Tukwila Formation—Andesitic to dacitic volcanic sandstone, siltstone, shale, tuff-breccia, tuff, volcanic mudflow (lahar), carbonaceous shales, and minor lava flows or sills. Typically massive; only local sedimentary interbeds indicate structure. K-Ar age on plagioclase from tuff-breccia at top of unit, about 3 km west of quadrangle, yielded an age of 42.0±2.4 Ma (Turner and others, 1983)