The delicate balance between land and sea swung upward at the end of San Andres time as these late Permian seas retreated to southern New Mexico. The deep Delaware basin was the only persistent marine body of water. It was rimmed by magnificent towering barrier reefs, the Goat Seep and Capitan reefs that now are host to Carlsbad Caverns. These reefs were similar to the present-day Great Barrier Reef of Australia, except that the Capitan and Goat Seep reefs surrounded an inland sea whereas the Australian reef borders a continent. The Capitan reef is about 400 miles long, and other than oceanward channels cut through to the south, completely encircled the 10,000-square-mile Delaware basin. At its heyday, the Capitan reef was barely awash, and teeming with life, in contrast to the silent, stagnant deeps of the Delaware basin which were about 2000 feet below sea level only a few miles away from the barrier reef. On the steep slope into the basin, huge slump blocks of fossiliferous reef limestone slid, mingling with fossil-hash sand. These “flank” beds dip steeply from the massive reef core to interfinger with the black sandy limestones of the basin. The Delaware basin was a marine feature throughout Late Paleozoic time; its northwestern border is now marked by the southeast-trending front of the Guadalupe Mountains southeast of Carlsbad; its north edge was east-northeast of Carlsbad, and it extended southward into West Texas. Shallow “shelf” seas reached irregularly and intermittently northward and northwestward from the Capitan reef and mingled with low islands throughout all but southeastern New Mexico. Landward, away from the Delaware basin, the rocks change from massive, thick, light-gray limestones of the reefs into thin units of thin-bedded dolomite, then abruptly into alternating beds of gypsum and redbeds, the Artesia Group of rocks, and finally, marking the distant shorelines, into thin units of red mudstone and red sandstone, the Bernal Formation. Evaporation of sea water was excessive, and average temperatures high; the climate varied from semiarid in northwestern New Mexico to subtropical in the Delaware basin area—a contrast and a similarity to today’s climate. [Illustration: Figure 9. Castile gypsum sample]