At Chaco Canyon, many parts of the pueblo walls were finely made. Different styles of decoration were produced by using sandstone blocks of various sizes. An unusual effect was achieved by alternating bands of large rectangular blocks with a series of bands of much smaller, finely laminated standstone blocks. The interior of the walls consisted of crude rubble in adobe mud, and, where some form of banding technique was not used in the outer or veneer wall, the chinks between the larger stones were filled with adobe mud and spalls or very small chink stones. When carefully done, this technique also produced an attractive appearance. Since both the interiors and exteriors of walls were usually plastered with numerous thin layers of adobe, it is something of a mystery why the Indians took the trouble to produce such pleasing effects in their stone work and then to cover it up with plain plaster. It may be that what we regard as decoratively charming was to them simply a structural and engineering feature. They may have considered carefully spalled and banded masonry to be structurally sounder than simple rock, rubble, and plaster walls. Usually the walls of the upper stories are successively thinner, and a similar idea was used in the beams which form the room ceilings (and thus the floors of rooms above)—the heaviest beams were in the lower rooms, and those in the upper stories were correspondingly lighter and smaller. In the Chaco-type great pueblo of this period, the majority of the rooms were large by pueblo standards. They were rectangular in shape (except for the kivas, which are circular) and often 8 to more than 12 feet long and 6 to 8 feet wide. Ceilings were 8 or 10 feet high, and doorways, usually with a raised sill, were 3 to 4 feet high and 2 to 3 feet wide. In comparison with the typical rooms in the Mesa Verde area, those in Chaco were very spacious. In the Mesa Verde region the people were also learning to build in sandstone, but the available standstone was coarser than that in Chaco and did not have the clean fracture planes, and the masonry was of a thicker and seemingly cruder sort. Walls were made of rectangular blocks of tan sandstone which quite often were carefully shaped and ground to give a pleasing effect. On the sloping green tabletop mountain, known as the Mesa Verde (from which the surrounding area gets its name), in the early part of this period (A.D. 1050-1200), the people built large unit-type pueblos upon the long finger-like mesatops which extend southward from an abruptly rising escarpment on the north. Most of these units were multistoried, and although they centered around a central plaza, they were much smaller, more tightly contained units than their Chaco Canyon counterparts of the same period. Frequently they contained at least one towerlike structure connected by an underground passage to a nearby kiva. One or more other kivas might be located in, or front on, the small central plaza.