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There was an amphitheater behind me as I took this shot, from which people
can watch the hundreds of thousands of Mexican free-tailed bats leave on
their evening feeding expeditions during the summer. Fortunately, the bats
take a left upon returning to the caverns, into their own section of the
caverns, rather than following the route down to the right, which is the
passage from this entrance into the main part of the caverns.
By late October, there are only a few hundred bats left to see, so we
were too late in the season for the full show.
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One cannot tour any cave without hearing the standard spiel about how to tell
a stalactite from a stalagmite. Even Nathan already knew that a stalactite
has to hold on tight to the ceiling or it might fall, while a
stalagmite might grow to reach the ceiling someday.
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Many bizarre formations are down there to be seen. These cave curtains form
along sloping areas of the ceiling where a source of water clings to the
ceiling for a time before dripping off.
All of the major formations consist of minerals dissolved from the overlying
rocks by groundwater percolating down and reaching the caverns. As it drips
from the ceiling, some minerals precipitate because of outgassing of carbon
dioxide, causing the minerals dissolved by the weak carbonic acid to be
deposited. Depending on how fast the water drips, simple evaporation
contributes to the process, as well.
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