The Central Cascade Mountains of Washington are composed largely of Tertiary continental and volcanic rocks. They lie between older metamorphic rocks to the north, Tertiary marine rocks to the west, and younger volcanic rocks to the south and east. An eastward-dipping reverse fault through Lake Kachess separates two Tertiary basins of the same general age. This fault cuts Eocene rocks and is older than the Snoqualmie granodiorite.
East of the fault the structures are open. The Swauk arkose unconformably covers pre-Tertiary peridotite and Easton schist. The Swauk is folded along northwest trends that become more east-west south of Mount Stuart. This change in trend and the local origin of some of the Swauk suggest that the Mount Stuart block was high during early Tertiary time. Locally the Silver Pass volcanic rocks overlie the Swauk. The conformable sequence of Teanaway basalt-Roslyn arkose lies unconformably above the Swauk and is deformed into a broad basin. The nearly horizontal Yakima basalt unconformably overlies the older rocks.
West of the Kachess fault, the structures are more complex. The oldest rocks are limy hornfels and marble of the Denny formation. They are overlain apparently unconformably by the sedimentary rocks of the Guye formation. Unconformably above the Guye is the extrusive Mount Catherine rhyolite; that is overlain by the tightly folded sedimentary rocks and basalt of the Naches formation. The mildly deformed Keechelus andesite overlies the Naches unconformably. The Snoqualmie granodiorite intrudes all the units in the western area.
Fossils are rare. A few vertebrate remains indicate that the Roslyn is probably middle or upper Eocene. Fossil leaves suggest that the Swauk and Guye formations are Paleocene or Eocene and the Naches is Eocene.