and thus created a second unconformity. Moenkopi and Chinle formations Clastic red beds were laid down in shallow-water on top of the eroded Paleozoic surface early in the Triassic. These sediments were deposited on flood plains by streams on an expansive lowland that was slightly sloped in the direction of an ocean to the west. Mud built up in tidal flats to become the mudstone of Moenkopi Formation. Examples of this formation, some that still show fossilized ripple marks and mudcracks, can be seen in the northern and western parts of the park. Another period of erosion returned, creating a third unconformity. The brightly colored shales of the slope-forming Chinle Formation were laid down on top of this eroded surface. Petrified wood from the Petrified Forest Member of the Chinle is sometimes found at the base of Chinle slopes. Glen Canyon Group The Glen Canyon Group of formations includes (from oldest - lowest - to youngest) the Wingate Sandstone, Kayenta Formation, and the Navajo Sandstone These formations are most prominently exposed in the western and northern sections of the park. Triassic climates progressively became dryer, prompting the formation of sand dunes that buried dry stream beds and their flood plain. This sand became the cliff-forming several hundred feet (many tens of meters) high and red-colored Windgate Sandstone. Outcrops tend to run for hundreds of miles (hundreds of kilometers) with few breaks, creating an impediment to human travel. For a time climatic conditions became wetter and streams cut channels through the sand dunes. Reddish-brown to lavender-colored sandstones interbedded with siltstones and shales constitute the resulting ledgy slope-forming Kayenta Formation. The youngest and therefore topmost formation in the Glen Canyon Group was formed after arid conditions returned to the region. A vast and very dry desert, not unlike the modern Sahara, covered 150,000 square miles (388,000 km²) of western North America. Cross bedded sand dunes accumulated to great thickness, especially in the nearby Zion and Kolob canyons area (see geology of the Zion and Kolob canyons area), forming the buff to pale orange Navajo Sandstone. Navajo outcrops form cliffs, temples, and under certain conditions natural arches (such as Millard Canyon Arch) in the area. A fourth unconformity was created by a period of erosion. San Rafael Group Mud flats developed on top of the eroded surface of the Glen Canyon Group, forming the Carmel Formation. The massive cliff-forming Entrada Sandstone in turn