Beautiful mineral deposits occur in some natural caves. Deposits that
look like icicles, called _stalactites_, are found hanging from the
ceiling of a cave. Other deposits, _stalagmites_, are like the
stalactites except that they jut upward from the floor. _Columns_ are
formed from stalactites and stalagmites that have joined together. In
addition, some caves contain sheet-like deposits that are spread along
the ceiling, floor, and walls. These deposits are called _flowstone_.
Calcite is one of the minerals that commonly form cave deposits.
Just a few of the caves in Texas contain these deposits. They occur
mostly in the limestone rocks that are south and southwest of the Llano
uplift area of central Texas. Some of the commercial caves that contain
good examples of calcite deposits are located near Boerne in Kendall
County and near Sonora in Sutton County. Calcite deposits also occur in
Longhorn Cavern, a large cave located in the Longhorn Cavern State Park
of Burnet County. These caves were formed by underground waters that
moved through cracks and pores in the limestone rocks and dissolved
passageways in them. After the cave passages were made, water containing
dissolved calcium carbonate dripped into the cave. As it evaporated,
this water left behind a deposit of calcium carbonate—the mineral
calcite.
You can better understand how the cave deposits are formed by watching
icicles grow in wet, freezing weather. First, small hanging drops of
water freeze, and a small icicle forms. Then, as more water drips over
it and freezes, the icicle grows longer and wider. Some of the water
drips completely over the icicle and falls to the ground. There, it
either freezes into a sheet of ice, or it begins to build upward to form
an upside-down icicle. The water dripping down in the caves evaporates
instead of freezing, and in doing so it leaves behind a deposit of
calcite.
[Illustration: Calcite stalactites and stalagmites in the Caverns of
Sonora, Sutton County, Texas. Photograph courtesy of the Travel and
Information Division of the Texas Highway Department.]