Huck Finn’s caves, beside the Mississippi, run for a few miles from a single well hidden entrance. The rock is limestone, layered horizontally when MO was under water. When the seas retreated the rock broke a little, in a very linear grid pattern of vertical cracks. We’re told that water and CO2 in the air make a weak Carbonic Acid solution that dissolves limestone and so opened up the cracks over time, creating what we now see as caves.
I never found that CO2 and water story very convincing until the excellent park guide shone her flashlight up on the ceiling where we could see the oily marks from the swarms of bats that gathered there in 2 particular spots. The photo below (looking straight up) shows the nearer of the 2 spots with a crack which lets in some rain water from above. The CO2 in the bat breath has created enough Carbonic Acid to disolve the rock at the crack and start to visibly open it up. (no bats today – they don’t like the light)
A large amount of corrosion, long ago, has made a “Boot” of stone which precariously hangs overhead and we all walk under holding our breath.
Outside where storms blow we saw:This repair sign was one of at least 4 in the tiny town of Hannibal. I guess it says something about the weather. We left just ahead of huge storms (no tornadoes thank goodness) and drove for 10 hours back to 341 happy to be far from Joplin, MO where the search goes on through the wreckage of last weekends disasterous tornado that hit the small city full on. Once, when stopped at a toll booth, we were caught by the edge of a storm with 1/2 inch size hail stones. They sound horrible bouncing off the car roof. It felt that if you were driving into them they could easily crack windshields.