My favorite desert is also the largest on the North American continent. Covering approximately 170,000 square miles, it straddles two nations -Mexico and the US- and several states within both nations. Like most bioregions, the Chihuahuan Desert does not recognize man-made boundaries, but man’s impact on the environment can be seen delineating national and local areas. Although a ‘desert’ is defined by more than one criteria, all share a common trait of aridity. But they weren’t always arid. Historical knowledge of any landscape enhances a sense of place and a realization of how places change. And change, they do.
Deep History of the Chihuahuan Desert
Learning about the deep history of the Chihuahuan Desert (or any bioregion) develops an appreciation for the desert as we know it today, and how natural and man-made changes impact all deserts of tomorrow. Although the Chihuahuan Desert is relatively young (~8,000 years), many people perceive it as being old and desolate. Unlike many regions of North American, various stages of the Chihuahuan Desert’s geological history are still visible. If you know what to look for, most of these features are like reading chapters of a book. As geologists tend to say, “Every rock tells a story.”
Rocks from the Chihuahuan Desert have been dated at about 980 million years ago (mya), long before the time of the supercontinent, Pangea (300 mya). For most of that time and periods later, much of the North American continent was covered by ocean water. Stand on most any place in the Chihuahuan Desert, sometimes at thousands of feet in elevation, and one can see deposits -limestone and fossils- of organisms that lived and died in the waters.
Alternate uplifitng of the continent and volcano activity periodically changed the surface of the North American continent, sometimes violently. As the tropical Western Interior Seaway, which separated the eastern and western halves of the continent (~100-56 mya), began to recede south towards the Gulf of Mexico, silt and sediments were deposited. Much of the sandstone and shale in the Chihuahuan Desert are representative of this period in geological time. Over millions of years the submerged continental crust rose above waters and natural landforming continued.
Formation of the modern Chihuahuan Desert landscape is geologically complex. Many periods of uplift and faulting formed the typical range and basin topography associated with most deserts of the North American continent. The extension and pulling apart of continental drift created the Rio Grande Rift and many other geological features in the Chihuahuan Desert. Volcanic activity subsided (55 mya to as early as 500 years ago), mountains eroded into basins, and their deposits can be seen in many areas of the desert.
As basins filled with sediments, the great ancestral waterway of the Rio Grande changed course several times. Over the next millions of years, erosion sculpted many of the topographical features that we see today. In the Big Bend area of the Chihuhuan Desert, the combined waters of the Rio Grande and Rio Conchos flowed in an ancient river channel that overlayed soft rocks. As the water became trapped in its own channel, it cut into the hard rock of the buried highlands. Eventually, the river carved through the massive soft Cretaceous limestone layers forming a series of gorges. Near Santa Elena Canyon in Big Bend National Park, the river separates what was once a large elevated block along the Terlingua fault line. The southern end of the mesa, Sierra Ponce, lies within the Mexico national boundary, and the northern end, Mesa de Anguila, in the US boundary. Today the Rio Grande (or Rio Bravo) is only a small paltry remnant of the majestic waterway of long ago. However, it still remains the major watershed and water source for animals, birds and humans that inhabit the Chihuahuan Desert.
As the great mashlands and shallow seas receded and dried up during the Cenozoic era, mountain erosion and streams deposited sediments while volcanic activity deposited blankets of ash and stacks of lava rocks. The climate was mild and moist, which allowed mammals and vast grasslands to develop and proliferate. However, a drying trend in the Miocene era (15-8 mya) shifted plant and animal communities and signaled the formation of the North American deserts. The uplift of the Sierra Madre Occidentalis in west Mexico, the Sierra Madre Orientalis in east Mexico and south Texas, and the American Rocky Mountains to the north, all created a rain-shadow effect, increasing aridity for the Sonoran, Mohave and Chihuahuan Deserts. Cold had a greater influence on the biology of the Chihuahuan Desert as the continental divide was uplifted, creating an open channel for icy Arctic air.
Distributions of many plants and animal species and closely related species suggest that all deserts of the NA southwest have past connections. Because the geological and climatic changes were relatively recent, the desert biota in the southwest are the youngest on the North American continent. Therefore, it is thought that speciation of many modern desert plants occurred earlier in tropical deciduous forests and thorn scrub. Changing climates and immigration rather than evolution may have had subsequent impact on changes on the desert biota, characterized by dramatically shifting species ranges and community compositions. Species differences likely reflect separation times and evolutionary mechanisms, while some species represent range splits. In fact, the relationships between Chihuahuan and Mohave Desert plants is quite strong.
While communities of life in the deserts may look stable and barren today, both geological and climatic histories of the North American southwest foster an awareness and understanding of changes affecting the interrelationships of land, climate and life. From a land beneath the sea, to periods of sometimes violent terrain alterations, from tropical marshes to ‘belly-high’ grasslands and then desert scrub, the desert was formed and continues to change. We are now in a period of change again, this time more rapid than human historical records have shown us. Although we are familiar with cyclic patterns of weather change, we may be at the advent of long-term climatic change. Observing changes in our surrounding plant and animal communities can provide us with information on these changes that we as humans are often less aware of. Because life in the desert is more sensitive to specific climatic changes, they may also serve as a bellweather of broader and more far reaching changes.
Next Section: Sub-regions of the Chihuahuan Desert (under construction)