![]() This member is exposed on Chinde Mesa at the northernmost border of the park. Lenses of selenite gypsum are scattered periodically throughout the Owl Rock Member representing the minerals left behind after evaporation of inland lakes. NPS The Owl Rock Member consists of pinkish-orange mudstones mixed with hard, thin layers of limestone. ![]() The erosion-resistant lava flows, such as Pilot Rock and the Hopi Buttes, protect the softer lake-bed deposits beneath. The Hopi Butte Volcanic Field, which can be seen from the northern overlooks of the park extending northwest, is considered one of the largest concentrations of maar landforms in the world, covering about 965 square miles (2,500 square km). The vent from one of these maars is exposed on the Painted Desert Rim across the park road to the east of Pintado Point. The resulting ash formed fine-grained deposits that were deposited within the lake sediments.Īfter a few million years of erosion, most of the Bidahochi Formation has been removed from the park area, leaving volcanic scoria cones and maars (flat-bottom, roughly circular volcanic craters of explosive origin). Many of the volcanoes were phreatomagmatic, when ground or lake-water mingled with eruptive material (magma) to cause explosive eruptions. ![]() Volcanoes, both nearby and as far as the Southwestern Nevada Volcanic field, spewed ash and lava over the land and into the basin. Fine-grained fluvial and lacustrine (lake related) sediment such as silt, clay, and sand represent the lower part of the Bidahochi Formation. The Little Colorado River and its tributaries, including the Puerco River, have cut their own valleys into the soft Chinle and Bidahochi Formations of the Painted Desert.ĭuring the Late Miocene and Early Pliocene Epochs of the Neogene Period (4-8 million years ago) a large lake basin with ephemeral lakes covered much of Northeastern Arizona. While not as numerous as the fossils of the Chinle Formation, fossils have been found even in the quaternary sediments, including fragments of an ancestral proboscidean (elephants and their relatives, such as mammoths). These dune deposits are largely stabilized by vegetation, especially grasses. The youngest dunes are found throughout the park, in all settings, deposited around a thousand years ago. Younger dunes, around 10,000 years old, are found in drainage areas that contain sand such as Lithodendron Wash. At higher elevations in the northern part of the park, 500,000-year-old dunes can be found. Pleistocene and Holocene Epoch (1.8 million years ago to present) deposits of windblown sand and alluvium (deposited by flowing water), now cover much of the older formations of the park.
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