[5] The Chuckanut and related formations "are all composed predominantly of fine- to medium-grained sandstones with lesser amounts of interbedded shale, conglomerate, and coal.

"[6] The sandstones consist of sand eroded from the Mount Stuart massif and probably from uplifted metamorphic sources in northeastern Washington[7] (this was well before the rise of the Cascade Range), and distributed by rivers across a low-lying coastal plain starting about 54 Ma.

[9] Deposition of the Chuckanut continued as convergence of the Crescent Terrane (Olympic Peninsula) initiated strike-slip motion on the Straight Creek Fault (48 Ma), displacing much of the original formation to the north.

Continued deposition across the Straight Creek Fault formed the Raging River and Puget Group formations east of Seattle; the former is partly marine, indicating it was probably a large delta, and locating the Eocene coast line.

Johnson (1984) estimated a total thickness of 6,000 metres (20,000 ft), which would make the Chuckanut Formation one of the thickest nonmarine sedimentary sequences in North America.