Apparently there is no uniformity or prearranged plan in the distribution of the kivas. As religious belief and custom prescribed that these rooms should be subterranean, the greatest number were placed in front of the rectangular buildings where it was easiest to construct them. When necessary, because of limited space or other conditions, kivas were also built far back in the cave and enclosed by a double wall of masonry, with the walls being spaced about two and a half to three feet apart. The section between the walls was then backfilled with earth or rubble to the level of the kiva roof. In that way the ceremonial structure was artificially made subterranean, as their beliefs required. In addition to their ability as architects and masons, the cliff dwellers excelled in the art of pottery making and as agriculturists. Their decorated pottery--a black design on pearly white background--will compare favorably with pottery of the other cultures of the prehistoric Southwest. As their sense of beauty was keen, their art, though primitive, was true; rarely realistic, generally symbolic. Their decoration of cotton fabrics and ceramic work might be called beautiful, even when judged by our own standards. They fashioned axes, spear points, and rude tools of stone; they wove sandals, and made attractive basketry. The staple product of the cliff dwellers was corn; they also planted beans and squash. This limited selection was perhaps augmented by pinon nuts, yucca fruit, and other indigenous products found in abundance. Nevertheless, successful agriculture on the semiarid plateau of the Mesa Verde must have been dependent upon hard work and diligent efforts. Without running streams irrigation was impossible and success depended upon the ability of the farmer to save the crop through the dry period of June and early July. Rain at the right time was the all-important problem, and so confidently did they believe that they were dependent upon the gods to make the rain fall and the corn grow that their prayers for rain probably developed into their most important ceremonies. From Dr. A. E. Douglass's tree-ring chronology the earliest date so far established for the Mesa Verde cliff dwellings is 1066 A.D. and the latest date 1274 A.D. While it should not be imagined that these are the all-inclusive dates representing the total time of the cliff-dweller culture, it is interesting to note that this same tree-ring story tells us that a great drought commenced in 1276 and extended for a 24-year period to 1299. It may logically be presumed that the prehistoric population was gradually forced to withdraw from the area as the drought continued and to establish itself near more favorable sources of water supply.