Drought seems to have always been one of the main controlling factors in the migrations of Southwestern Indians. The study of tree-rings tells us this. By matching ring patterns formed by the annual growth of certain kinds of trees, pines chiefly, archæologists are able to determine the years in which age-old timbers were cut. Those they are interested in are the ones used by prehistoric Indians long years ago for building roofs on their houses. So naturally, if an Indian had cut a tree down with a stone axe and laid it across the walls of his house, then the year that the tree was cut would correspond to the approximate time his house was built and occupied. Indians did not cut timbers until they were ready to use them. Felling timbers with crude stone axes was somewhat of a chore. Old beams from houses show that long periods of drought reigned in the Southwest. It is thought that these dry spells caused Indian families to leave their homes and seek new lands for settlement and cultivation. Such a condition seems to have existed in the entire San Juan area of northwestern New Mexico, northeastern Arizona and southwestern Colorado. The greatest of the large centers of Indian population, which may have numbered hundreds or even thousands of people, were the towns of Chaco Canyon in northwestern New Mexico and the Mesa Verde cliff dwellings in southwestern Colorado. Many of these towns, it seems, were abandoned when the great drought was at its height between 1276-1299, a period of twenty-three years. And so we find shifts in population. It is believed that some of these shifts were toward the Valley of the Rio Grande. Before this time small individual groups or migrant bands took to wandering. Other Indians could have remained even after the time of the twenty-three year drought period, dreading to leave their homes as we would ours today. No, there was no great exodus of population. The people from the great towns in the west did not move out all at one time and completely abandon their homes and desiccated lands. They moved out in small bands, or even families. In some way, a traveler might have reported high mountain ranges, water and fertile lands to the east—the next best to the places they knew as home which they and their ancestors had occupied for hundreds of years.