“The Rio Grande River—at least the section that runs through northern New Mexico—is not a typical river that has carved out its own valley. Rather, the valley appeared first and the river followed. This “rift valley” is a separation in the earth’s crust caused by faulting and other earth movements when the North American and Pacific plates scraped against each other some twenty-nine million years ago.

The Rio Grande Rift is not just the canyon, or gorge, that holds the river, but an area of more than 160,000 square miles reaching from central Colorado almost to Big Bend National Park in Texas. Taos Plateau is part of this “rift system” and lies in the San Luis Basin, which is nearly 100 miles long and about 47 miles wide. The San Luis Basin is one of four major basins created by the faulting and volcanic energy. Some scientists believe that several million years from now the Rio Grande Rift may become an ocean.”

(http://www.taos-history.org/unit/lt-06-1.html)