Columbia River Basalt Group map shows main regions of basalt exposure in Washington, Oregon, Idaho and Nevada, USA. (usgs.gov)
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/places/mountain-ranges/blue-mountains/blue-mountains.md
The North Fork [Stillaguamish] rises as several branches in a remote area of Skagit County near Finney Peak, approximately 10 miles (16 km) north of Darrington.