During the Permian Period, about 260 million years ago, a shallow, tropical sea covered what is now southern New Mexico, West Texas and northern Mexico. A broad arm of this sea, known to geologists as the Delaware Basin, was rimmed by a massive reef, formed by the calcareous shells and body parts of marine organisms.
As Pangea broke apart, tectonic uplift cut off this sea's connection with the ocean and, over thousands of years, it evaporated, leaving the reef encased in halite, gypsum and other deposits. Additional layers of sediment and wind-blown deposits covered the region througout the Mesozoic Era and Tertiary Period, obscuring all evidence of the Permian reef. Then, during the Miocene-Pliocene Uplift, some 20-10 million years ago, the reef and its encasing sediments were raised and erosion gradually uncovered portions of the reef's Capitan limestone, with its cargo of marine fossils.
Today, much of the 400-mile long, U-shaped reef remains buried below the landscape of West Texas and southeastern New Mexico but its massive limestone formations are exposed as the Apache, Guadelupe and Glass Mountains. In addition, one section of the buried reef, in New Mexico, has been hollowed out by acidic groundwater, yielding Carlsbad Caverns, among the largest and most spectacular cave systems on our planet.
