The Northwest Central States: The Big Picture

The geologic history of the Northwest Central United States is a story of the repeated assembly and disassembly of a large continental mass. By around 600 million years ago, the core of what would eventually become most of North America was a separate continental block. 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 level. 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 did not begin until around 100 million years ago, and reached its peak around 65 million years ago, at the very end of the Mesozoic era. These episodes of orogenic activity formed the 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 this volume, the Northwest Central States are divided up into five different geologic provinces or regions (Figure 1.3): the Central Lowland (1), Great Plains (2), Rocky Mountains (3), Columbia Plateau (4), and the Basin and Range (5). Each of these regions has a different geological history and thus varies in rocks, fossils, topography, mineral resources, soils, and natural hazards.

Figure 1.3: Geologic regions of the Northwest Central.