Four hundred twenty-six miles is a long distance. It’s a full day’s drive from your home to anywhere. Now, instead of driving, think of walking, crawling, climbing, and maybe at times wading through high water that same distance using only a carbide lamp or LED lamp on your helmet. That’s what explorers have done to survey and map the more than 426 miles of Mammoth Cave, making it the longest known cave system in the world.

Crawling through a tight space on the Intro To Caves Tour, Mammoth Cave National Park / NPS - Basia Gowen

How did Mammoth Cave form? What unique geological processes occurred to create what Arthur M. Palmer, in his book A Geological Guide to Mammoth Cave National Park, says is “… a natural underground plumbing system of colossal proportions, in which active and abandoned passages interconnect in a three-dimensional labyrinth so large that no one person has seen more than a part of it.”

Mammoth Cave is a solution cave, the product of a karst landscape. Karst is a topography formed from the dissolution of soluble rock like limestone by weak acidic water. Sinkholes, ridges, springs, sinking streams, and caves are characteristic karst landforms. Almost all the United States have areas of karst topography, and it’s quite prevalent within the southern U.S., including Kentucky, Florida, Alabama, and Georgia.

According to park staff:

Solution caves form when rainwater percolates through the soil, picks up carbon dioxide (from both the air and soil), forming a weak acid. This acidic water squeezes between the small cracks and layers of bedrock, such as limestone, and dissolves out a small channel in the rock for the water to flow.

Three hundred million years ago, a shallow sea covered the southern part of North America, including what is now west-central Kentucky. Sediments typically found in marine environments (limestone, dolomite, silt, shale, sandstones) were deposited during that time, ultimately forming various rock layers to which geologists have given names: St. Louis Limestone, Ste. Genevieve Limestone, Girkin Formation, and the Big Clifty Sandstone (the caprock overlaying the three limestones). Mammoth Cave extends through these three limestone layers.

Why limestone?

Limestone is a sedimentary rock composed of calcium carbonate, which constitutes the seashells littering the bottom of that ancient shallow sea as the organisms living within those shells died.

Calcium carbonate is easy to dissolve with a weak acid created from rainwater and snow percolating through the soil and picking up carbon dioxide along the way. In the case of Mammoth Cave, this weak acid dissolved small holes and cracks in the limestone, widening these openings. Over a long period of time, those holes became larger channels through which flowed a larger amount of water. This larger flow widened and deepened the channels even more, creating passageways, many of which were sizeable enough for cavers to navigate.

According to park staff:

The extensive regional underground drainage system is represented by many karst valleys, sinking streams, and springs. The abundant annual precipitation (132 cm [52 in.]) passes rapidly underground through sinkholes and joints to join the base-level flow of the Green River

While the rock layers of the Mammoth Cave system appear horizontal, they do tilt slightly enough to allow springs, streams, and rivers to continue cutting downward while abandoning upper passageways like the ones you will see during one of Mammoth Cave National Park’s cave tours.

The great length of the Mammoth Cave System comes from the many hours of cave exploration leading to discoveries of connections between Mammoth Cave to the Flint Ridge and Roppel Cave systems. In addition to this linked series of cave systems, 300 smaller caves have been discovered and mapped within the park, all a part of the unique karst geology so prevalent in the area.

Types Of Cave Passages

While embarking on one of the guided cave tours in the park, you may notice different passageway shapes. There are three main types from which form numerous variations in Mammoth Cave:

Cleaveland Avenue, an example of a tubular passage, Mammoth Cave National Park / NPS file

Tubular/Tube passages – these are tunnels with round or elliptical cross sections. Cleaveland Avenue and Gothic Avenue are great examples of tubular passages. Tubular passages formed while completely filled by water from the Green River flowing through them. Wider than they are tall, these passages can measure 100 feet (30.48 m) wide and 30 feet (9 m) tall. As the Green River eroded downward, these passages were abandoned in favor of lower openings through the limestone.

Boone Avenue, along the Grand Avenue Tour, is an example of a canyon passageway, Mammoth Cave National Park / NPS file

Canyon passages – these are passageways formed above the level of the Green River by underground streams flowing downward through the limestone. They tend to be taller than they are wide, just like surface canyons except with cave roofs. Canyon passages have a consistent width from the top to the bottom, where the water may still be cutting a downward path. Boone Avenue along the Grand Avenue Tour is an example of a canyon passage.

Cathedral Domes is an example of a vertical shaft, Mammoth Cave National Park / NPS - Jackie Wheet

Vertical shafts – these tall, narrow passages were created by water flowing straight down through vertical cracks in the limestone. Cathedral Domes, seen along the park’s Wild Cave Tour, and Mammoth Dome, seen along the Historic Tour, are examples of vertical shafts. the Mammoth Dome vertical shaft is over 190 feet (57.9 m) tall.

Fat Man's Misery is a great example of a keyhole passage, Mammoth Cave National Park / NPS - David Kem

Variants may occur from these three main passageway types, such as the keyhole passage, which looks like a keyhole, with a wide oval top from which extends downward a narrow, vertical passage. This passage type is formed from a combination of a tube passage and a canyon passage and give a good indication of the changing water levels.

According to park staff:

The tube at the top forms at or just below the water table, but when the water table lowers, the stream that was in the tube cuts a canyon in the floor as it goes down toward the new water level. Fat Man's Misery on the Historic Tour is an excellent example of a keyhole passage.

Mapping The Cave

How do we know Mammoth Cave is as lengthy as it is? How do we map and measure the cave and those distances? Cavers and cave surveys!

Cave surveyers like Dick Market use a compass and measuring tape to get the distance and height of a room or passage at Mammoth Cave National Park / NPS - Rick Olson

Detailed notes and sketches are written on waterproof graph paper while surveying a passage or room at Mammoth Cave National Park / NPS file

Cave surveying is literally dirty work. Cavers (the commonly-applied term for “spelunkers”), must be prepared to walk, climb, clamber over cave breakdown (piles of fallen rock from cave ceilings), get on their knees and crawl through dirt and mud, squeeze through tight openings, and even wade, sometimes up to their necks, in muddy water. All along the way, cavers are making detailed notes and sketches of the size and shape of passages, indicating areas of breakdown and cave formations (speleothems) on waterproof grid paper. At the same time, cavers are measuring passage lengths using a compass, and measuring tape. Upon return to the surface, the process of refining the sketches and “number crunching” the compass readings will create an accurate map and view of that passage or series of passageways.

Click here to read more details about surveying and mapping Mammoth Cave and caves in general.

Speleothems And Solutional Features

One of the most delicate speleothems found within the cave are gypsum flowers, Mammoth Cave National Park / NPS - Thomas Digiovannangelo

Stalactites hanging from the ceilings like icicles, and stalagmites rising from the ground are two of the most common speleothems (cave formations) seen during a tour of Mammoth Cave. But there are plenty of other formations, many with descriptive and colorful names, like soda staws, cave popcorn, rimstone dams, gypsum flowers, and flowstone. Temperature, humidity/wetness, jointing and other structures in the rocks themselves, and soluble minerals such as calcium carbonate and gypsum have a hand in speleothem creation.

In a nutshell, water underground is under pressure, carrying a super-saturated solution of calcium carbonate. Once that water enters an area of less pressure, like the opening into a cave passageway, the pressure is released and a thin film of calcite precipitates out and covers the surface where ever the water drips. Layers and layers of calcite build up over thousands of years to create the features you will see on cave tours like the Frozen Niagara Tour or the Domes and Dripstones Tour.

Forming some of the most delicate features in Mammoth Cave, gypsum flowers occur in dry passageways within the Mammoth Cave System. The mineral-rich water evaporates, and delicate, curving fronds of white-to-gray gypsum grow outward in clusters from ceilings and walls.

You can read more about speleothems in general here, and Mammoth Cave speleothems in particular here.

In addition to speleothems, you will see interesting features created by water action in the cave. These solutional features include scallops, anastomoses, and flutes, and solution pockets.

An example of cave scallops seen in River Hall, Mammoth Cave National Park / NPS - David Kem

Scallops are spoon-shaped hollows dimpling the cave walls. They form from slow-moving water through the passageways. Scallops indicate the direction of water flow and even water velocity. You can see scallops at River Hall along the Historic Tour route.

An example of anastomoses solution feature / Wikimedia Commons

Anastomoses are small winding tubes that interconnect with each other in a maze-like pattern. Opening into the walls of tubular passages, they generally occur at ceiling level.

According to A Geological Guide to Mammoth Cave National Park:

They may be remnants of earlier solution channels formed during the earliest stages of cave development, or they may be formed by the periodic flooding of passages located at, or slightly above the water table when water is forced into partings under pressure.

Anastomoses occur in the ceiling of Swinnerton Avenue within the Flint Ridge Cave System, and in Diamond Caverns, located right outside the boundary of Mammoth Cave National Park.

An example of fluting seen at Mammoth Dome, Mammoth Cave National Park / NPS - David Kem

Flutes are parallel grooves carved by water dripping vertically or flowing down a deep slope. The grooves follow the direction of the water. They are common in vertical shafts and where waterfalls flow in other passageways. You can see examples of flutes near the bottom of Mammoth Dome along the Domes and Dripstones Tour.

Solution pockets seen in the Rotunda, Mammoth Cave National Park / James St. John via Flickr

Solution pockets are irregularly-shaped cavities extending into the walls, formed by dissolution of the limestone along bedding planes or joints. A large example of a solution pocket may be seen in the ceiling of Sparks Avenue, along the Historic Tour.

For a more detailed look at the geology of Mammoth Cave National Park, you can read and download the park’s 2011 Geologic Resources Inventory Report.