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.