Beginning about 3 million years ago and into recent times, the uplift of the local Guadalupe Mountains and changing climates lowered the water table. Water that had been inside the caverns drained away and was replaced by air. Most solution stopped, but large sections of partly dissolved walls and ceilings collapsed under their own weight. Stability was finally achieved, however, and probably no rock has fallen within the caverns during the last several thousand years. Even before the collapsing ended, another phase of cavern development had begun. Rain water and snow melt slowly seeped into the caverns. Droplets of water, each holding a minute quantity of dissolved limestone, appeared upon the ceilings. Exposed to the air, the droplets evaporated and left their mineral content as calcite and aragonite—crystalline forms of limestone. Over centuries, this process of evaporation and deposition has built a myriad of crystalline stalactites of all shapes and sizes. Water that dripped to the floor evaporated and deposited the calcite and aragonite to build stalagmites. When joined together, stalactites and stalagmites become columns, or pillars. In the scenic rooms, conditions existed that brought about the creation of helictites—twisted formations that seem to defy gravity in their growth. Color in the cave formations, shades of brown, red, and yellow, result from the presence of small amounts of iron oxide and other minerals. Carlsbad Caverns National Park offers more than the caverns themselves. Each evening from April to October, bats in incredible numbers spiral upward out of the Caverns’ entrance and fly southward over the rim of the escarpment to feed in the valleys of the Black and Pecos rivers. They return just before dawn, diving swiftly and from high altitudes into the entrance. Flying directly to the bat cave, they spend each day hanging head downward in dense clusters from the walls and ceilings. The bat cave itself is not open to visitors. A park naturalist explains the bat flight and discusses the bats in detail in a talk given at the entrance to the Caverns each evening before the flight begins. The nature trail, a half mile loop that begins and ends near the entrance to the Caverns, guides the visitor to many of the desert plants of the region. The park is visited by more than half a million people each year, and yet the delicate cave formations and the natural beauties remain unmarred. White Sands National Monument White Sands National Monument, fifteen miles southwest of Alamogordo, is another area of great interest to the geologist and biologist. But its strangeness, the graceful contours of its snow-white dunes, and the peculiar adaptations of some of the plants and animals that live among its dunes are appreciated as fully by the nonscientist. More than 370,000 visitors come to marvel at the White Sands each year.