. . Crystal Onyx Cave . . . We're Not Mammoth, but we are Spectacular

Crystal Onyx Cave Geology

Geologic map of the area

Geologic Map

This geologic map shows Prewitts Knob as the pink blob in the center, with Cave City to the north.

The green is St. Louis limestone from the Mississippian period, about 350 million years ago.  The blue is St. Genevie limestone, a younger Mississippian limestone and the lime green around the knob is Girkin limestone, from sometime later.

The pink at the top of the knob is Big Clifty sandstone, a still later Mississippian deposit.This whole area of Kentucky was once a layer cake of sedimentary deposits from an ocean that existed in what is now the Midwest during the Mississippian period, 350 million years ago.  Much of the limestone has been eroded or dissolved away.

The sandstone atop Prewitt’s Knob protected its limestone, but water seeping through it has created caverns in it.  Other nearby caves are located in similar formations – Mammoth, Diamond, and Onyx for example.

Chemistry of Caves

Rain water picks up carbon dioxide from the air and soil becoming a weak acid. It goes through cracks in the impermeable sandstone to the limestone. This slowly dissolves the limestone along joints, bedding planes and fractures; sometimes making caverns and tunnels.  The water then also carries calcium from the dissolved limestone. When this water drips and is exposed to the air some of the calcium solidifies in the area of the drip, becoming a calcite stalactite. 

When the water hits bottom, more calcium is deposited to make a stalagmite. If you look at a stub where one has broken off, you can see growth rings from the times it has been active. 

You will see that they grow very slowly, and not all at the same time, as one route gets clogged, the water finds another path. The growth is not more than an inch a century.

Soda straws are thin stalactites which are hollow. The water runs through them and drips off the end.

A growing formation
Stalactites form the appearance of a Tyranasaurus

Tiny Creatures

Limestone is a sedimentary rock formed mainly of the shells of marine creatures. During the Mississippian age, Kentucky was covered by shallow, tropical seas. Although these seas contained crinoids, trilobites, and many other creatures, the limestone on Prewitt’s Knob is made mainly from tiny creatures called ostracods and condonts.  

Other sedimentary layers found in Kentucky include coal beds and oil. The deposits in Prewitt’s Knob include layers of clay that have not solidified, they add red to the color of the cave formations.  

Topographic map of Prewitts Knob

On the topographic map  you will see blue ponds that are identified on other maps as springs. These could be fed by the water which percolates through Prewitt’s knob.  You can also see many sink holes that are characteristic of karst topography.

There are other smaller caves within the property that haven’t been developed. Also many sink holes that could be connected to caves.

Topographic map of Prewitts Knob
Spectacular view of the Great Wall along the Bench Trail

Description written by James St. John, Ohio State University Geologist. The pictures he refers to are on his Flickr page.

Crystal Onyx Cave is a relatively small, speleothem-rich cave on the eastern flank of Prewitts Knob, southwest of the town of Cave City, Kentucky, USA. The walls of the cave are limestones and dolostones of the Ste. Genevieve Limestone (upper Middle Mississippian).

Crystal Onyx Cave was discovered in 1960 as a sinkhole. The name of the cave refers to the whitish-colored travertine (composed of calcite). Fifty-three Indian bones dating to about 680 B.C. were found wrapped up at the bottom of the main shaft of this cave.

The structures shown are examples of “speleothem”, which is the technical term for “cave formations”, and refers to all secondary mineral deposits in caves. The most common speleothem-forming mineral is calcite (CaCO3 – calcium carbonate), the same mineral in limestone, which is the host rock for almost all caves on Earth. Speleothem composed of calcium carbonate is given the compositional rock name travertine. Some travertine forms at the surface, at hot springs or cold springs. Cave travertine forms in many specific ways, and genetic rock names have been established for the many known varieties (e.g., dripstone, flowstone, helictites, coralloids, shelfstone, rimstone, etc.)

Profile and overview diagrams of the cave.

Now Open Daily

One of the most beautiful caves in Kentucky!

Open from 11 a.m. to the last tour at 8 pm.  Come and visit us!

Crystal Onyx Cave * 425 Prewitt’s Knob Road, Cave City, KY 42127 * (270)773-3377