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. WHAT TO DO IN BIG BEND NATIONAL PARK Approximate Round Trip Time