Crusty, porous, like pale plaster, travertine forms when dissolved limestone (calcium carbonate) settles out of ﬂowing water and becomes solid again. Around hot springs, travertine creates rinds, mounds, cones, and terraces like those veiled beneath the hot-spot vapors of Yellowstone. Off the silty main stem of the Colorado River in the Grand Canyon, Havasu Creek ﬂows in a necklace of milky turquoise pools over terracotta rock, terraced by scalloped ledges of travertine. The travertine forms when the creek’s calcareous precipitate coats roots, branches, and other trapped matter, then builds on itself even as the organics rot away. Smoothing the lip of each apron of travertine, the creek cascades into pools, its downstream energy checked by stair step rather than by meander. Severe ﬂash ﬂoods have periodically ﬂushed out Havasu’s travertine. Then the encrustation begins again—though it takes time for the turquoise to come back.