The age of the loesses and associated paleosols that buried the original Rainwater basins and the fluvial sand in which they formed have been dated by either radiocarbon dating, optically stimulated luminescence dating, or both. The oldest loess overlying original Rainwater basins have yielded ages from about 28,000 to 36,000 BP radiocarbon years and optically stimulated luminescence up to 36,000 BP calendar years. The fluvial sands of a pleistocene Platte River, which overlain by the Gilman Canyon formation and younger loesses, yielded optically stimulated luminescence dates from about 36,000 to 50,000 BP calendar years. These dates demonstrate that the Rainwater basins were formed prior to the Middle Wisconsinan, Marine Isotope Stage 3, and the accumulation of the Gilman Canyon Formation. Thus, Rainwater basins and their associated ridges are relict, pre-Last Glacial Maximum landforms that have migrated upward in elevation as successive layers of glacial-age loess have accumulated and intercalated soils sporadically formed.