A LARGE bolide impact, such as that thought to have occurred at the Cretaceous/Tertiary (K/T) boundary, should produce large amounts of light-attenuating debris, thereby causing an 'impact winter'1–3. Because of thermal buffering in the oceans, evidence for a brief (1–2 months2–4) impact winter would be found only in terrestrial environments. Aquatic leaves in the K/T boundary section near Teapot Dome, Wyoming, preserve structural deformation that can be duplicated experimentally in extant aquatic leaves by freezing. Reproductive stages reached by the fossil aquatic plants at the time of death suggest that freezing took place in approximately early June. Both the existence of the structurally deformed plants and the high abundance of fern spores occur in a horizon containing sparse impact debris, but below the horizon containing abundant impact debris; I therefore suggest that the lower horizon represents debris and effects from a large, distant bolide impact, and the upper horizon represents a small, nearby bolide impact.