Broken Top is a glacially eroded stratovolcano in central Oregon’s Cascade Range, rising 9,177 feet in the Three Sisters Wilderness near the Three Sisters peaks. It formed beginning about 300,000 years ago through repeated lava and pyroclastic eruptions from main and side vents, then became inactive by about 100,000 years ago. Subsidence, hydrothermal alteration, and especially glaciation stripped away much of its summit and southeastern side, leaving an amphitheater-like opening that exposes layers of scoria, pumice, tuff, and lava. The mountain and its surroundings also record later glacial history, including moraine-dammed lakes and a destructive 1966 flash flood caused by a failed moraine dam near Crook Glacier.