Geology
After the Pangaean supercontinent sundered during the Cretaceous period, the region became part of a shifting transition zone between a shallow inland sea—the Western Interior Seaway—and a band of plains and low hills to the west. A sandy and swampy coastline oscillated east and west, alternately submerging and uncovering the area atop the present Colorado Plateau that Chaco Canyon now occupies.
As the Chaco Wash flowed across the upper strata of what is now the 400-foot (120 m) Chacra Mesa, it cut into it, gouging out a broad canyon over the course of millions of years. The mesa comprises sandstone and shale formations dating from the Late Cretaceous, which are of the Mesa Verde formation. The canyon bottomlands were further eroded, exposing Menefee Shale bedrock; this was subsequently buried under roughly 125 feet (38 m) of sediment. The canyon and mesa lie within the "Chaco Core", distinct from the wider Chaco Plateau, the latter a flat region of grassland with infrequent stands of timber. As the Continental Divide is only 15.5 miles (25 km) east of the canyon, geological characteristics and different patterns of drainage differentiate these two regions both from each other and from the nearby Chaco Slope, the Gobernador Slope, and the Chuska Valley.
Read more about this topic: Chaco Culture National Historical Park