Ancient hunters entered the Fort Irwin area more than 10,000 years ago, first hunting Late Pleistocene animals such as mammoth with large fluted spear points and living in small extended family groups along streams and lake shores while men hunted and women gathered seeds and plants. As the environment gradually shifted from moist pluvial wetlands to arid desert and large game became extinct, hunters adapted to smaller animals such as bighorn sheep, deer, and rabbits, and their primary weapon changed from the jabbing spear to the atlatl-thrown spear from roughly 5000 B.C. to 500 B.C., then to the bow and arrow around 500 B.C., which remained in use well into the 1800's. Rock circles, stone tools, and petroglyphs remain from these generations of prehistoric hunters and gatherers in the Mojave Desert. Less than 200 years ago, the area was a turbulent frontier where Father Francisco Garcés became the first European explorer to enter the Mojave Desert in the late 1700's by following the Indian trail later called the Old Spanish Trail, linking Santa Fe and the Spanish Southwest with Alta California. In the early 1800's, mountain men, fur trappers, traders, and immigrant wagon trains passed through, while prospectors arrived after discoveries of gold and other minerals. After recurring Indian attacks on miners and travelers, the U.S. Army carried out the Pah-ute Indian Campaign in 1860 and established a redoubt at Bitter Springs, the first U.S. Army presence at Fort Irwin. Mining remained important until the creation of the Mojave Antiaircraft Artillery Range in 1939–1940. In 1942 the post was renamed Camp Irwin for Major General George LeRoy Irwin, became home to the U.S. Army Armor and Desert Training Center in 1951, received fort status in 1961, passed to the California National Guard in 1972 for training, saw activation of the National Training Center on October 16, 1980, and was placed on rolls as an active installation on July 1, 1981. Fort Irwin occupies 1000 square miles of the north-central Mojave, the smallest North American desert, a high, somewhat quadrangular region marked by low rainfall, low humidity, hot summers, cold winters, strong seasonal winds, northwest-southeast mountain ranges, broad shallow basins, and dry lake beds that formed during the Pleistocene and still periodically fill in wet years before drying again.