The controversial turtleback surfaces of Death Valley may be colossal fault mullions resulting from severe crustal extension which were localized along undulating and northwest-plunging zones of weakness that were in existence prior to this deformation. Supporting evidence includes (1) a coincidence between the surfaces and carbonate layers in folded metamorphic rocks beneath the surfaces, and (2) striations, slickensides, and extensional fractures with orientations compatible with northwest extension.