The geologic history of the Southwestern United States spans more than two billion years. This history is unusually well exposed throughout the area’s extraordinary geological landscape, including spectacular mountains, canyons, and countless other striking features. The Southwest’s dynamic geologic history has resulted in the formation, preservation, and exposure of rocks from a wide variety of ages and sources.

Rocks exposed in the Southwest reveal that a large continental mass—a supercontinent—was repeatedly assembled and disassembled over hundreds of millions of years. By around 600 million years ago, the core of what would eventually become most of North America was a recognizable entity. Over the next several hundred million years this continent was mostly tectonically stable and flat, and was repeatedly flooded and exposed by rising and falling sea levels. Around 300 million years ago, episodes of tectonic activity and volcanism added land to the continent along what would become the West Coast. Major mountain building (orogenesis) began around 100 million years ago, and reached its peak around 65 million years ago, at the very end of the Mesozoic era. These orogenic episodes formed the modern Rocky Mountains, which have dominated the geology and landscape of western North America ever since. At the same time that the Rockies were rising, globally high sea level caused an enormous shallow sea—the Western Interior Seaway—to form across what is today the Great Plains, from Texas to Alaska. This seaway disappeared in the early Cenozoic era, and was replaced by a changing landscape of forest and grasslands filled with an amazing diversity of life, especially mammals, which replaced dinosaurs in most of the ecological niches for large terrestrial vertebrates. In the late Cenozoic, glaciers shaped the mountain landscape and the creatures that inhabited it.

In this volume, the Southwestern states are divided into four different physiographic provinces or regions (Figure 1.3): the Colorado Plateau (1), the Basin and Range (2), the Rocky Mountains (3), and the Great Plains (4). Each of these regions has a different geological history and thus varies in terms of rocks, fossils, topography, mineral resources, soils, and other geological features.

Figure 1.3. Physiographic regions of the Southwest: 1) Colorado Plateau, 2) Basin and Range, 3) Rocky Mountains, and 4) Great Plains.