A stratigraphic and pollen analytical study of a thick sequence of alluvium at Chaco Canyon National Monument has provided a detailed history of late Quaternary alluviation, channel trenching, vegetation, and climate for the eastern Colorado Plateau. The alluvium at Chaco Canyon consists of five informal units, each separated by an erosional unconformity: Fajada (late Pleistocene), Gallo (7000 to 2400 B.P.), Chaco (2200 to 850 B.P.), Post-Bonito (600 B.P. to 1860 A.D.), and Historic (1935 to present). The Fajada paleosol occurs throughout northwestern New Mexico and is radiocarbon dated as older than 6,700 yr.

Pollen analysis of the four younger alluvial units provides evidence for a mid-Holocene interval of aridity that occurs later than recognized elsewhere in the Southwest. Around 5,800 yr ago both pinyon and ponderosa forests were rapidly diminished by a decrease in precipitation. Maximum reduction of the pine forests and woodlands and the greatest aridity of climate persisted from 5600 to 2400 B.P. Ponderosa forests in the mountains adjacent to Chaco Canyon had begun to expand their range by 2200 B.P., and the major expansion of pinyon woodlands in the Chaco area began about 850 yr ago.

The climate at Chaco Canyon when the Basketmaker and Pueblo peoples lived there about 600 to 1150 A.D. was drier and warmer than it is today. The ponderosa pine logs used in pueblo building were obtained from the mountains east, south, or west of Chaco Canyon. Abandonment of the pueblos coincided with the incision of the valley floor by the post-Bonito channel system.

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