The Trans-Pecos region, located in the westernmost part of the State, is
an area of mountains and plateaus with broad basins between the major
mountain ranges. Many different types of rocks are exposed in
Trans-Pecos Texas and these include marine, fresh-water, and terrestrial
deposits. In many areas igneous rocks flowed out on the surface and now
overlie sedimentary rocks. There are also many places where igneous
rocks have been injected into the surrounding rocks, and these igneous
rocks have been exposed by later erosion.
Included within this area is the Van Horn uplift of southern Hudspeth
and Culberson counties, the Solitario uplift of southern Presidio and
Brewster counties, and the Marathon uplift of northeast Brewster County.
This region also includes the Big Bend area of Texas, a part of which
has been set aside as a National Park where many interesting and
important geological features may be seen.
The Trans-Pecos region is one of rugged topography with elevations as
high as 8,700 feet, at Guadalupe Peak in the Guadalupe Mountains of
northern Culberson County, and as low as 1,500 feet, in the Rio Grande
valley.
Numerous invertebrate fossils occur in the Cretaceous limestones and
shales of the Trans-Pecos region and in the Paleozoic rocks of the
Marathon uplift. The Gaptank formation of Pennsylvanian age and the
Permian reef limestones of the Glass Mountains are especially
fossiliferous. In addition, many vertebrate fossils have been collected
in Trans-Pecos Texas, particularly in and around Big Bend National Park.