A crack extending vertically through one of the red sandstone units, probably the Slickrock Member of the Entrada Sandstone, capping a ridge outside Moab, Utah. Water coming down through the crack was forced laterally on encountering an underlying impermeable layer, and the lateral flow has eroded the basal part of the crack into a flared cross-section. The flared part is tall enough to stand up in (note the walking shadowed figure), and provides a tunnel underneath the sandstone ridge. The feature is about 83 ft/25 m long and is a popular trail destination--my son and I came there as part of a commercial ATV tour.