The Rocky Mountains were primarily formed during a geological event called the Laramide orogeny, which occurred between about 80 million and 55 million years ago. This mountain-building event was caused by the subduction of the Farallon Plate beneath the North American Plate at a shallow angle, pushing the crust far inland and causing sheets of crust to be thrust upward to form the broad and high Rocky Mountain range. The formation process involved the uplifting of ancient Precambrian metamorphic rocks along with sedimentary rock layers that had been deposited in earlier shallow seas. The Laramide orogeny caused tremendous thrusting and piling of crustal sheets, akin to pushing a rug together on a floor, which created the rugged mountain landscapes seen today. Earlier ancestral mountain ranges also existed in this region much before the Rockies, including the Ancestral Rocky Mountains formed about 300 million years ago, but they were largely eroded away before the modern Rockies were raised. In summary, the modern Rockies resulted from complex tectonic activities mostly related to an unusually shallow subduction of the Farallon Plate, causing deformation far inland from the plate boundary and uplifting ancient geologic formations.
