| Summary: | Riverbluff Cave, in Greene County, Missouri, is a short single passage between the James River and its direct tributary, Ward Branch. Before stream incision the cave functioned as a spillway/piracy between the two streams during high-discharge events and accumulated a sequence of stratified fluvial sediments within the cave. Five cosmogenic-nuclide burial ages for these sediments range from 0.984 to 0.570 Ma. These ages are consistent with both the stratigraphic order of the samples and the inferred position of the Matuyama/Brunhes paleomagnetic boundary. These ages indicate that sandy channel-facies deposits derived from Ward Branch entrances began to accumulate within the cave as early as 0.984±0.065 Ma. This facies is capped by highly fossiliferous gravel beds dated at 0.658±0.065 Ma, which contain abundant mammoth bones (possibly <italic>Mammuthus trogontherii</italic>) and other vertebrates. The high concentration implies that this deposit may record some type of mass-mortality event. By 0.570±0.072 Ma, all Ward Branch entrances had been abandoned because of incision, and a laminated red clay derived from backflow from flooding along the James River capped the older channel sediments.
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