in funneling rain-bearing storms into the canyon and boosting local precipitation levels. The principal Chacoan complexes, such as Pueblo Bonito, Nuevo Alto, and Kin Kletso, have elevations of . The alluvial canyon floor slopes downward to the northwest at a gentle grade of ; it is bisected by the Chaco Wash, an arroyo that rarely bears water. The canyon's main aquifers were too deep to be of use to ancient Chacoans: only several smaller and shallower sources supported the small springs that sustained them. Today, aside from occasional storm runoff coursing through arroyos, substantial surface water—springs, pools, wells—is virtually nonexistent. 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. The Chaco Wash flowed across the upper strata of what is now the Chacra Mesa, cutting into it and 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 Mesaverde Group. The canyon bottomlands were further eroded, exposing Menefee Shale bedrock; this was subsequently buried under roughly of sediment. The canyon and mesa lie within the "Chaco Core"—which is distinct from the wider Chaco Plateau, a flat region of grassland with infrequent stands of timber. As the Continental Divide is only 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. Climate An arid region of high xeric scrubland and desert steppe, the canyon and wider basin average of rainfall annually; the park averages . Chaco Canyon lies on the leeward side of extensive mountain ranges to the south and west, resulting in a rainshadow effect that fosters the prevailing lack of moisture in the region. The region sees four distinct seasons. Rainfall is most likely between July and September, while May and June are the driest months. Orographic precipitation, which results from moisture wrung out of storm systems ascending the mountain ranges around Chaco Canyon, is responsible for most of the summer and winter precipitation, and rainfall