The Nacimientos are of some geological interest despite their modest elevation, as they form the western boundary of the Rio Grande Rift, a major rift valley containing the Rio Grande. The corresponding range on the east side of the rift, the Sangre de Cristo Mountains, is significantly higher and more rugged than the Nacimientos, which consist of long, gentle ridges never reaching above tree line. The Jemez Mountains lie between the Nacimientos and the Sangre de Cristos and are sometimes mistakenly thought to be part of the structure of the rift, but in fact are of volcanic origin rather than part of the graben of which the Nacimientos and Sangre de Cristos are the obvious manifestations.