The San Andres Formation is a Permian geologic unit in New Mexico and Texas and the most widely exposed Paleozoic formation in New Mexico. Composed mainly of thick marine limestone, it stretches in a broad belt from west Texas into central New Mexico, reaches about 400 meters thick, and also extends in the subsurface across major basins. Its rocks record a late Leonardian marine transgression, preserve characteristic late Permian fossils, and help trace changing marine conditions across the region. Long studied and repeatedly reclassified during the twentieth century, it is now a key reference for understanding Permian seas in the American Southwest.