The thinolites in the Quaternary tufas and tufa mounds of the Lahontan and Mono Lake basins of the western United States are identified as being pseudomorphous after crystals of the mineral ikaite, CaCO36H2O. Ikaite is a metastable mineral in the sedimentary realm. It crystallizes from solution at temperatures close to zero Celsius but rapidly decomposes at warmer temperatures and becomes pseudomorphed in calcite by in situ redistribution of the calcium and carbonate ions. The likely implication is that the thinolites record ice-cold episodes during deposition of the tufas.