The sweetest little syncline in the Chuckanut

SMALL CHUCKANUT SYNCLINE ON NORTH STATE ST, BELLINGHAM

This view looks northeast at the west-dipping syncline. The fresh Chuckanut rock is gray; oxidation has discolored some rock to tan. Click to enlarge.

By Dave Tucker. Posted Dec. 20, 2009 Excavation at a construction site across from 705 State Street in Bellingham has revealed a beautiful little west-dipping synclinal fold in Chuckanut rocks. This is located just south of the complicated intersection of Forest, State, Wharf, and  Boulevard at the south edge of downtown. Thin siltstone beds are nicely exposed here. This is the best place I know of to see a small-scale fold structure in Chuckanut Formation rocks, and the synclinal axis is obvious. The upper (dip) surfaces of some of the layers are exposed on the right side (see the photo), and are brownish pink rather than the dark gray typical of fresh Chuckanut rocks. That’s right, you may be familiar with the ubiquitous dun- colored Chuckanut sandstone, but fresh, unweathered rock is gray. The discolored bedding plane surfaces probably reflect water percolation along those surfaces for millenia before the excavation cut into the rock. The water chemically weathered the rock surfaces and discolored them with iron oxide minerals.

The Chuckanut rock is covered by glacial drift. However, at the south corner of the exposure, note a sequence of  4-5 horizontally bedded, well-sorted Pleistocene sediment strata. They vary from fine grained silty-clay to silty sand, and look lacustrine. These must date to the end of Vashon glaciation, and were perhaps deposited in a kettle as ice receded. I have not seen a sequence like this in the Bellingham area before. Because of brush covering the middle part of the bank, I can’t determine whether these beds are inset against the thicker till a few meters away.

Sometimes plastic tarps completely cover the rock face, at other times they seem to have blown away or something, so go back from time to time if the face is too covered to see well. I haven’t seen any new work at this construction site for a while, and it  is may be a  temporary victim of the economy. This exposure will likely be covered once construction recommences, so don’t tarry too long!

Bus map to the syncline outcrop

Getting there: This locality is on N. State Street, at the intersection of State, the Forest Bypass, Boulevard, and Wharf Streets. You can walk from downtown on State St, or down the hill from WWU campus on Cedar and then north on State, or from Fairhaven, turn right at State just after the complex intersection. If you are on the WTA bus, the 401 to Fairhaven stops on State just north of the intersection; the 401 to downtown goes right past the exposure. If you are driving, there is parking on the street right in front of the outcrop. From downtown, head toward Fairhaven on State St. Stay in the left lane and cross at the weirdo intersection with Boulevard. The site is just up the hill. From Fairhaven and points further south, go north on 12th and the Boulevard, then up the hill at S. State. The outcrop is over the hillcrest and most of the way down the other slope.