The Monument, some 230 square miles in extent, preserves the most impressive part of the world’s largest gypsum desert. This is a glistening sea of pure gypsum sand that the wind has drifted into huge dunes that are almost bare of vegetation except along the fringes. Wavelike, the restive dunes move slowly before the prevailing winds, covering and uncovering the few plants that lie in their way. And wavelike, their surfaces trace the vagaries of indecisive breezes in tiny parallel ripplelike ridges. Because of the almost constant wind and resulting gradual advance of the dunes, most of the plants that are able to establish themselves in the open flats between the dunes eventually become buried. A few species, however, are able to survive the irresistible march of the sand. Through rapid growth and elongation of the stems, the struggling crowns remain on top of the rising crests of the dunes. Plants with stems more than forty feet long have been found. As the dunes continue forward under the pressure of the wind, they leave the plants elevated on columns of compacted gypsum bound by their adventitious roots. Animals, too, have become adapted to their unusual surroundings. The small creatures, lizards, mice, and others, are picked off easily by such predators as foxes, coyotes, and hawks when they are conspicuous. Thus, through the centuries, only the lighter-colored individuals have survived among the dunes and, through many generations, have developed pale and elusive animals that blend inconspicuously with their white surroundings. Pocket mice are a good example of this. Among the white dunes, the pocket mice are white; in the nearby red hills, they are a rusty color; and on the beds of black lava a few miles north of the sand, the pocket mice are very dark. A visitor to White Sands National Monument should stop at the visitor center, where exhibits explain the geology of the duneland and others describe the plants and animals that are able to live there. But how and from where did this natural wonder come? The dunes of gypsum lie in the Tularosa Basin, which stretches for more than 100 miles between two north-south mountain ranges. All sides of the valley slope gently inward, forming the basin, with Lake Lucero, its lowest point, at the southwest end of the Monument. This valley was formed hundreds of centuries ago, when a great section of the earth’s crust settled to form the type of basin known geologically as a graben. High above the basin floor, beds of gypsum are found to the north in the mountain ranges flanking the valley. Similar gypsum beds lie far beneath the floor of the basin. Thus, it was once a part of the high plateau, a great blocklike section that slowly sank to form the basin at its present level.