Figure 8: Rio Grande rift: Sandia Mountains and Rio Grande viewed from Bernalillo, New Mexico. The Sandia mountains are called fault-block mountains. The entire range has been lifted up along the eastern marginal faults of the Rio Grande rift, as the rift formed, and the rocks have tilted toward the east like an open trap door. The river took advantage of the low regions of the rift as it developed into a major waterway between one and four million years ago. Photograph by Jayne C. Aubele, New Mexico Museum of Natural History and Science.