Geology of Mammoth Cave Part 1 – Laying the Groundwork.
I never thought I would have so much fun starting a blog with a dumb writing trope. But here we are. Well, here I am writing it, and you, later, reading it.
Anyway, Trope waring
“Search Geology of Mammoth Cave on either google or Bing, and the top result will be:”
Limestone.
And fair enough, it’s true. There is a lot of Limestone in Kentucky, and you can blame it for most of the caves in the world. But I feel we can delve deeper, break through the rocky shell to reach another layer of understanding. Insert 3rd rocky pun here.
Anyway, I once went to school, the university kind. I spend a decent amount of my time at that school (a degree’s worth,) studying geology. Don’t worry. I won’t inflict all of that on you here. Just this blog. And an article which will be, at best, 3 pages plus some pictures.
However, I do think that the general geology of Mammoth Cave is interesting enough to warrant it’s own article or two. Perhaps even a whole book!
However, However, I didn’t write a book titled The Geology of Mammoth Cave, I wrote book titled The Specters of Mammoth Cave. There may be a character that book, who might be really cool, who might have an interest in Geology. They might have a thing or two to say about rocks in the book, but I could not fit as much geology as I would like. (Spoiler for a book which isn’t even out yet. The Rocks didn’t do it!)
So here I am, just a guy on the internet, writing about a couple of the things I think are cool about rocks and stuff.
It all began several billion years ago. But we can skip ahead to the Mississippian. (known as the early carboniferous in countries without a Mississippi.) This period lasted roughly 360-323 Million Years ago. This is when the Appalachian Mountains were growing into one of the tallest mountain ranges on earth. When tree ferns, horse tails and conifers were replacing the earlier Devonian trees. When amphibians, giant millipedes, and flying insects roamed. And before massive swamps began their long geologic transition to modern coal reserves.
Horn corals above, Brachiopods below.
But we don’t care about those. What we care about are the oceans. Because the region now known as Mammoth Cave was too far west, and too low and flat to be above the water line. It was an ocean. Much of Kentucky used to be an ocean. This ocean was full of organisms we only know of because of the geologic record. (And the ancestors of modern living things.) Ammonites, and Trilobites, (on their way out but not gone yet,) Crinoids, Brachiopods, Bryozoans, Rugose corals which could be solitary horn corals or colonial, and fish including sharks, lung fish, and coelacanths.
Mississippian sea, as re-imagined by Me. Too many Ammonites, since most died out in the Devonian, also that ichthyosaur-looking thing was supposed to be a coelocanth.
The waters were warm, the seas were relatively shallow, and lots and lots of creatures which lived in them made calcium carbonate shells (the chemical component of limestone.) The perfect environment for, you guessed it: Limestone. But not just limestone, also shale, dolomite, chert/flint, sandstone, all of which are found in the bedrock layer of Mammoth Cave. Also known as the Saint Louis Limestone which, despite its name, includes amounts of all of these.
This rock layer is the deepest exposed layer in Mammoth Cave, and the oldest. That’s usually, but not always, how it works with sedimentary rocks: (older=deeper). And as you can guess from the name, the St. Louis Limestone is a pretty extensive unit. Spanning a large portion of the land west of the Appalachian to land east of the Mississippi. It also goes north into Pennsylvania and Indiana, and South as far as Mississippi. Limestone is any sedimentary rock which is mostly calcium carbonate. Dolomite is made of Calcium Magnesium Carbonate. It’s also typically harder. Chert/flint is made of fine grained (microcrystalline) Quartz. Meaning it’s like sandstone but which micro-grains. (It is also used in a lot of prehistoric tools because it is hard and can make sharp edges.)
These rocks, despite their different compositions all record of a vast ocean which lasted millions of years. Above them is another, similar layer known as the Ste. Genevieve Limestone. Which takes up a similar geographic span, is also Mississippian, (but newer,) and is “Composed of similar types of Rocks to the St. Louis Limestone.” Similar enough that it’s tempting to leave it at that, because I don’t want this to be a billion-word blog.
Crinoids: Not trees, but filter feeding animals related to starfish.
However, there are a few things which stands out. It is said to contain even more fossils, especially crinoids, compared to the older St. Lois. One of the members in the Ste. Genevieve Limestone is called the Karnak Limestone, which, I have not researched, but I am going to put a wild guess in now, that this is probably also related to the Ruins of Karnak, a cave formation in Mammoth Dome, inside Mammoth Cave. Also, This layer is noted to have oolitic limestone near the top, which is a special favorite of mine.
Oolites are basically tiny little balls (spherical concretions) made through physical action of waves, and the chemical action of precipitation. Waves break up larger sediments and wash them around. Then, as these small fragments are washed around by the waves, chemicals precipitate out of the water accumulating on them. Because they are constantly being rolled around by wave action, they typically take on a spherical shape. This process requires two things: Waves, and supersaturated water. The first is obvious, the second works just like any home chemistry experiment which has you make a hyper saturated solution by boiling water and adding stuff to it. (I like to make rock candy, myself.)
I’ve taken liberty with the color, oolites are often white.)
In the Florida Keys today, oolites are still being made. If they are later turned into rock, they will be oolitic limestone. (Maybe the Bahama Sand Banks would be a better modern example.) – (no picture here because I am trying to mostly use my own pictures in this blog.
Why do I find oolitic limestone so cool? Basically, because if you find oolites, you get a very clear and immediate picture of the environment recorded in the rocks: A shallow intertidal sandy dune sea. Limestone itself is a marine record, but I like the added imagery of a crinoids or bryozoans rooted in ground as small sands roll back and forth around them in the waves. Also, oolites are popular in aquariums.
The final layer of rock in Mammoth Cave is the Girkin Formation. It is full of pickles! Not really. Actually it is a Crystalline Limestone layer with lots of oolites. It’s also a much smaller formation than the previous 2, and isn’t as well-resourced online. Apparently the thing which most distinguishes it is a different fossil content, but this is not the blog about fossils. Since I’ve already covered oolites, let’s talk about crystalline limestone.
Much of the limestone in Mammoth Cave is crystalline limestone. That just means that the rock is at least 50% calcium carbonated -what herein there makes it limestone—and also has crystals of other stuff. You have to look much closer at the rock, (magnifying glass, microscope, x-ray spectrometer,) if you want to know what those other crystals are. It’s usually sparkly when you break off the weathered exterior and can be confused with other sedimentary rocks if you desperately want to find something which is not limestone. However, if you drop HCl on it, and it fizzes, you, unfortunately, still have limestone. Which, I guess, in this context is a positive since limestone makes the longest and biggest caves with the mostest formations. (rule of 3 thing here, I’m letting the spelling error ride for comedy.)
But there’s one more thing you need to make a cave. There is limestone all over the surface of the earth. But usually it erodes- wearing down physically and chemically. (more on that when we talk about cave formation!) There must be something which sets Kentucky apart from Guanxi Province China, otherwise the Bluegrass state would have some dope Karst Towers instead of the worlds longest cave.
Sunset in Xingping, China atop one of those dope Karst Towers I mentioned. Honestly, here because I will one day write more about this trip. (Reminder that this is, ostentatiously, a blog and not Geology Digest.)
You thought we were done with the rock talk? Too bad. There is Another Skywalker Layer. Its (mostly) not in Mammoth Cave. It’s above it.
Because every cave needs a protective caprock, something which keeps most of the weathering and erosion out, so that cave formation can happen in the cracks and cavities.
The protective caprock over Mammoth Cave is the Big Clifty Formation. It’s a layer of sandstone, shale, and siltstone which serves as the caprock for Mammoth Cave. It keeps the water out (mostly,) Protecting the limestone layers underneath from more complete erosion. Sandstone is typically silica, Siltstone is typically smaller bits of silica and other minerals. And shale is typically made of clay minerals. I don’t want to spend many more words in this blog talking about them, so for today it will suffice to say that they are important to Mammoth Cave for one reason more than any other:
They don’t let water through easily. Because of this, the Big Clifty Formation protects the rocks underneath (and keeps the water in Sloan’s pond from draining into the groundwater.)
So, there you have it. The Groudwork for Mammoth Cave. It’s rocks. Specifically, rock left behind by a Mississippian Ocean. Three layer (and a sandstone caprock,) which leave any cave visitors with a decent deeper into the sands (more accurately sediments) of time.
When you step into mammoth cave, know that you are surrounded by a record of oceanic life from more than 300 Million years ago, and it only gets older the deeper you go.
But how did the cave get there? More on that in a future article!
Oh!!!!
I have sources this time. If you want to read more about any of this, or see better pictures than those I have drawn, check them out!
RESOURCES:
About the Mississippian Plateau
Geology of Kentucky: Chapter 15, Mississippian Plateaus (uky.edu)
https://www.uky.edu/OtherOrgs/KPS/goky/pages/gokych15.htm
About the Geology of Mammoth Cave
Geology of Mammoth Cave National Park | U.S. Geological Survey (usgs.gov)
https://www.usgs.gov/geology-and-ecology-of-national-parks/geology-mammoth-cave-national-park
About the Mississippian
Mississippian Subperiod | Natural History Museum (humboldt.edu)
https://natmus.humboldt.edu/exhibits/life-through-time/visual-timeline/mississippian-subperiod
About oolites (admittedly I was lazy and used Wikipedia for this one.
https://en.wikipedia.org/wiki/Oolite
About the different rock formations (oldest to youngest)
St. Loise Limestone
St. Louis Limestone | Indiana Geological & Water Survey
https://igws.indiana.edu/compendium/st-louis-limestone
Ste. Genevieve Limestone
Ste. Genevieve Limestone | Indiana Geological & Water Survey
https://igws.indiana.edu/compendium/ste-genevieve-limestone
Girkin formation.
https://thekitchencommunity.org/gherkin-vs-pickle/
(actually, I took most of the information from geology of mammoth Cave source mentioned earlier.)
The Big Clifty Formation
Big Clifty Formation | Indiana Geological & Water Survey
https://igws.indiana.edu/compendium/big-clifty-formation
(Fun Fact: many horned corals were preserved when their shells were replaced with silica, meaning that their fossils are quartz. There are small animals today who make their shells out of silica instead of Calcium Carbonate. For years, I got these two disparate facts mixed up in my head, and thought that Rugose Corals actually made their bodies out of silica.)