In the arid basin-and-range country of the southwestern United States and Mexico, cloudbursts send torrents of runoff down ravines cut into the flanks of sparsely vegetated mountains. The flash floods may deposit enormous quantities of sand, gravel, and mud at the mouth of a given ravine, where the sediments fan out across the basin floor and are compressed over time into a gently sloping carapace of conglomerate rock. Flowing water may subsequently carve deep, parallel grooves into the alluvial fan until all that remains of it is an array of elongated humps. These remnant conglomerate humps, which resemble the backs of immense whales breaching in unison, are known colloquially as ballenas—Spanish for whales.

Jon Krakauer