Although the rooms are smaller than the ones in the great communal houses of Chaco, they are solidly built of double course standstone blocks. Because in most cases the mesas are sloping southward, many of these unit houses were built upon one or more terraced flats. Frequently the only entrance into the pueblo was by staired entranceway leading from the south into the small interior court or plaza. At Mesa Verde, in the latter part of this period (about A.D. 1200-1225), the people who had been living on the mesatops in the unit house type of dwelling seem suddenly to have abandoned these dwellings and taken up residence in the nearby caves. Here they built great pueblo-type structures, often of several hundred rooms with numerous associated kivas. Since they were limited by the ceiling of the caves to two and three-story structures, and not so exposed to the elements, it was not necessary to use such thick, strong walls, roofs, and ceilings. The general construction at Mesa Verde was therefore thinner than at Chaco, and the rooms and doorways are considerably smaller. In fact, with the warm southern exposure of the caves which were used, the people must have done most of their living and daily chores outside, in the small plaza areas and on the roofs of the lower tiers of rooms. The rooms themselves could be small, for they were probably used only for sleeping and storage. Because the native sandstone at Mesa Verde is coarser grained and does not fracture as easily into blocks and spalls, the style of alternating large and small banded masonry found at Chaco was not adopted there. But much of the stonework at Mesa Verde is nonetheless excellent; perhaps some builders had greater artistry than others, for some of the rooms, especially the circular towers, contain blocks which have been carefully pecked, ground, polished, and fitted into exact position with loving care. In this period, also, a structure known as a Great Kiva comes into prominence. It usually has an entrance on the north side (instead of through the smoke hole), often with a stairway. It has a large raised firebox in the center of the south side, and occasionally another entrance there. In addition, on the east and west sides of the floor are large, rectangular stone-lined pits, built up above the floor. Their exact use is still a mystery, and perhaps they served more than one purpose. It has been suggested that when covered with boards, they would make excellent foot drums for the dances, or good places for the medicine men to conceal themselves while performing certain magical rites during initiation ceremonies. Finally, four large posts set into the floor of the kiva supported the roof. Great Kivas are fairly common in the Chaco area, but in the Mesa Verde vicinity they seem to be very rare.