Whatever the causes, the end of the Great-Pueblo period was marked by a redistribution of population and a general trend toward concentration in places where conditions were most favorable. While the chief movement was from the north, there was also some withdrawing from the south. By the beginning of the following period, which is sometimes known as the _Regressive-Pueblo_ phase, much territory throughout the Plateau area was deserted. Main population centers were confined to the central area of the Plateau. This includes the Little Colorado drainage, particularly the section in the vicinity of the Hopi mesas and the Zuñi region, and the Rio Grande drainage. Although there were certain traits which characterized the culture as a whole during the Great-Pueblo period, there was a somewhat different development in each of the three main culture centers which flourished at this time. In each of these there was an intense local specialization in architecture and in pottery making. The latter, in fact, became so highly specialized that products of the various areas may be identified no matter where they may be found. No two pieces of pottery of each kind will be exactly alike, but they all conform to a common ideal. It must be stressed that, by _culture center_, we do not mean an entirely restricted area, but rather a nuclear section in which specialization was most intense and from which influence spread, often over a large area. The oldest settlement, and one which continued to be a cultural leader with far-reaching influences for centuries, lies in the Chaco Canyon of New Mexico.[61][73][95] The Chaco River is a tributary of the San Juan which flows through northwestern New Mexico. Within the canyon are found twelve large ruins, which include some of the most spectacular of the ancient buildings erected in North America, and innumerable smaller ruins. The twelve great communal buildings were more or less rectangular, oval, or D-shaped structures, with up to four stories on three sides, and a single-storied row of rooms which bowed out to the southeast. Within the walls was a great open court or plaza which contained numerous kivas. Other kivas were incorporated within the building mass. It is interesting to note that the traditional underground character of the ceremonial chamber was preserved through filling in the space between the circular walls of the kiva and the straight walls of the other rooms. [Illustration: Fig. 29—Pueblo Bonito, Chaco Canyon National Monument, New Mexico. (Courtesy National Park Service.)]