Ten million years ago, as the Miocene-Pliocene Uplift of the Intermountain West continued, fracture lines developed from southern Colorado to southern New Mexico and a long block of crust stopped rising with the surrounding terrain. Volcanism developed along some margins of the block and at other faults within the rift zone, creating lava flows, producing volcanic mountain ranges and showering the region with eruptive debris. In other areas, fault-block mountains formed, their steep walls facing the rift while their opposite slopes drop gently to the surrounding landscape. In concert with these events, erosion of the rising terrain along the margins of the block spread aprons of sand and gravel across the rift valley.
In southern Colorado, these events created the broad San Luis Valley, flanked by the San Juan and La Garita Mountains to the west and the high spine of the Sangre de Cristos to the east. Further south, the volcanic Jemez Mountains of northern New Mexico rise along the west edge of the rift while the fault-block Sandia and Manzano Ranges tower above its eastern edge, just east of Albuquerque. To the west of that city, a north-south line of volcanic peaks rise above a fault within the rift valley and, in southern New Mexico, volcanic and fault-block ranges mark the borders of the depression.
As noted above, thick layers of volcanic and erosional debris now fill the rift valley; 10,000 feet of this loosely compacted material has produced the flat landscape of the San Luis Valley and Albuquerque sits atop 5000 feet of sediment. The Rio Grande River, rising in the San Juans of southwestern Colorado, flows southward through the heart of the rift, slicing through the valley fill, including a thick lava bed west of Taos.
