Ice-rich glaciolacustrine sediments near Mayo, Yukon Territory, reveal a thaw unconformity in the form of truncated ice wedges and abrupt changes in cryotexture. The unconformity has been radiocarbon dated at 8870 ± 200 years BP, which is within the Holocene period of optimal climatic conditions in northern Yukon and the Mackenzie Delta area reported by other workers. Analysis of the mineralogy of the sediments indicates that the material above the unconformity is enriched in minerals that are the products of a more intense weathering environment than those deeper in the profile. Oxygen-isotope ratios of ground ice in the sediments suggest the presence of two genetically distinct ice units above and below the unconformity. An average rate of upward permafrost growth in this area of 0.1–0.2 mm year−1 is calculated for the period since the climatic optimum.