U. S. Department of the Interior / U. S. Geological Survey Deserts: Geology and Resources by A. S. Walker [Illustration: Great Sand Dunes National Monument, Colorado (photograph by John Keith).] _Beauty is before me_ _And beauty behind me,_ _Above and below me hovers the beautiful,_ _I am surrounded by it,_ _I am immersed in it._ _In my youth I am aware of it,_ _And in old age_ _I shall walk quietly_ _The beautiful trail._ _from a Navajo benedictory chant describing the desert_ [Illustration: Cacti dominate the Sonoran Desert vegetation near Tucson, Arizona (photograph by Peter Kresan).] What Is a Desert? Approximately one-third of the Earth’s land surface is desert, arid land with meager rainfall that supports only sparse vegetation and a limited population of people and animals. Deserts—stark, sometimes mysterious worlds—have been portrayed as fascinating environments of adventure and exploration from narratives such as that of Lawrence of Arabia to movies such as “Dune.” These arid regions are called _deserts_ because they are dry. They may be hot, they may be cold. They may be regions of sand or vast areas of rocks and gravel peppered with occasional plants. But deserts are always dry. [Illustration: Ripples on a dune in the Gran Desierto, Mexico (photograph by Peter Kresan).] Deserts are natural laboratories in which to study the interactions of wind and sometimes water on the arid surfaces of planets. They contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Because deserts are dry, they are ideal places for human artifacts and fossils to be preserved. Deserts are also fragile environments. The misuse of these lands is a serious and growing problem in parts of our world. [Illustration: DISTRIBUTION OF NON-POLAR ARID LAND (after Meigs, 1953)] [Higher-resolution map] There are almost as many definitions of deserts and classification systems as there are deserts in the world. Most classifications rely on some combination of the number of days of rainfall, the total amount of annual rainfall, temperature, humidity, or other factors. In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least 12 consecutive months without rainfall, arid lands have less than 250 millimeters of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 millimeters. Arid and extremely arid land are deserts, and semiarid grasslands generally are referred to as steppes. [Illustration: Gran Desierto of the Sonoran Desert, Mexico. Surrounding the dark 25-kilometer-long and 5-kilometer-wide Sierra del Rosario mountains (upper right) are dunes and sheets of sand.]