November 23, 2017: In Easterbrook’s book, “A Walk Through Geologic Time From Mt. Baker to Bellingham Bay”, he says that “volleyball-sized” chunks of Dunite from the Twin Sisters range can be found in the Middle Fork of the Nooksack River at the Mosquito Lake Road bridge crossing. Dunite is a rock composed largely of Olivine but with black flakes of Chromite. The green Olivine weathers to a dun color, hence the rock’s name of Dunite. You can also look for Jasper and perhaps Serpentine.
There is a parking area and access to the river on the southwest side of the river. If you are heading south on Mosquito Lake Road from the Deming area, the parking area is on your right immediately after you cross the one-lane bridge. See the aerial view below with a P for parking. For best results, go in summer or fall when the water level is lowest.
Please take only a few Dunite specimens. After all, there are no longer any glaciers to transport new rocks down from the Twin Sisters. Consider taking a rock to the club’s retail store/lapidary room where, for a nominal fee, you can get a slab cut off to expose the nice green Olivine and black Chromite.
There are bathrooms at South Fork County Park near where Mosquito Lake Road joins Route 9 at Acme.