AUDUBON AVENUE &
Audubon Avenue (formerly called Big Bat Room) is a large to immense canyon passage that is significantly filled with underground river sediments and breakdown. It makes up part of Mammoth Cave's level B, the 2nd-highest set of passages in the system, and formed before 3.25 million years ago (Late Pliocene or before). When an underground river went through this passage, flow direction was from east to west (from the Rotunda Room eastward toward the Green River).
The only modern cave tour that allows examination of most of Audubon Avenue is the “Mammoth Passage Tour”. It enters through the Historic Entrance, through Houchins Narrows to the Rotunda, and into Audubon Ave. The final portions of the “River Styx Tour” and the “Historic Tour” go through a portion of Audubon Ave.
Audubon Avenue (above & below) - gray limestones of the lower Girkin Limestone line the walls and ceiling of this giant canyon passage. It's been estimated that well about half to two-thirds of the height of this passage is buried by cave sediments and breakdown. The eastern end of Audubon Avenue, at the Rotunda, has a >80' thick interval of unlithified material filling the passage. Geologic evidence indicates that Audubon Avenue (and all other passages at levels A & B) were once completely sediment-filled at about 2.3 million years ago.
The ceiling of Audubon Avenue is the lower Beaver Bend Member (Girkin Ls.), which is the same unit seen in the Rotunda ceiling and along the cave mouth at the Historic Entrance. The thin, dark interval of argillaceous limestone in the upper walls, a little below the ceiling, represents the Bethel Member (Girkin Ls.). Below that is gray limestones of the Paoli Member (basal Girkin Ls.).
Audubon Avenue ends in a moderately large room called Rafinesque Hall. Cave passages extend from this room to the southwest & west for relatively short distances. Rafinesque Hall is the farthest accessible point on regular cave tours.
Rafinesque Hall (above & below) (looking S) - the limestone walls of Rafinesque Hall have the same stratigraphy as the rest of Audubon Passage - a ceiling of lower Beaver Bend Member; a thin, dark interval of Bethel Member near the ceiling, and walls of mostly Paoli Member (all Girkin Limestone, lower Upper Mississippian).
Dismal Hollow & Lookout Mountain (looking SW from Rafinesque Hall) - when level B had flowing water, an underground river flowed northwest from Broadway Avenue and turned a corner at the Rotunda Room, headed west through Audubon Avenue, turned another corner here at Rafinesque Hall, and continued to the southwest (= view direction of above photo). The SW end of Dismal Hollow is filled to the ceiling with rubble (= huge debris pile in this picture). The debris pile is called "Lookout Mountain". It coincides with the location of a surface sinkhole near the hotel on the southern side of the NW end of Mammoth Cave Ridge. The sinkhole is approximately a third of a mile away from the Green River.
Audubon Avenue (near Rafinesque Hall) (above & below) - the stratigraphic succession exposed along Audubon Avenue walls includes the uppermost portions of the next unit below the Girkin Limestone - the Ste. Genevieve Limestone (upper Meramecian Series, upper Middle Mississippian). A little above the floor of the passage is a noticeably dark, very thin, recessive-weathering interval of (presumably) argillaceous limestone. This dark layer usually has a bit of a bench developed immediately beneath it. The dark layer, plus the overlying gray limestones, are the basal Paoli Member (Girkin Limestone). The underlying gray limestones are the uppermost Levias Member of the Ste. Genevieve Ls. Reported Levias Member lithologies include lime mudstones (micrites; micritic limestones) and intraclastic limestones.
Audubon Avenue (near Rafinesque Hall) (above & below) - Paoli Member (lower Girkin Ls.) overyling Levias Member (upper Ste. Genevieve Ls.).
Rills - these subvertical grooves are incised on a sloping limestone surface just above floor level in Rafinesque Hall. The rocks are Levias Member limestones (upper Ste. Genevieve Limestone). Rills are dissolution features formed above the water table (in the vadose zone). Slightly acidic water running downward along the rock surface will dissolve some of the limestone rock, leaving a series of parallel grooves.
Little Bat Avenue (aka Little Bat Room; Little Bat Cave) (above & below; looking S) - an opening along the southern wall of Audubon Avenue leads to Little Bat Cave. Audubon Avenue used to be called Big Bat Cave. These names indicate the abundant presence of bats here in former times. Bats still occupy the cave, but the populations are highly reduced nowadays. Currently, the “Historic Tour” and the “River Styx Tour” allow visitors to see some of Mammoth Cave's lower levels. These tours eventually emerge here at the mouth of Little Bat Cave and enter Audubon Avenue (& then on to the Rotunda & out through the Historic Entrance).
Little Bat Cave is part of level B and once acted as a drain for Audubon Avenue waters. Note the thin, dark interval above the mouth of Little Bat Cave. That's the basal Paoli Member (Girkin Limestone). The gray limestones below that are upper Levias Member (Ste. Genevieve Limestone).