Layers of History/Layers of Rocks
When visitors tour Bluff Dwellers Cave most of the conversation is about the cave itself. How old is the cave? What lives in the cave? What people used the cave? While it is discussed, the rocks themselves are often overlooked. There are many layers of history at Bluff Dwellers Cave, and today I would like to discuss the layers of rock that allowed the cave to even be possible.
Karst is a type of landscape characterized by dissolving bedrock and caves are just one feature of a karst landscape. Sinkholes, springs and caves are all places where water has dissolved and/or eroded away the bedrock. In order for a cave to form the rock has to be a type that is soluble. The types of rocks that form caves include limestone, dolomite (a limestone with high levels of Mg), marble (which is limestone that has undergone metamorphism) or gypsum. The most common rock to find caves in is limestone, which is exactly what Bluff Dwellers Cave has formed in. The limestone itself tells a very interesting story: it is the result of millions of years of sediment depositing at the bottom of an ocean.
Bluff Dwellers Cave is formed in limestone from the Mississippian period (or Carboniferous if you were taught the geologic time periods outside of the U.S.). Limestone is a sedimentary rock that forms at the bottom of the ocean. The sediment is created from the shells, skeletons and other hard parts of animals that live in the ocean. It is all made of calcium carbonate (CaCO3). As the sediment builds up in horizontal layers at the bottom of the ocean it eventually gets buried. The sediment then lithifies into the rock we call limestone. Yes, that means that limestone is made almost entirely out of fossils. Amazing!
There are several layers of limestone and one layer of shale that you will see in Bluff Dwellers Cave, all are from the Mississippian Period. Rock layers are named by geologists to help separate different prehistoric environmental conditions. There are 4 layers in Bluff Dwellers. The oldest is on the bottom, Compton Limestone. A thin layer of shale called the Northview Shale is on top of this. Above this is the Pierson Limestone, most of Bluff Dwellers Cave’s passageways are within this rock. On the roof of our cave is the Reeds Spring Limestone, which has a lot of insoluble chert nodules. You will see fossils throughout the Pierson limestone. Brachiopods and crinoids are abundant.
Each layer of limestone and shale tells a different story. The size of the sediment/fossil pieces tell us a story about the oceanic environment: Was the water calm, or was it churning violently? Was the water deep or was it shallow enough for sunlight to penetrate to the bottom? These different environments would allow different animals to live there, allow different size of sediment pieces to settle to the ocean floor.
Next time you visit Bluff Dwellers Cave take a moment to look at the layers of rock. Touring a cave gives you the unique opportunity to see INSIDE a rock. Layers of rock represent layers of history that built up over millions of years. What an amazing planet we live on.
-Nicole
Bluff Dwellers Cave Manager & Resident Geologist
