Earliest of the Chaco sites, small villages of crude pit houses were occupied during 600 to 800 A.D. Archeologists, studying changes in masonry style, pottery types, and other remains, have pieced together the development of the culture of the inhabitants of Chaco Canyon over a period of six centuries. After Chaco Canyon’s cultural high point was reached in the 1100’s, a gradual decline set in. The population decreased, and in the 1200’s, the region was abandoned. During this same century, other Pueblo centers within the San Juan Basin were also deserted. Various theories have been advanced to explain this exodus: soil erosion, warlike enemies, poor facilities for urban sanitation, drought, and intervillage warfare. One or a combination of these factors might have been instrumental in bringing about the abandonment of the region. Aztec Ruins National Monument Aztec Ruins National Monument, at Aztec in the northwestern corner of the state, was set aside to preserve an outstanding example of classical Pueblo construction, one of the largest pre-Spanish villages in the Southwest. More than 40,000 visitors come to the Monument each year. Tree-ring dates indicate that most of the big central pueblo was rapidly constructed during the period 1106 to 1121. At this time, the people of Aztec were influenced by the cultural center to the south, Chaco Canyon. During the last half of the 1100’s, after the decline of Chaco Canyon had begun, the people of Aztec began to look toward the north, and by 1200, Mesa Verde ideas dominated the region. Aztec was abandoned by about 1300, as was the rest of the general region. Of particular interest at Aztec Ruins National Monument is the reconstructed Great Kiva. The Pueblo Indians built separate rooms, now known as kivas, for ceremonial purposes. During the period of Chaco influence, the people at Aztec constructed a very large circular building, 48 feet in diameter, in their plaza. Its features differ from those of the small kivas and establish it as a Great Kiva, similar to those at Chaco. The Great Kivas represent the peak of religious architecture among the Pueblos. The Great Kiva at Aztec was completely restored in 1934 by Earl H. Morris, of the American Museum of Natural History. Bandelier National Monument Bandelier National Monument, forty-six miles west of Santa Fe, preserves the ruins of dwellings and other structures that were erected by Indians who lived in the area until about 1550 A.D. Almost 100,000 people visit the Monument each year. When many of the ancient Pueblo Indian centers were abandoned in the late thirteenth century, the people moved to locations where the water supply was more constant. A favorable area was the upper Rio Grande Valley in what is now New Mexico. One of the later flowerings of Pueblo culture occurred here. The ruins within Bandelier are representative of this phase of Pueblo development.