The ground gives way ahead of you here on the Soldier Pass Trail, deep in the Coconino National Forest outside Sedona, Arizona. Formed in the late 1880s — with an initial collapse noted in the early 1880s — this is one of at least seven sinkholes surrounding the city. Before you is a chasm roughly 660 feet (200 m) deep that continues into a cave adding about 180 feet (55 m) for a total of roughly 840 feet (260 m). Its interior even contains vegetation, a surprising green pocket in the hollow.
This hollow began as subsidence into caverns of paleokarst made of Redwall Limestone of the Pennsylvanian subperiod. After a second collapse a large block was displaced; many broken edges are very angular, though the south wall shows rounded rocks and a patina of manganese oxide, which infers the exposure has lasted a few hundred years. Notice that later collapses tend to widen the hole rather than deepen it. The upper walls you see are sandstone of the Schnebly Hill Formation, while lower reaches are shaly siltstone of the Hermit Formation.
Trace the sandstone rim down to the manganese-streaked south wall, and notice how the green vegetation clings on far below.