The Ohanapecosh Formation is a widespread belt of volcaniclastic rock in Washington that preserves an early phase of Cascade volcanism during the middle Oligocene, about 36 to 28 million years ago. Underlying much of Mount Rainier National Park and nearby mountains, it formed the older foundation later built upon by Mount Rainier. Made mainly of volcanic clastic rocks, lava flows, and especially coarse mudflow deposits, it records a large, long-lived volcanic landscape whose strata reach up to 3 kilometres thick. Its broad extent, major lava flow-mudflow complexes, and contacts with older and younger formations make it a key record of the geologic history beneath the modern Cascades.