The San Juan Mountains of southwest Colorado overlie the remnants of Uncompahgria, the western uplift of the Ancestral Rocky Mountains that rose late in the Paleozoic Era, some 300 million years ago (MYA). Throughout the Mesozoic Era, erosive forces decimated this range, spreading sediments across the region. Then, during the Cretaceous Period (about 100 MYA), a seaway covered the area, adding marine and coastal deposits. As the Cenozoic dawned, 65 MYA, the Laramide Orogeny was underway, folding the Northern and Central Rockies and buckling the crust of the San Juan region as well.
During the Oligocene, about 35 MYA, volcanism developed in southwest Colorado; this mountain building process continued for 5 million years and was followed by a phase of explosive eruptions that produced the massive calderas of the San Juans. Finally, during the Miocene (25 MYA), a second phase of volcanism, basaltic in nature, developed across southern Colorado and northern New Mexico in concert with crustal stretching and the formation of the Rio Grande Rift; igneous plutons and laccoliths also rose with the Colorado Plateau during this Period and have since been uncovered by erosion.
The current topography of the San Juan region, which is bisected by a westward curve of the Continental Divide, is the product of Pleistocene mountain glaciers that sculpted the layered and folded strata described above. Though less intense, the erosion continues today, conducted by the upper tributaries of the Rio Grande, Gunnison, Uncompahgre, Dolores, San Juan and Animas Rivers.
