File:Limestone wall (Main Cave, Mammoth Cave, Kentucky, USA) (38015050006).jpg

Western Kentucky's Mammoth Cave is the longest cave system on Earth, with 412 miles known and mapped as of fall 2017. The name does not refer to the early discovery of fossil mastodon or mammoth bones here. Rather, the name refers to the immense size of many rooms and passages.

Shown here is an intensely vandalized limestone wall in Main Cave, the principal trunk passage in the Mammoth Cave Ridge portion of the system. The original bare wall was gray, marine limestone of the upper Ste. Genevieve Limestone (upper Middle Mississippian). A crust of gypsum speleothem later formed on the limestone wall. In prehistoric times, American Indians entered the cave, often traveling miles from the entrance. They used cane reed torches, which gave off smoke. In historic times (late 1700s and 1800s), mining operations and cave tours used lanterns that also released smoke. The smoke stained the originally-white gypsum crusts a dark gray to black color - the upper wall in the photo shows this.

The brownish-colored band in the middle of the photo represents where gypsum crust has been chipped away by prehistoric American Indians. They entered the cave at least 2000 years ago, and probably before & after that. Some old, mummified bodies have been discovered in Main Cave. Exactly why the gypsum was chipped away and collected is unknown. They also engaged in flint mining by whacking chert nodules that occur in some limestone horizons. Flint/chert was used in making various tools, such as arrowheads and spearpoints.

The "Mat 1850" graffiti near the bottom of the photo is a scratch signature made by one of the most famous guides in Mammoth Cave history - Mat Bransford (<a href="https://www.nps.gov/maca/learn/historyculture/images/Old_Mat_Bransford_CMS.JPG" rel="nofollow">www.nps.gov/maca/learn/historyculture/images/Old_Mat_Bran...</a>).