Tumtum Mountain is a small but prominent volcanic cone in southwestern Washington at the eastern end of Chelatchie Prairie. Rising 2,004 feet, it stands out because it towers about 1,400 feet above the surrounding flatland. Part of the Cascade Volcanic Arc, it is the youngest and westernmost volcano in Washington’s Cascades. Geologists identify it as a Pleistocene dacite lava dome about 70,000 years old, likely formed along the Chelatchie Fault. Its lack of glacial drift and erosion shows it arose after the last major regional glaciation, making it a strikingly young volcanic feature amid much older surrounding rocks and a landscape also shaped by later timber ownership.