Rio Grande Rift

Most of the high mountain country in northern New Mexico lies along a north-south line which more-or-less splits the state in two. The Rio Grande Rift forms this dividing line. This is a huge fracture in the earth's surface. About 29 million years ago, which is actually pretty recent, geologically speaking, Basaltic magma began welling up from the earth’s mantle. This heat and pressure caused the earths crust to arch up. The crust spread apart until it collapsed in on itself. In other words, the reason the eastern slope of Sandia Mountain is so gradual is because the entire western edge fell into the earth. It’s for this reason that people often refer to the Sandia Mountains as a trapdoor into the center of the earth. If you dug a hole in Albuquerque 20,000 feet deep (that’s almost three miles below sea level by the way), you would find the same rocks and the same fossils that you would find one the eastern surface of the mountains.