Examination of pack rat middens revealed that at the time that Pueblo Bonito was built, Chaco Canyon and the surrounding areas were wooded by trees such as ponderosa pines. Evidence of such trees can be seen within the structure of Pueblo Bonito, such as the first-floor support beams. Scientists hypothesized that during the time when the pueblo was inhabited, the valley was cleared of almost all of the trees, to provide timber for construction and fuel. This tree removal, combined with a period of drought, led the water table in the valley to drop severely, making the land infertile. This explains why Pueblo Bonito was inhabited for only about 300 years and is a good example of the effect that deforestation can have on the local environment. The Anasazi, no longer able to grow crops to sustain their population, had to move on.