ROTUNDA
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).