in identification. Bob and Adena collected a couple of chips which they think had fallen off the erratic, though they say that there are others which may be ‘urban erratics’ related to construction. Adena took some great close-up details of the rock. Between the chips Bob and the photos, I can see that the rock is a medium grained gneiss, a metamorphic plutonic rock consisting of whitish quartz and plagioclase grains interspersed with black biotite and hornblende. Chlorite gives the rock a slight pale green tinge. 1 mm white veins cross the small sample I have, and gneissic banding is evident in the photos. I think I see a few very tiny pyrites with my hand lens.
The two big trees are sequoias. The ‘Big Rock geochache’ webpage tells about them, and has another photo. Coordinates at the rock are 47° 43.542’N, 121° 59.189’W. On the south side of the Safeway building, in a grassy depression, are a cluster of other large rocks, probably erratics that were relocated here when the Safeway building and parking lot were constructed. This is about 150 ft east of Big Rock.
As an added bonus, John tells us about a nearby place. John wrote “Its the ‘Duvall Grill & Tap Room’, on the main drag of Duvall about 3/8 mi N of the Duvall erratic. I didn’t stop in but noted its proximity to a point of geologic interest. Considering the natural affinity geologists have to beer, I’d expect hordes of them to descend upon Duvall, attempting to kill two birds with, er, one rock.”