Capitan Reef

Guadalupe Mountains National Park, Texas, and Carlsbad Caverns National Park, New Mexico, both incorporate features of the Capitan Reef within their boundaries. About 250 million years ago, West Texas and southwestern New Mexico were covered by an embayment of a tropical Permian sea. Unlike modern reefs, which are built primarily by corals and their symbionts, Capitan Reef was built by sponges, algae, and other organisms that incorporated "lime" into their structures. These organisms thrived in the shallow waters near the coast, and, as they died, their calcium carbonate structures accumulated on the sea floor. Deposits of these organisms, along with calcium carbonate precipitated from the water, built the horseshoe-shaped reef that paralleled the ancient shoreline of the embayment, separated from the land by a relatively shallow lagoon. Eventually, the channel connecting the embayment to the sea was closed. Over millions of years, sediments from the adjacent land filled the basin and buried the reef.

El Capitan, Capitan Reef, Guadalupe Mountains National Park, Texas
Photograph by Mark Eberle, November 1999

Heat and pressure converted the reef materials into limestone. The weight of the accumulated materials and movements of the Earth's crust caused cracks to form in the buried limestone. Slightly acidic water percolated into these cracks, and sulfurous gases rose to acidify water in the fractures. These acids slowly dissolved the calcium carbonate and began the process of forming extensive caverns, such as those located within Carlsbad Caverns National Park.

Natural Entrance, Carlsbad Caverns National Park, New Mexico
Photographs by William Stark, March 2000

Over millions of years, segments of the Capitan Reef were uplifted, and much of the sediment was eroded away from the more resistant limestone reef materials. The Guadalupe Mountains, stretching to the northeast from El Capitan in Texas to near Carlsbad, New Mexico, are one of these uplifted portions of the reef. The southern end of the range is included within Guadalupe Mountains National Park. As this uplift continued, groundwater that supported some of the material in many of the caverns drained away, and material collapsed to form large underground chambers, such as those in Carlsbad Caverns National Park. Because the reef was built, in large part, through the accumulation of dead marine organisms that incorporated calcium carbonate into their structures, fossils are abundant in the reef deposits throughout the exposed portions of the Guadalupe Mountains.

Return to Geology Page  /  Return to Fieldtrip Highlights  /  Return to Course Homepage