Millions of years ago, the ocean which extended from the Gulf of Mexico
to the Arctic Circle covered this part of west Texas. Muds, sands, and
gravels were deposited in the bottom of that ocean. In these sands and
gravels were preserved the remains of fish, shellfish, and various other
forms of sea life.
Tremendous pressures within the globe’s interior caused the earth’s
surface to rise in this region and the waters to recede. Much of the
area became swampy, forming suitable habitat for the giant dinosaurs.
Peat beds were compressed to form coal deposits. The remains of these
coal beds as well as of petrified trees and dinosaur bones may be seen
in the rock exposed in various parts of the park. At a still later date
there were more extensive earth movements which caused great breaks to
occur in the earth’s surface.
[Illustration: _Sierra del Carmen in Mexico_]
Generally speaking, the geologic structure in the Big Bend area is a
“sunken block” that might be compared to cutting through the elevated
landscape twice. One cut is along the Sierra del Carmen mountains on the
east, and the other at Santa Elena Canyon on the west. The area between
the two cuts has sunk an average of several thousand feet. The central
part of this block has been arched upward forming the Mariscal, Chisos,
and Christmas Mountains.
Part of this uplift was caused by volcanic activity. Large masses of
molten rock pushed toward the earth’s surface and arched and deformed
the overlying sedimentary layers that had long before been deposited on
the floor of the ocean. The Chisos Mountains were formed by this
volcanic pressure.
As you look south or north, you see an extensive plain that is underlain
very largely by sedimentary rock layers. The isolated peaks for the most
part were caused by hot lava that pushed up from below, then cooled to
make the present highlands. This is also true of the Chisos Mountains
which are a much larger mass. Cooling of these melted rocks caused
fracture joints, or shrinkage cracks, and weathering along these breaks
caused the columns, spires, buttresses, and pinnacles that you see in
all directions near you. Forces of erosion have done most of the
carving, forming the various rugged features near at hand.
The geological history is very complex and cannot be even summarized in
a few paragraphs. Park Rangers can give you additional information
regarding the geological history, or explain the origin of features you
may wish to know more about.