The Guadalupe Mountians are part of one of the finest examples of an ancient marine fossil reef on earth. Geologists visit from around the world to marvel at this extraordinary natural phenomenon, which formed during the Permian Age about 250 million years ago. During this time, a vast tropical ocean covered much of southeastern New Mexico and West Texas. Over millions of years, calcareous sponges, algae, and other lime secreting marine organisms, along with calcium carbonate that precipitated from the water (a source of lime only recently discovered to be primary in this process) built up to form the 400-mile-long, horseshoe-shaped Capitan Reef.
Through time the ocean floor slowly sank, while the reef continued to grow, remaining relativly near the water's surface. A broad, shallow lagoon 20 to 50 miles wide formed behind the reef. Sediments deposited in the lagoon make up the backreef. Forereef is composed of material from the front (seaward) side of the reef that broke away from the steep slope, slipped to the bottom, and collected as sediments.
Eventually the sea began to evaporate as it was cut off from the main body of the ocean. The water became saltier, killing the reef-building organisms. Debris washed in with the rivers and buried the seabed and reef thousands of feet deep in a thick blanket of sediments and mineral salts. It became entombed for millions of years. Then, some ten to twelve million years ago, uplifting occured. Wind and rain eroded away the softer overlying sediments, leaving the more resistant limestones of the reef exposed. This ancient reef complex towers above the Texas desert in Guadalupe Mountains National Park.