The Big Clifty Sandstone is a geologic formation

Geologic formation

A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...

 in Kentucky

Kentucky

The Commonwealth of Kentucky is a state located in the East Central United States of America. As classified by the United States Census Bureau, Kentucky is a Southern state, more specifically in the East South Central region. Kentucky is one of four U.S. states constituted as a commonwealth...

. It is sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 that is Pennsylvanian

Pennsylvanian

The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...

 in age. At Mammoth Cave National Park

Mammoth Cave National Park

Mammoth Cave National Park is a U.S. National Park in central Kentucky, encompassing portions of Mammoth Cave, the longest cave system known in the world. The official name of the system is the Mammoth-Flint Ridge Cave System for the ridge under which the cave has formed. The park was established...

 it overlies the Girkin Formation

Girkin Formation

The Girkin Formation is a geologic formation located in the Chester Escarpment of central Kentucky, USA. It comprises a level of Mammoth Cave and lies above the Ste. Genevieve Limestone and St. Louis Limestone and below the Big Clifty Sandstone in that area...

, the Ste. Genevieve Limestone

Ste. Genevieve Limestone

The Ste. Genevieve Limestone is a geologic formation named for Ste. Genevieve, Missouri where it is exposed and was first described. It is a thick-bedded limestone that overlies the St. Louis Limestone. Both are Mississippian in age and part of the Meramecian series.Members of the Ste...

, and the St. Louis Limestone

St. Louis Limestone

The St. Louis Limestone is a large geologic formation covering a wide area of the midwest of the United States. It is named after an exposure at St. Louis, Missouri. It consists of sedimentary limestone with scattered chert beds, including the heavily chertified Lost River Chert Bed in the Horse...

, providing a cap for the entire cave system. The existence of Big Clifty and similar sandstones at Mammoth Cave is the cause for the sheer size of the cave passages below it due to water flow through them and subsequent dissolution. Once this cap of the Big Clifty erodes, the cave will begin to shrink in size.

The Big Clifty Sandstone also appears as sandstone knob

Knob

Knob may refer to:* A rounded handle that one pulls or twists:** Doorknob, a round handle one turns to open any door** Control knob, controls a device** Brodie knob on the steering wheel...

s throughout south-central Kentucky, including the 200 feet (61 m) Pilot Rock on the border of Todd County

Todd County, Kentucky

Todd County is a county located in the U.S. state of Kentucky. As of 2000, the population is 11,971. Its county seat is Elkton. The county is named after Colonel John Todd, who was killed at the Battle of Blue Licks in 1782...

.