Best known in this valley area are the cave and open sites that Morris excavated north of Durango and which contained the remains of early Basketmaker peoples already mentioned and the great pueblo of Aztec, near the town of the same name about 15 miles above the confluence of the Animas and San Juan Rivers. As described elsewhere, this latter structure was also excavated by Morris in 1916-21. Without doubt, parts of the valley were more or less continuously occupied from early Basketmaker times until the final abandonment of the Four Corners country about A.D. 1300. Although we have no firm data on which to base conclusions, it would be safe to assume that the Developmental Pueblo Period in the Animas Valley lasted from about A.D. 750 or 800 to 1050 or 1100, and that conditions in the living patterns of the people elsewhere were reflected in the Animas Valley.
As the Developmental Pueblo Period progressed, house arrangements became more complex. The next step seems to have been an extension of the earlier linear or crescent-shaped alinement of contiguous houses by adding on one or more wings, so that the resulting plan was L-shaped or formed a rectangular U. In these cases, the semisubterranean kiva was still retained in the courtyard as a definite religious structure. These types of planned communities are called "unit houses." Most were single storied, though some may have had a second story added on the back tier of rooms.
Changes in pottery styles, and especially in decoration, are very marked during this period. Although plain gray ware was still made, pottery with black designs on a white background shows up in great quantities. In the western part of the San Juan area, painted pottery with a pinkish-orange background and red designs makes its first appearance; examples of this type show up as trade pieces in eastern San Juan sites. The differences between culinary and nonculinary wares become more marked. The former are usually corrugated vessels, formed by pinching or indenting the clay coils while they were still plastic and before the pot was fired. Later in the period, this type of corrugation became quite decorative in itself and some of the better cooking ware aesthetically rivals the painted wares.