File:Rotunda (Mammoth Cave, Kentucky, USA) 2 (25492402707).jpg

(photo by Ljubomir Risteski) (looking ~south)

The Rotunda (also known as the Vestibule) is the first cave room encountered after entering Mammoth Cave's Historic Entrance. The Rotunda is an impressively large cave room having a rounded shape - this is best seen in the ceiling. The Rotunda is at the intersection of three passages: Houchins Narrows (behind & to the left of the photographer here), Broadway Avenue (off to the left in the background), and Audubon Avenue (off to the right in the background).

This room has wooden artifacts from an 1810s saltpetre mining operation. These remains principally consist of old leaching vats. During wintertime in the past, cave 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.

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 left in this photo) into the Rotunda, and then into Audubon Avenue (off to the distant right). For a while, water also flowed into Houchins Narrows (~behind the viewer). Level B is currently well above present water table. The Rotunda & the three passages extending from it are now dry.

Most of the bedrock in the walls of the Rotunda Room consists of gray limestones of the Paoli Member of the Girkin Limestone (lower Chesterian Series, lower Upper Mississippian). A thin interval of dark, recessive-weathering, argillaceous limestone occurs near the ceiling (= shadowed parting in the picture) - that's the Bethel Member of the Girkin Limestone. The Rotunda ceiling has gray limestones of the lower Beaver Bend Member of the Girkin Limestone.

The circular ceiling of the Rotunda is a breakout dome.

Blocks of limestone that have fallen naturally from the ceiling and walls are common in many parts of Mammoth Cave, including the Rotunda. Such detached limestone blocks are called breakdown. The Rotunda and nearby Audubon Avenue & Broadway Avenue are filled with an estimated >80 feet 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 lighter-colored area of the ceiling in this photo). The slab shattered into numerous pieces and damaged some of the 1810s saltpetre works. The broken slab remains where it fell. No one was in the cave during this event. The slab detached from the near-basal Beaver Bend Member.

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).