They were abandoned by active water the longest ago, and they have had the most time to develop the secondary mineral deposits that form in the absence of liquid water.

The cap of Big Clifty Sandstone above the limestone is crucial: it is relatively impermeable, which means rainwater doesn’t soak through it easily.

Below the sandstone, the limestone passages are protected from intrusion of water from above.

This is what keeps the upper passages dry and allows the gypsum and other sulfate minerals to form and persist.

Without the Green River cutting its valley, there would be no levels, no dry upper passages, and no cave environment suited to the sustained formation of the mineral deposits the ancient miners sought.

The river is the indirect cause of everything that happened inside Mammoth Cave for thousands of years.