My next post about the Geology of the National Parks Through Pictures is a park we visited back in 2002 on a Spring Break trip. Sorry for the terrible quality of the pictures from the cheap-even-then camera.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
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Long before I ever intended on doing geological snippets on the National Parks, we visited Mammoth Cave National Park. As the longest cave system in the world, this in and of itself, is a geological wonderland. We had managed to go to the caves twice, but I only seem to have pictures from our first trip.
The Mammoth Cave system is located within central Kentucky, along with a lot of other cave systems. However Mammoth Cave boasts the largest interconnected cave network in the world, totaling more than 405 miles of caves. Most caves form within a type of rock known as a limestone. Limestones are rocks formed by the deposition of animal life within a marine environment. Many marine animals form shells made out of mineral called calcite or similar minerals, all of which are carbonate minerals (Calcite = CaCO3). These animal life deposits could take the form of shells (such as clam shells), or even just what is called "lime mud", created by algae or the break down of shells.
The main thing about carbonate minerals is that they dissolve in acid. When acid is added to calcite or other carbonate mineral, the acid combines with the CO3 part of the mineral to produce carbon dioxide (CO2), water (H2O), and some ions. The acid doesn't even need to be that strong. As rain water travels through the soil it picks up carbon dioxide, producing a weak acid known as carbonic acid. This type of acid, now in the groundwater, is strong enough to slowly dissolve away limestones located below the surface over the millions of years that the rocks are exposed to the groundwater. As the limestones are dissolved away, any surface features slowly collapse due to the now empty voids below. This type of landscape is called karts topography. You can see an overview of the Mammoth Caves karst here but there are many other areas where is can be better seen.
Not all of the landscape collapses down. Frequently in cave systems, the overall strength of the limestone is strong enough to maintain the structures for a long while, until too much of the rocks are dissolved away. There is also usually a caprock, which is an indissolvable rock (often sandstone) that protects the limestone below from getting too much of the rainwater. Here is a field fairly close to the natural cave entrance of Mammoth Caves where the ground surface is fairly stable due to the caprock. However there is still active dissolution of the limestone within Mammoth Cave by not only the groundwater but active river systems in the cave. Currently flowing thorough the cave system are the Green and Nolin Rivers, totaling 23 miles in the lower caves.
Seen here is the historic natural entrance to Mammoth Caves. Overall, the cave travels through four different rock units. Going oldest on the bottom to the youngest at the top, the rock units are the St. Louis Limestone, the Ste. Genevieve Formation, the Girkin Formation, and the Big Clifty Formation, all of which are Mississippian in age (~330 million years old). All of these formations formed on the shores of or within an ancient ocean that used to be located across the middle of North America. Most of the caves that visitors are likely to visit are within the Ste. Genevieve and Girkin Formations.
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| Mammoth Caves cross section. Image courtesy of the Kentucky Geological Survey. |
The Big Clifty Formation (AKA the Big Clifty Sanstone) is composed mostly of sandstone and shale, forming the indissolvable caprock to the Mammoth Cave system. You can see the Big Clifty just below the soil as you enter the Historic Entrance pictured above. Although the Big Clifty Formation can reach 50 to 60 feet thick, generally you can only see the lower part of the formation. Below the Big Clifty at the Historic Entrance is the Girkin Limestone. The Girkin Limestone is 135 to 140 feet thick and is composed mostly of fine to coarse grained crystalline limestone, formed from the inorganic precipitation of calcite directly from the water.
Water first entered the cave system about 10 million years ago, entering in through the upper Girkin Formation. By 3.2 million years ago, the upper cave systems were completely formed. Below the Girkin Formation, Ste. Genevieve Formation is 110 to 120 feet thick and composed of interbedded limestone and dolostone with fossils of corals, bryozoans, brachiopods, shark teeth, gastropods, and crinoids.
Below the Ste. Genevieve Formation, the St. Louis Limestone is 175 to 200 feet thick and is composed of a mix of limestone, dolostone, sandstone, siltstone, and shale, as well as an abundance of chert nodules. The St. Louis Limestone also has a similar fossil assemblage as the Ste. Genevieve Formation.
Within the cave systems, the cave formations are formed not only from the dissolution of limestone by carbonic acid, but also the precipitation of limestone. As the groundwater travels through cracks in the limestone and enters cave systems in the lower levels, any calcite that is dissolved within the water is left behind as water drips down. These deposits form the classic stalactites (on the ceiling), stalagmites (on the floor), and columns. They also produce other interesting formations known as flowstones as the water flows down various surfaces of the cave. Here water is both dissolving limestone as well as precipitating limestone. The specific variety of limestone that is produced by this precipitation is known as travertine.
Here you can see a flow stone formation that forms a "curtain" where it flows down the wall to an overhang where stalactites are precipitating off of.
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