Like many of Yellowstone's waterfalls, Tower Fall began as a low ledge at the junction of two different bedrocks. Rock at the brink and underlying the fall is a tough volcanic breccia; the weaker downstream rock erodes faster. Where Tower Creek drops into space, imagine the missing streambed?a channel of softer rock long since worn away. Just downstream from the base of the Fall, the Yellowstone River enters a narrow, swift-running gorge. Rower Creek cannot downcut fast enough to keep pace?and is left hanging high above the river.