At the very apex of the cultural efflorescence of the Great Pueblo period, some unknown circumstance or series of events caused abandonment of most of the urban centers of the San Juan Anasazi and Mogollon areas. A prolonged drouth is recorded by tree rings formed during the period from 1276 to 1299 A.D. Surely a drouth of this magnitude would have had serious effects on people as dependent upon agriculture as were the Pueblo Indians. There are some indications that alien and perhaps enemy people were drifting into these areas at about this time, perhaps predecessors of the modern Navajos and Apaches. The congested living conditions of Pueblo villages undoubtedly contributed to unsanitary conditions and social pressures that also may have contributed to shifts in population. Many of these migrants moved into the homeland of the Rio Grande Anasazi in the upper Rio Grande Valley, and to a lesser extent in the central Rio Grande Valley. Some may have relocated at Zuni and other areas along the Little Colorado River drainage extending westward into Arizona. [Illustration: (_Photograph courtesy of the National Park Service_) Figure 5. Pueblo Bonito, Chaco Canyon National Monument A multistory pueblo containing 800 rooms.] During the Great Pueblo period, another group showing characteristics of both the Anasazi and of non-Puebloan people occupied an area on both sides of the Continental Divide along the eastern margin of the San Juan Basin. This cultural phase, known as the Largo-Gallina because of the distribution of sites in the Largo and Gallina drainage basins, is represented by pit houses, surface rooms, and towerlike structures with some similarities to the dwellings of the Anasazi. The black-on-white pottery also suggests Puebloan relationships, whereas the culinary vessels resemble those of the Navajo. Beginning at about 1300 A.D. or shortly thereafter, a different cultural group began to move westward from the Texas Panhandle into the plains of northeastern New Mexico. These people of the Panhandle Aspect built single-room structures and contiguous room pueblos resembling those in use by the Anasazi of the Rio Grande and were skilled bison hunters, farmers, and traders. This combination of traits suggests both Puebloan and eastern traditions. Panhandle Aspect villages were abandoned suddenly not long after 1400, perhaps as a result of the drouth recorded by tree rings for the period from 1439 to 1454.