Upper Cretaceous and lower Tertiary rocks exposed in the Raton basin, New Mexico and Colorado, contain a thin, indium-rich, kaolinitic clay bed that marks the palynologically defined Cretaceous-Tertiary boundary. The clay bed is presumed to have been deposited as the result of a catastrophic event, possibly the impact of an asteroid, at the end of the Cretaceous; it is preserved in sediments laid down in the quiet waters of ponds and coal-forming swamps. Occurring in a conformable sequence of nonmarine rocks, this extraordinary marker bed enables observation of depositional conditions of an instant of geologic time throughout a wide area in the basin. Investigations made at various sites where the boundary clay is present indicate that component elements of a dynamically aggrading fluvial system existed at the close of the Cretaceous. This depositional system was characterized by meandering rivers, broad floodplains, and levees. The levees were breached locally by crevasse splays that periodically invaded the floodplains and associated poorly drained and well-drained swamps.