Navajo rugs, blankets and weavings
Jurassic period
Monument Valley Navajo Reservation

At the beginning of the subsequent Jurassic period, a brief wetter era was supplanted by the sudden reappearance of arid, desert-like conditions. Navajo Sandstone, as much as 1,000 feet deep in fossilized cross-bedded sand dunes, was the primary feature of this time. Apparently supporting little biotic life, this 25-million-year era ended with the emergence of a new regime, characterized by extensive tidal flats that periodically covered the landscape.

During that new era, the 125 million years that composed the remainder of the Jurassic and Cretaceous periods, large faunal life and a complex animal community appeared. Attracted by the abundance of small animals and plants, dinosaurs and other large creatures began to inhabit the swampy fringes of the region. As the end of the Jurassic Period neared, a more temperate climate appeared.

A marine environment followed the temperate one, inundating tidal flats with advancing beaches and shallow seas. The late Mesozoic environments, characterized by Dakota sandstone and Mancos Shale, played a significant role in shaping modern landscapes throughout the region. This era created layers of deposits, one atop the other, many of which are in evidence across the Colorado Plateau.

The geologic structure of the region changed dramatically after the series of deposits. In a geologic instant, region-wide orogenic uplift caused the creation of plateaus and monoclinal folds, which in turn changed as a result of volcanism and erosion. The existing river drainages, home to most prehistoric habitation, were the result, and the general outline of the modern plateau was formed.

The area that became Navajo National Monument represents many of these moments in the geologic past. Its lowest elevations show the Wingate Formation, the 300-foot-deep sandstone formed during the time nearly 200 million years when the region was a great inland desert. The red and purple sandstones of the Moenave and Kayenta formations are also present in the monument, as is the Navajo Sandstone of the beginning of the Jurassic period, more than 190 million years ago. These are the rocks so exquisitely shaped by wind, rain, snow, and sun.

In geologic time in the American Southwest, Navajo National Monument represents a middle period between the much older Grand Canyon environment and the younger Mesa Verde Group. Tsegi Canyon itself has eroded into a series of Triassic-Jurassic rock layers, making it look more open and less vertical than nearby places such as Canyon de Chelly. The principal formations within the monument all have differing degrees of resistance to erosion, which helped create the relatively open look of Tsegi Canyon as well as the rock shelters in which Keet Seel and Betatakin ruins stand. Most of the rock shelters in the monument are at the base of the Navajo sandstone layer, the opposite of such places as Mesa Verde, where alcoves form on the upper reaches of Cliff House Sandstone.

Tsegi Canyon is the primary drainage of the eastern part of the Shonto Plateau. The canyon contains three major branches and countless side branches, all cut deeply into the Navajo sandstone characteristic of the area. Betatakin and Keet Seel are located in two of the arteries of the canyon, while the side canyons contain numerous other prehistoric ruins.

• Navajo history
• Navajo Nation
• Canyon de Chelly
• Chaco Culture

Colorado Plateau Monument Valley Navajo Reservation - page 4

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