Organic-rich "muck" deposits, which blanket auriferous gravels in the Klondike area, Yukon Territory, comprise two principal stratigraphic units: (i) a silty Late Pleistocene deposit, and (ii) Holocene organics lying unconformably on the silt. The deposits are found predominantly in valley bottoms and, if undisturbed, are normally perennially frozen. Field and laboratory investigations of particle size, mineralogy, and morphology, as well as organic matter and sedimentary structures, indicate that the silt is both primary (massive) and redeposited (bedded) loess (by weight 87% medium and coarse silt and fine sand). Radiocarbon dates indicate that the loess was deposited during Late Wisconsinan McConnell glaciation, beginning after 27 000 14C years BP. The loess was likely derived from the floodplain of the Yukon River during periods of low flow. Turf in growth position and organic matter in the silt similar to that of loessal grasslands near Kluane Lake suggest a grassland environment for the area during McConnell glaciation. A mummified carcass in the silts indicates that some of these sediments have been frozen since shortly after deposition. Ice wedges are commonly found in the upper portion of the silt, but these wedges rarely extend into the overlying organic material. Separate, smaller ice wedges are found in the Holocene unit. Radiocarbon dates indicate that peat growth began at the start of the Holocene, as in other unglaciated portions of Yukon, when the climate became abruptly wetter.