The Chuckanut Mountains were formed by the folding of the Chuckanut Formation (which is predominantly made up of layers of 55-million-year-old sandstone, conglomerate, shale, and bituminous and sub-bituminous coal) and the later Huntingdon Formation (predominantly shale and sandstone) on top, as well as an exposed section of pre-Jurassic-age phyllite
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/places/formations/huntingdon-formation/huntingdon-formation.md