Houchins Narrows ends at an impressively large cave room (see map below). The ceiling has a rounded shape to it, which inspired the name for the room - the Rotunda (also known as the Vestibule). This room is at the intersection of three passages: Houchins Narrows, Broadway Avenue, and Audubon Avenue.
Rotunda (looking NE) (from an old postcard) - Houchins Narrows extends to the left along the back wall. Broadway Avenue is a large canyon passage extending to the right along the back wall. Audubon Avenue is a short, but large, canyon passage extending behind the photographer. The area surrounded by railings is the remnants of the 1810s saltpetre mining operation, principally consisting of old leaching vats. During wintertime in the past, visitors could witness thousands of hibernating bats on the walls & ceiling of the Rotunda. Paleontologic evidence of bats in Mammoth Cave also suggests that former bat populations were likely in the millions.
Rotunda (looking NE - same view direction as above) - the Rotunda and the three passages extending from it are part of Mammoth Cave's level B, the 2nd-highest level of passages, formed before 3.25 million years ago (Late Pliocene or before). When level B was flooded, water flowed from Broadway Avenue (from the back right in this photo) into the Rotunda, toward the viewer, and into Audubon Avenue (behind the viewer). For a while, water also flowed into Houchins Narrows (toward the back left in this photo). Level B is currently well above present water table. The Rotunda & the three passages extending from it are now dry.
Rotunda (looking ~SSE) (from an old postcard) - Broadway Avenue is in the distance, off to the left. Audubon Avenue extends off in the distance to the right. Houchins Narrows is behind the photographer.
Rotunda (above & below; looking ~S) - almost the entire height of the walls of the Rotunda Room consists of gray limestones of the Paoli Member (Girkin Ls.). A thin interval of dark, recessive-weathering, argillaceous limestone occurs near the ceiling (see near-top of these photos) - that's the Bethel Member (Girkin Ls.). The Rotunda ceiling (top right of photos) is gray limestone of the lower Beaver Bend Member (Girkin Ls.).
Solution pockets (Paoli Member, Girkin Limestone) - the walls of the Rotunda (& the walls of many Mammoth Cave passages) have irregularly-shaped cavities extending into the walls. These dissolutional features are usually preferentially developed along certain bedding planes (most likely representing minor unconformities). They are also well developed along joints in cave ceilings. Solution pockets appear to form principally by episodic flood events, resulting in slightly acidic water infiltrating bedding planes and joints. Solution pockets like these along Broadway Avenue have been called "Pigeon Boxes".
Leached dirt pile - numerous old piles of dirt are present along the walls of the Rotunda Room (and along Broadway Avenue to the southeast of here). These are leftovers from the 1810s saltpetre mining operation. “Saltpetre” is potassium nitrate (KNO3), one of the ingredients of gunpowder. However, the material that was extracted from Mammoth Cave sediments (cave dirt) was not saltpetre, despite what many folks say. Cave dirt from here, and other nearby cave localities, was processed to extract calcium nitrate (Ca(NO3)2) (sometimes called “false saltpetre”). The calcium nitrate was later converted to potassium nitrate. America won the War of 1812 against Britain because of the supply of potassium nitrate derived from Mammoth Cave and other caves.
Early records indicate that miners observed dirt piles becoming “rejuvenated” with saltpetre after a period of time. Modern research has indicated that the calcium nitrate in Mammoth Cave floor sediments is likely biogenic in origin (some blame pack rat urine reacting with limestone debris; some blame bat guano; I'd be surprised if bacteria didn't played a role).
Saltpetre leaching vats (“hoppers”) - the center of the Rotunda Room has several old wooden leaching vats from the 1810s saltpetre mining operation. Cave dirt from Mammoth Cave was dumped into these vats and water was added to them. The water was delivered here via hollowed-out tulip poplar wooden pipes from the waterfall at the cave mouth. The water would dissolve (“leach out”) the calcium nitrate component from the sediments. The water was then sent back to the mouth of the cave via more wooden pipes propped up high near the Rotunda ceiling. Gravity delivered the water back to the surface. There, the calcium nitrate-rich water was heated and mixed with wood ashes & then evaporated, leaving potassium nitrate crystals. These were delivered to gunpowder-making factories.
In the early 1800s, some Mammoth Cave tour guides would take wood from the abandoned saltpetre works and burn them to create a bonfire, illuminating the Rotunda Room for the benefit of visitors.
Breakdown - blocks of limestone that have fallen naturally from the ceiling or walls of a cave are called breakdown. The Rotunda Room and Broadway Avenue and Audubon Avenue (see map at top of this page) are filled with a >80' thick interval of limestone breakdown and subterranean fluvial sediments.
The only known breakdown event in Mammoth Cave in recent historical times occurred in January 1994. At that time, a fierce winter snow storm shut down Mammoth Cave and the state of Kentucky. Freeze-thaw processes affected the Rotunda room, resulting in the collapse of a large piece of Rotunda ceiling rock. The slab shattered into numerous pieces and damaged some of the 1810s saltpetre works. The broken slab remains where it fell (see above photo). No one was in the cave during this event. The slab detached from the near-basal Beaver Bend Member (Girkin Ls.).
Early 1800s breakdown events in Mammoth Cave were observed & noted by saltpetre miners during the powerful December 1811 to February 1812 earthquakes along the New Madrid Fault Zone (Mississippi River Valley).