The Guadalupe mountains were created by the Capitan reef, an accumulation of algae, sponges, and other ancient marine life during the Permian period (about 250 – 300 million years ago). At the time, much of present-day west Texas and New Mexico was under the Permian Sea.
In more recent (drier) times, El Capitan has been used as a landmark and signal mountain for centuries. Being essentially a fossilized reef, the Guadalupe range is made up almost entirely of limestone. This means it has little surface water, but is home to some really spectacular caverns — like, for instance, Carlsbad Caverns (which I’ve written a bit about previously).