Wave-like bedforms of large amplitude and wavelength (6 m, 60 m) occur in an exposure at the base of the Fish Canyon Tuff, Colorado, an ignimbrite (ash-flow tuff) of great volume (> 3,000 km3). The bedforms are located some 30–40 km from the center of the source caldera. They are interpreted to be deposits from a series of large pyroclastic surges, precursors to the main pyroclastic flows. The pyroclastic surge beds have bigger wave forms than those previously recognized. Such megawaves were perhaps produced by violent explosions related to major unroofing of a magma chamber. The depositional sequence of this great ignimbrite-forming event is similar to that of much smaller eruptions and implies a similar eruptive sequence.