A big part of understanding caves is learning about the rock that makes up the cave. Learn the different types of rock found at Mammoth Cave and why they are the perfect combination to hold the longest cave system in the world.
The most common rock in Mammoth Cave is limestone. Limestone is a sedimentary rock that allows cave passages to easily form because of its chemical makeup and ability to be dissolved by acidic groundwater. Mammoth Cave’s limestone formed about 330 million years ago at a time when a warm, shallow ocean covered much of the southern United States, including parts of Kentucky.
Limestone forms when calcium carbonate (CaCO3) minerals in water cement together fossil fragments and other sediments gathered on the ocean floor. Over millions of years, this process creates limestone bedrock. Geologists classify Mammoth Cave’s limestone into three distinct formations (from youngest to oldest): Girkin, St. Genevieve, and St. Louis.
Shale forms when wet swampy clays and muds are compressed over the course of millions of years. The sediments that make up shale are tightly packed, this arrangement means that shale, unlike limestone, is not a porous rock.
One of the shales at Mammoth Cave formed about 320 million years ago as the shallow sea receded and returned, depositing layers of shale above the limestone beds the cave is formed in. This shale is a caprock that serves as a sort of “roof” that protects the limestone layers and cave system below from leaks. This is one of the reasons that much of Mammoth Cave is dry.
Sandstone forms when tiny particles of sand, minerals, weathered rocks, and organic materials are compressed together tightly. Unlike limestone, sandstone is formed from rocks and minerals from the continental crust, rather than from minerals in the water of the ocean.
One of the sandstones at Mammoth Cave, named the Big Clifty Sandstone, formed about 320 million years ago, when sand was deposited over top of Mammoth Cave’s shale and limestone beds by a large river delta. This sandstone is usually the top layer of the park’s many ridges, like Flint Ridge, Mammoth Cave Ridge, and Joppa Ridge because it is resistant to erosion.
Other Rocks Found in Mammoth Cave
While limestone makes up the majority of Mammoth Cave and shale and sandstone serve as it’s protective layer, other rocks contribute to the cave’s beauty and geology.
Dolomite – looks very similar to limestone and it is made of calcium-magnesium carbonate. Dolomite usually forms when magnesium-rich water seeps through pre-existing beds of limestone, creating a smooth rock surface with many small pores. For this reason, some people may use the phrase “dolomitic limestone” to describe it.
Siltstone – Siltstone is a light-grey, gritty rock, made of microscopic quartz sediments. In it’s pure form, it may be found above-ground in some high bluffs. In the cave, silt is found in impure, “silty” limestone beds. Silt allows this limestone to weather more easily than purer limestone. You may notice silty limestone in recessed ledges where limestone is very crumbly.
Chert – more commonly known as flint, chert is a hard, blackish-grey rock. Because Chert resists being dissolved by water, it can be found jutting out from limestone passages in formations called nodules. Early Native peoples used chert to form tools and weapons due to its hardness and ability to be sharpened.
Conglomerate – Conglomerate is like sandstone, but contains larger grains like pebbles. It represents areas that saw faster moving water since slow moving water does not move pebbles. An important conglomerate is the youngest rock found in the park. It was deposited about 310 million years ago.
In the book A Geological Guide to Mammoth Cave National Park, the renowned cave geologist, Art Palmer, invites his readers to “[I]magine making a vertical slice through the rocks of central Kentucky, so that you can view them from the side.”
If you took a slice of Mammoth Cave, from it’s highest to lowest points, you would have yourself a stratigraphic column.
Stratigraphy, or the study of rock layers, gives us an idea of how our world formed. Because of Mammoth Cave’s spacious, tall passages, you can observe over 300 feet of stratigraphy in your visit to the park.
There are three limestone formations that make up the cave system: the Girkin, St. Genevieve, and St. Louis. The Big Clifty Sandstone formation acts as protective layer at the top of ridges. Mammoth Cave’s passages descend through a total thickness of 300 feet of limestone, occupying all of the Girkin and St. Genevieve, and half of the St. Louis.
Within each of these formations, geologists have identified additional “members” (or beds of limestone with similar characteristics), and even sub units. Studying Mammoth Cave’s rock layers, or stratigraphy, gives us a picture of the geologic events that formed the earth, helping us understand the past.