The Big Blue Rock Pod · Kentucky Geological Survey

Ep. 5: Geology of Mammoth Cave

April 25, 2022·1 hr 9 min·5 clips

Discover how the same weak acid in your soda pop dissolves limestone to create massive caves underground.

This episode focuses on the geology and formation of Mammoth Cave National Park in Kentucky. Hosts Matt Crawford and Doug Curl are joined by hydrogeologist and co-host Sarah Arpin, who previously worked at the park and studied its regional geology. Mammoth Cave is the world's longest known cave system, with over 420 miles of mapped passages. The cave formed in three Mississippian-aged limestone units: the St. Louis, St. Genevieve, and Girkin formations. A key geological control is the Big Clifty Sandstone, which acts as a protective caprock over the limestone plateau. Water becomes a weak carbonic acid by absorbing carbon dioxide from the atmosphere and soil, slowly dissolving the limestone. The average dissolution rate for the cave rock is roughly one centimeter per hundred years. A regional dip of the bedrock of about one-third to one-half degree northwest guides water toward the Green River, creating long flow paths. The sinkhole plain south of the park, lacking the protective caprock, funnels vast amounts of water underground into the system. Research by geologists like Art and Peg Palmer identified multiple cave levels, correlating their formation with cycles of glacial advance and retreat. A seminal study by Granger, Fabel, and Palmer used cosmogenic dating of quartz pebbles to determine some upper cave passages were already forming over 3.5 million years ago. The sandstone-capped ridges erode extremely slowly, at about 1.7 meters per million years, preserving the cave structure. In contrast, periods of rapid glacial meltwater incision occurred at rates around 30 meters per million years. Cave features like scallops on passage walls indicate past water flow direction and speed. The establishment of Mammoth Cave as the world's longest system in 1972 resulted from both its unique hydrology and decades of exploration and mapping. The "Kentucky Cave Wars" were a historical period where landowners competed for tourists by promoting private caves, sometimes damaging formations and artifacts. Modern exploration continues, including cave diving at springs where the system discharges into the Green River. The tone is conversational and educational, blending casual host banter with detailed geological explanations. Listeners interested in earth science, Kentucky's natural history, or cave systems will find this episode fascinating. Those seeking high-drama narrative or non-geological topics might want to skip it.

As heard by us

A grounded look at Mammoth Cave's rock layers, river setting, and the harder question of its geologic age.

Mammoth Cave comes through as a real place, not just a landmark name. The discussion ties it to the rock layers, the sinkhole plain, the Big Clifty sandstone, and the Green River, which keeps the geology easy to picture even when the details get technical.

Read the full review in PlayNext →

Why you'd press play

Want Mammoth Cave explained from the bedrock up, clearly?

Read the full recommendation in PlayNext →

Listen to the show on

Apple Podcasts Spotify

Play clips from this episode More from The Big Blue Rock Pod