Geography and Geology

Much of northern New Mexico’s landscape is the product of volcanic activity, beginning hundreds of millions of years back and ceasing (at least for now) only about 3,000 years ago. The region’s main mountain ranges are relatively young, pushed up in the Eocene era between 55 million and 34 million years ago, when shock waves from the collision of the North American plate and the Farallon plate caused the continent to heave. Just a few million years later, the Rio Grande Rift—one of the biggest rift valleys in the world—formed, as eras’ and eras’ worth of accumulated rock was pulled apart by shifting faults, leaving the perfect path for the river when it began to flow about three million years ago. The water carved deep canyons, perfect slices of geologic time, where you can see layers of limestone, sandstone, clay, and lava stacked up. The best example is the 800-foot-deep Rio Grande Gorge [1], cut through the lava plateau that stretches west from Taos [2]. Just 1.2 million years back, a volcano’s violent eruption and subsequent collapse created the huge Valles Caldera, and the jagged edges of the crater have barely softened in the intervening time. A very tangible benefit of the state’s volcanic activity is the numerous hot springs, especially in the very young Jemez Mountain [3]s (15 million years old).

Below all this, though, is evidence of a more stable time. For about four billion years, the land was completely underwater, and then it spent hundreds of millions of years supporting a range of prehistoric sea life—hence the marine fossils found at the top of the Sandia Mountains, 10,000 feet above the current sea level. Dinosaurs, too, flourished for a time—in particular, one of the first, Coelophysis, a nimble meat eater, lived during the Triassic area around Abiquiu [4]. In 1947, paleontologists found the largest collection of Coelophysis skeletons in the world, about 100, all of them seemingly killed at once, perhaps by a flash flood. Another product of the dinosaur age (specifically, the Jurassic) was the lurid red, pink, and orange sandstone. It began as a vast desert, then petrified into the very symbol of the American Southwest, and is visible around Abiquiu [4] and Jemez Pueblo [5].