How Mammoth Cave Formed

 

Modern tour trails travel down Broadway, one of the upper cave passages.

NPS Photo/ Jackie Wheet

When you visit Mammoth Cave for the first time, you may find yourself marveling at its towering passageways, mountainous heaps of fallen rock, and its maze-like sprawl. But how did it all form?

 

Solution Caves

There are many types of caves that form in different, incredible ways. Mammoth Cave, the world’s longest known cave, is a well researched example of a “solution cave.”

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.

The water actively dissolves minerals in the limestone. Over a long, long period of time, the channels enlarge so that water can flow more freely. As more water passes through the passageways grow. When these passages are large enough for humans to enter and explore, they officially become caves!

 

River Styx, one of the underground rivers, is illuminated by flashlights.

NPS Photo

How old is Mammoth Cave?

Researchers have dated Mammoth Cave’s rock beds to the Mississippian Period.

The rock beds formed about 320 – 360 million years ago. However, the passages of Mammoth Cave did not start forming until about 10 – 15 million years ago, when streams and rivers that were flowing over the surface allowed water to sink in and enter the rock beds through small cracks.

The highest passages formed until about 2 million years ago, when research speculates that ice age activity altered the amount of water flowing into the cave and the remaining cave streams descended to lower levels of the cave.

There are currently five distinct “levels” of passageways within Mammoth Cave. These consist of four fossil levels and the modern river level, which is the lowest level located over 300 feet beneath the surface.

The cave is still forming today as water moves downward through these large and small passages to reach the water table, and underground rivers continue to flow out of the cave and into the Green River.

 

Types of Passages

Most of Mammoth Cave’s passages are dry, only occasionally letting water seep into specific areas where stalactites, stalagmites, and other formations can be found. Depending on what part of Mammoth Cave you visit, you may notice different types of cave passages that you travel through. Here are a few common passage types and the unique ways they formed.

 

Boone Avenue along the Grand Avenue tour is an example of a canyon passage.

NPS Photo

Canyon Passages

These passages are formed by underground streams that follow the slightly tilted layers of limestone, eroding channels downward through the bedrock. These passages are usually taller than they are wide, and resemble canyons you might explore above ground, except that they have a cave ceiling over top!

Some canyons can be quite tall as they have carried massive amounts of water throughout their formation.

 

Broadway, one of the upper cave passages. The dirt trail has since been replaced by modern paved tour trails.

NPS Photo/ Jackie Wheet

Large Canyon Composite Passages

Large Canyon Composite cave passages form from several passages eroding together. They can be formed from the ceiling or floor of stacked tube passages breaking down and joining together to form a larger open passage with additional canyon components. Many of the upper tour routes in Mammoth Cave follow these large canyon composite cave passages.

 

Cleavland Avenue is a classic example of a Tube Passage.

NPS Photo/ Thomas DiGiovannangelo

Tube Passages

When you think of a tube passage, think of a pipe full of water. Tubes are wide, oval-shaped tunnels that formed while filled up completely with flowing water.

In order to be full of water, these passages must form in lower levels of the cave, where the water table would have been found. When the water table drops, the older, higher placed tubes empty out and become dry, allowing new passages to form in lower levels of the cave. Today, active tubes form the underground springs that transport water from the lowest levels of the cave out to the Green River. Visitors can walk through tubes passages in places like Cleveland Avenue on the Grand Avenue or Accessible Tours or Gothic Avenue in the Historic section.

 

Cathedral Domes, seen on the Wild Cave Tour, is great example of a large Vertical Shaft.

NPS Photo/ Jackie Wheet

Vertical Shafts

Where limestone beds display vertical cracks through multiple layers of rock, the water flowing through the rock beds will obey gravity and flow straight down.

Vertical shafts form when this happens. Vertical shafts are tall, narrow passageways that go straight down into the cave. In Mammoth Cave, vertical shafts can range in size from 30 feet to 200 feet tall. One particularly large vertical shaft can be seen on the Historic Tour. Know as “Mammoth Dome”, this vertical shaft is over 190 feet tall.

 

Fat Man's Misery, seen on the Historic Tour, is a small winding Keyhole Passage.

NPS Photo/ David Kem

Keyhole Passages

A keyhole passage has a shape like its name suggests: It has an oval top and then has a narrow passage extending down vertically from the oval. These passages are really just a combination of a tube passage and a canyon passage. They can tell you about changing water levels that occurred as the cave was forming. 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.