Editor’s note: Caprock Chronicles is written or edited by Paul Carlson, emeritus professor of history at Texas Tech. This week’s essay reviews a 2,000 year-long drought on the Southern High Plains and its causes and results. “Altitherma” is a geological and climatic designation for an extensive period of drought that occurred across the Great Plains beginning about 6,500 years ago (or 4,500 BCE). Also known by experts as the Atlantic Climate Phase or the Holocene Climate Optimum, the long — 2,000-year —i ntermittent dry spell brought changes to what some call the North American shortgrass steppe, including, writes Margaret Bickers, “the South Plains, the Llano Estacado and the High Plains of eastern New Mexico and Colorado into northern Kansas.” Bison and other large fauna left the region for a while and people followed, moving east to wetter country or west to higher elevations. Many did not return to the plains, writes Bickers, “for several thousand years.” During the long drought, tallgrass prairie characteristic of the region gave way to shortgrass steppe, and in the most severe instances the short grasses tended to give way to desert shrubs and near-desert conditions. Some places became nearly bare of vegetation, and, accordingly, a few scientists argue that during the long Altithermal a western Great Basin desert culture emerged on the Great Plains. The Altithermal event occurred on the Llano Estacado during what many archaeologists call the “Archaic” period, about 8,500 years ago to about 2,000 years ago. For our area, many scholars subdivide the Archaic into three chronological parts with the big drought coming in the middle of the three (6,500 years ago to about 4,500 years ago), well after glaciers at the end of the “ice age” had retreated. Not unlike drought in our area in the 1930s and 1950s, during the Altithermal wind picked up soil from the thinning ground. This time, however, the wind through two millennia deposited the material in dunes, draws, and valleys, helping to create some of the huge local dunes and choking off and filling-in the upper Yellow House and Blackwater draws of the ancient Portales River Valley. Moreover, as grasses thinned, bison and other large mammals on the plains declined in number and moved elsewhere. Archaic-age people living in the area adapted. Many left. Others turned more and more to plant foods and hunted smaller animals, such as pronghorns and jackrabbits. Those who remained probably tended to stay near streams and rivers that held permanent water sources. The Lubbock Lake site, for example, held water during the Altithermal, and the archaeological record suggests it supported a human population throughout the long drought. Causes for the Altithermal are not precisely known. Increased sun spot activity is one theory. Most theories suggest changes in atmospheric air masses and wind patterns. Three air masses characterize much of the interior of North America: the Arctic, the Tropical Maritime and the Mild Pacific. The dry Mild Pacific air mass dominates the Great Plains for about 50 percent of the year and probably accounts for the grassland environment. It occurs through much of the winter and part of the summer, times when the region receives very little rainfall. Arctic and Tropical Maritime air masses carrying greater precipitation intrude on the Mild Pacific in late winter and early summer, bringing rain in the spring. Also the Tropical Maritime, which brings rains to the eastern prairies, sometimes intrudes in early autumn into the Mild Pacific, bringing rain in late summer and early fall. Seasonal changes in the three air masses, governed in part by the waxing waning of the upper tropic and sub-tropic jet streams, impact rainfall amounts. A year-round dominance of the Mild Pacific results in severe drought. In the case of the Altithermal, a major shift in the three…