It is named after the Tukwila area, which is close to the formation.

The formation consists of various fossils of marine origin.

[1] The Tukwila Formation is mainly composed of andesitic to dacitic volcanic sandstone, siltstone, shale, tuff breccia, tuff, lahar, and carbonaceous shales.

The tuff breccia has an approximate age of 42 Ma.

[3][4] Are the following:[citation needed] This article about a specific stratigraphic formation in the United States is a stub.