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Natural Bridges National Monument

Geologic Setting

 

Figure 1. Natural Bridges National Monument relative to Colorado Plateau physiographic features. Light gray area signifies the aerial extent of the Colorado Plateau. Dark gray and black areas represent uplifts and mountains.

In 1904, National Geographic referred to the ‘Colossal Bridges of Utah’, grabbing the public interest in the first decade of the 20th Century. Theodore Roosevelt established Natural Bridges as Utah’s first National Monument on April 16, 1908, to protect both the natural bridges and the archaeological ruins in the area (Huntoon et al., 2000). The primary natural resources of the park are the bridges, cliffs, canyons, springs, and surface water.

This park is part of the Geologic Resource Evaluation Program because of the unique geologic resources and human impacts to these resources. Information gathered at this park may also be used to represent other parks with similar resources or patterns of use, especially when the findings are evaluated for Servicewide implications At an average elevation of 2,000 m (6,500 ft) above sea level, the 7,636.49 acres of Natural Bridges National Monument are located on a high pinyon- juniper mesa bisected by deep canyons. The monument, located in southeastern Utah, is part of a geological feature called the Colorado Plateau Province (figure 1). Covering parts of Colorado, Utah, Arizona, and New Mexico, the Colorado Plateau is a region of high plateaus and broad, rounded uplands separated by vast rangelands. The rangelands are underlain by large elliptical stratigraphic basins.

The structural fabric of gently warped, rounded folds contrasts with the intense deformation and faulting of the terranes bordering the Colorado Plateau. Northeast and east of the Colorado Plateau are the jagged peaks of the Rocky Mountains. The Mesozoic- age overthrust belt marks the west- northwest edge of the Colorado Plateau (figure 1). The extensional, normal- faulted Basin and Range Province borders the Colorado Plateau to the west and south. The Rio Grande Rift, tearing a ragged scar in the landscape, forms the southeast border.

The Colorado Plateau is also known for its laterally extensive monoclines that formed during the Late Cretaceous – Tertiary (figure 1). The basins adjacent to the steep limbs of the monoclines have been filled with sediment eroded from these folds. The La Sal Mountains and the Abajo Mountains lie north of Natural Bridges National Monument, the Ute Mountains lie to the east, and the Carrizo Mountains are south, across the border into Arizona.

The bridges and other features present on the Colorado Plateau today were molded by the processes of erosion. The destructive forces of wind and rain, running water, and freezing temperatures attacked the uplifts as soon as all the tectonic havoc started in the Late Cretaceous. The Colorado Plateau has been uplifted about 3,660 m (12,000 ft) since the end of the Cretaceous about 66 million years ago (Fillmore, 2000). Some of this uplift occurred geologically rapidly.

As the rate of uplift increased, so did the rate of erosion. The Colorado River, for example, carved its present course within the last 6 million years. With uplift, streams throughout the Colorado Plateau began to dissect the topography into the landscape we see today with unprecedented vigor, carving the rocks and carrying away the dismantled strata into the landscape we see today.


References:

Fillmore, R., 2000, The Geology of the Parks, Monuments and Wildlands of Southern Utah: The University of Utah Press, 268 p.

Huntoon, J. E., Stanesco, J. D., Dubiel, R. F., and Dougan, J., 2000, Geology of Natural Bridges National Monument, Utah, in D.A. Sprinkel, T.C. Chidsey, Jr., and P.B. Anderson, eds., Geology of Utah’s Parks and Monuments: Utah Geological Association Publication 28, p. 233- 250.

updated on 08/10/2007  I   http://nature.nps.gov/geology/parks/nabr/geol_setting.cfm   I  Email: Webmaster
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