American cockroach (Periplaneta americana) on the limestone wall of Altar Cave, southwestern San Salvador Island, eastern Bahamas.

Altar Cave is hosted in aragonitic, oolitic calcarenite limestones of the Cockburn Town Member of the Grotto Beach Formation (Upper Pleistocene). This cave is nearly intact, breached flank margin cave. Flank margin caves form at the flanks of carbonate islands and at the margins of fresh-groundwater lenses. As such, flank margin caves are phreatic features. In the vicinity of the seawater-freshwater mixing zone (halocline) of a groundwater lens (see diagram - www.carbonatecreek.com/caves/images/carbisland.gif), relatively rapid dissolution of limestone occurs due to the high aggressivity of the water. Limestone dissolution at Altar Cave occurred during the MIS 5e highstand (early Late Pleistocene ~119-131 ka).

Flank margin caves have no surface entrances. Access to them occurs only after surficial erosion intercepts chambers or passages. Altar Cave was erosionally intercepted by the development of a wave-cut, paleo-sea cliff formed during the mid-Holocene, at about 5000 years ago.

The cave entrance is low and requires crawling. The entrance passage has a sand-covered floor. The bedrock floor occurs at up to 2 meters below the present sediment-covered cave floor.

Altar Cave consists of one main chamber. A large mound of travertine speleothem occurs near the northeastern corner of the main chamber - it somewhat resembles a ceremonial altar.

Periplaneta americana, the American cockroach, is not native to the Bahamas or to the entire Western Hemisphere. It was introduced to the New World from Africa by humans starting in the early 1600s.