Breakdown (Standing Rocks, Main Cave, Mammoth Cave, Kentucky, USA) 2 by James St. John

Western Kentucky's Mammoth Cave is the longest cave system on Earth, with 412 miles known and mapped as of fall 2017. The name does not refer to the early discovery of fossil mastodon or mammoth bones here. Rather, the name refers to the immense size of many rooms and passages. In caves, "breakdown" refers to all rocks that have detached from ceilings and upper walls. It does not include sediments deposited by fluvial action when subterranean rivers were present in passages. All significant breakdown events in the Mammoth Cave system occurred not long after the water table lowered below each set of passages. Of the 6 or 7 levels at Mammoth Cave, the uppermost passages (Level A and Level B) are the oldest - they formed during the Pliocene. The bottom level (Level F) is the youngest, with subterranean rivers present today. Minor breakdown events occurred during major historic earthquakes - for example, the December 1811-January 1812-February 1812 events along southeastern Missouri's New Madrid Fault Zone. Saltpeter mining was being conducted in Mammoth Cave at the time - "saltpeter" is an ingredient of gunpowder. Miners reported breakdown during the earthquakes. In modern times, a decent-sized limestone slab detached from the ceiling of the Rotunda, a large, subcircular room near the Historic Entrance. This occurred during a fierce winter snow storm in January 1994. The park was closed, as was the entire state of Kentucky. The wind chill in Houchins Narrows, the passage between the Historic Entrance and the Rotunda, reached minus-41° Fahrenheit. 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 saltpeter mining 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 of the Girkin Limestone (lower Upper Mississippian). In January 2011, a tour guide heard a rock fall in the Rotunda-Audubon Avenue area. A new rock, pitted and sponge-like, was found in the middle of the tourist trail. That rock likely detached from the Bethel Member of the Girkin Limestone. In Mammoth Cave Ridge, the Bethel consists of crumbly, shaly, argillaceous limestones. In nearby Flint Ridge, the Bethel Member is a shale interval. The rocks shown above are called "Standing Rocks". They are upright breakdown slabs in Main Cave, a giant canyon passage and the main trunk in Mammoth Cave Ridge. I'm not sure if their upright position is natural. The grayish areas are bare limestone rock. The light brown areas are limestone covered with cave dust. The rocks are from the upper Ste. Genevieve Limestone (upper Middle Mississippian). Near Standing Rocks is an early 1800s ox tie-up area. Beasts of burden were used during the old saltpeter mining operations. Oxen were tied up using nearby solution pockets in the walls. The sides of some of the solution pockets are worn smooth from the rope tie-ups. Old corn cobs also occur at the ox tie-ups, but some early guides brought in corn cobs and tossed them on the ground and said they were original artifacts. An original ox track from the 1810s still remains near Standing Rocks. Other 1810s ox tracks near the corner of Methodist Church (in Broadway Avenue, "downstream" from the site shown here) are now destroyed. Locality: Standing Rocks, due west of Wandering Willie's Spring, Main Cave, Mammoth Cave Ridge, Mammoth Cave National Park, western Kentucky, USA