El Camino Real de Tierra Adentro, the road from Mexico City to Santa Fe, which was also the original path of Route 66; it involves a tortuous series of 23 switchbacks up the bajada road, which is now closed. La Bajada traditionally forms the southern boundary of the or upper river area of New Mexico, also known as Northern New Mexico. Geology The Caja del Rio plateau is a monogenetic volcanic field, which includes approximately 60 cinder cones, spatter cones, and basalt outflows. The volcanism can be explained by the field's location, which is very close to the intersection of the Rio Grande rift and the Jemez Lineament. These structural weaknesses create a thinned crust, and pathways for intrusion by magma originating in the mantle. The result has been a series of intracontinental basaltic eruptions. The Rio Grande rift is a result of extensional, (or divergent) tectonic forces exerted upon the American Southwest. This feature runs southward from the vicinity of Leadville, Colorado, through the entire state of New Mexico, through the vicinity of El Paso, Texas, and into Chihuahua, Mexico. The rift began forming approximately 30 million years ago during the late Oligocene Epoch. The Rio Grande rift takes the form of a series of basins, each offset to the right from the previous basin as one travels along the rift. The Caja del Rio volcanic field lies almost on top of the Bajada Constriction Zone, which is the zone of offset between the Albuquerque Basin to the southwest and the Espanola Basin to the northeast. The Jemez Lineament is another, older feature, which also represents a linear weakness in the crust. The Lineament trends southwest to northeast, and underlies a string of volcanic features across Arizona and New Mexico. Those features include the San Carlos volcanic field, Springerville volcanic field, Zuni-Bandera volcanic field, Mount Taylor volcanic field, the Jemez volcanic field, the Taos Plateau volcanic field, and the Raton-Clayton volcanic field. The Caja del Rio volcanic field lies in the southeastern part of the Jemez volcanic field. The lavas erupted in the Caja are diverse in their characteristics, ranging from basalt with a silica content of 49% to dacite with a silica content of 63%. Most are alkali basalts and hawaiites, which are basaltic lavas low in silica and mildly enriched in alkali metal oxides. These originated in the Earth's mantle but mingled with melted crust on their