Late Quaternary history and paleoecology of a small oxbow wetland on glaciated terrain were investigated using sediment lithology (cores, bulk samples, backhoedug trenches), ground-penetrating radar, vascular plant and moss macrofossil stratigraphies, and accelerator mass spectrometric radiocarbon dating. A nearly complete mastodon skeleton was recovered from late Pleistocene detrital peat and peaty marl near the top of the sediment sequence. Sedimentation in the basin began with silt and clay over dense cobble outwash transported southward from the nearby Hyde Park Moraine. Overbank sediment deposition occurred between ∼13,000 and 12,220 yr B.P. during a period of tundra vegetation, which ended with a sharp rise in spruce needle abundance and a shift to autochthonous marl and finally peat deposition. Fossils of aquatic and wetland plants began to accumulate before the tundra-spruce transition and increased after it. Rich fen wetland began to infill the pond with peat, while the upland supported open white spruce and later white spruce–balsam fir–tamarack forest. The mastodon, 11,480 ± 40 radiocarbon years old, was contemporaneous with spruce–balsam fir–tamarack forest and rich fen wetland. Many mastodon bones were articulated or nearly so, indicating that the animal died in the basin and that postmortem bone dispersal was slight.