The causes which led to the abandonment of the great houses and which resulted in the end of this phase of Pueblo development are just as difficult to understanding as are those which led to their being constructed in the first place. By 1300 A.D., the entire northern section of the Plateau had been deserted. This was not the result of a single mass migration, but rather of a wide general movement. First one big center and then another was deserted. Even in these centers themselves, all the inhabitants did not leave at the same time; rather it seems that small groups drifted away, a few at a time. Eventually, though, the entire northern frontier was deserted, and no living person who had contributed to the growth and flowering of the culture remained. Naturally, this strange departure has given rise to much conjecture. It would be pleasant to be able to say that such and such a cause produced this result. Unfortunately, anything connected with the human race is rarely quite so simple. The invaluable tree-rings have not only provided us with dates for various events, but have given us information about climatic conditions which undoubtedly had a tremendous effect on the movements of the people with whom we are concerned. From tree-ring records we know that during the centuries when the hopes and fears of the prehistoric Pueblo Indians were centered on their crops there were bad years as well as good ones. We know of periods when rainfall was below normal, and of others when there were real droughts. Most of these were of short duration, however, until the disastrous period between 1276 and 1299 when there was practically no rain, and the Southwest suffered an extremely severe drought. It was during this period that the northern frontier was finally abandoned, and the people moved to new localities. Some archaeologists have felt that the disappearance of the Pueblos from their old homes can be traced entirely to this disastrous drought. If all the communities had been abandoned at the same time, this would be a logical assumption. Actually, the time of the abandonment of all of the main centers does not fall between these two dates. Some were deserted prior to the beginning of the great drought and a few continued to be occupied after the dry period had begun.