Last updated: March 21, 2024
Article
NPS Geodiversity Atlas—Natural Bridges National Monument, Utah
Geodiversity refers to the full variety of natural geologic (rocks, minerals, sediments, fossils, landforms, and physical processes) and soil resources and processes that occur in the park. A product of the Geologic Resources Inventory, the NPS Geodiversity Atlas delivers information in support of education, Geoconservation, and integrated management of living (biotic) and non-living (abiotic) components of the ecosystem.
Introduction
Established in 1908, Natural Bridges National Monument is the oldest national monument in Utah. The site was established to preserve three of the largest known natural bridges in the world as well as Ancestral Puebloan cultural sites.
Natural bridges are erosional features formed by stream action (flowing water). The monument’s three natural bridges are in White and Armstrong canyons and they are carved in the Permian Cedar Mesa Formation.
Geologic Significance & Geodiversity Highlights
The monument’s primary geologic significance is its namesake natural bridges: Sipapu, Kachina, and Owachomo. The natural bridges were given Hopi names in 1909 to reflect the Indigenous heritage of the region.
In 1904, Century Magazine and National Geographic magazine articles first brought the bridges to the nation’s attention. At the time, the area was extremely remote at the time and required several days of travel by horseback to visit. The first written description of Sipapu Bridge described its size:
“This bridge would overspan the Capitol at Washington, and clear the top of the dome by 51 feet; and if the loftiest tree in the Calaveras grove of giant sequoia in California stood in the bottom of the canyon, its topmost bough would lack 32 feet of reaching the underside of the arch.”
Sipapu Bridge
Sipapu Bridge stands 220 feet (67 m) high and has a span of 268 feet (82 m). It is the largest bridge in the monument. Its name means “the place of emergence” in Hopi.
Sipapu formed when a meander in White Canyon was cut off and a new shorter course for the stream was established underneath the bridge and leaving an abandoned meander.
Kachina Bridge
Kachina Bridge is the second largest in the monument. It is stout with a thick span. Kachina Bridge is 210 feet (64 m) tall with a span of 204 feet (62 m). It is located near the confluence of White and Armstrong canyons.
Kachina Bridge also left an abandoned meander in White Canyon when it was formed when a new course to the confluence was made.
Owachomo Bridge
Owachomo is the smallest bridge in the monument, with a height of 106 feet (32 m) and a span of 180 feet (55 m). It is very thin with a thickness of just 9 feet (3 m). It was named for the small rock dome that rises above the bridge on one end (owachomo means "rock mound" in Hopi).
Owachomo formed near the confluence of Armstrong Canyon and a tributary. The tributary twice made new cut offs into Armstrong Canyon, leaving the bridge no longer spanning the main channel.
Geology and Geoheritage Values of the Natural Bridges
The close proximity of these large natural bridges to one another as well as their varying age and geomorphic history adds to the geodiversity significance of Natural Bridges National Monument. It is difficult to get an exact age on a landscape feature like a natural bridge, but clearly they are very young geologically, perhaps on the order of tens of thousands of years.
Natural rock spans like natural bridges and arches form through rock decay, weathering, and erosion with large rockfalls playing a prominent role. For example, in 1992, an approximately 4,000-ton rockfall occurred at Kachina Bridge. Groundwater sapping also likely played a role in bridge formation given the high permeability and porosity of the Cedar Mesa Sandstone.
Relative ages of the three bridges are based on the size, shape, size of their openings, and thickness of their spans. Kachina Bridge is the youngest with a thickness of 93 feet (28 m). Sipapu Bridge is considered middle-aged with a well-shaped opening and a thickness of 53 feet (16 m). Owachomo is the oldest bridge based on its thin span, large opening, and the fact that it now isolated from the main channel.
Natural bridges form when streams cut through narrow canyon walls. Most natural arches and bridges in Utah are in massive sandstones like the Cedar Mesa Sandstone, with their uniform grainsize, few bedding planes, and ability to hold up vertical and overhanging slopes.
Sipapu and Kachina are among the largest natural spans in the world. The world’s largest known natural bridges are in China and are karstic. The famous bridge in Virginia in Natural Bridge State Park, a National Park Service affiliated area, is also karstic in origin.
In Utah, only Rainbow Bridge in Rainbow Bridge National Monument and Morning Glory Bridge near Moab have longer spans than Sipapu Bridge.
Geologic Setting
Natural Bridges National Monument is in the heart of the Colorado Plateau, one of 25 physiographic provinces in the contiguous United States. The Colorado Plateau has an overall high elevation, thick continental crust, and has experienced relatively little structural deformation. The arid to semiarid environment as well as the immense erosional power of the Colorado Rivers, which is approximately 29 miles (47 km) west of the monument, and its tributaries has led to excellent exposures of sedimentary rock units in the park.
Colorado Plateau
The Colorado Plateau covers approximately 130,000 square miles (337,000 km2) across the Four Corner states of Utah, Colorado, New Mexico, and Arizona. The greatest concentration of national park sites in the country, including 30 units of the National Park System, is found on the Colorado Plateau.
The Colorado Plateau is divided into six sections, each with a distinctive character. Natural Bridges is within the Canyon Lands section of the Colorado Plateau, which is dominated by gently tilted sedimentary rock layers that have been intricately carved into canyons.
The Colorado Plateau has a complex geologic history and experienced multiple periods of uplift. Much of the uplift of the Natural Bridges region occurred during the Laramide orogeny, a mountain-building event that impacted most of the interior west, between 70 and 40 million years ago. Unlike the neighboring Rocky Mountains, the Colorado Plateau was relatively undeformed, experiencing general uplift and the formation of broad upwarps throughout the region.
Monument Uplift
Natural Bridges National Monument is located within the Monument Uplift, one of the large regional folds on the Colorado Plateau formed during the Laramide Orogeny. The Monument uplift extends from Monument Valley just past the Utah-Arizona border to the south and tapers out a bit north of the confluence of the Green and Colorado Rivers in Canyonlands National Park.
The east limb of the Monument uplift is a steep monocline known as Comb Ridge which is about 18 miles (29 km) east of the park. The Permian Cedar Mesa Sandstone is exposed in the center of the Monument uplift. It is resistant to erosion, and it holds up an elevated plateau known as Cedar Mesa which stands approximately 6,500 feet (2,000 m) in elevation. Cedar Mesa is the geologic centerpiece of Bears Ears National Monument which surrounds Natural Bridges. Bears Ears National Monument is managed by the Bureau of Land Management.
Cedar Mesa is a broad highland deeply incised by canyons, including White Canyon and Armstrong Canyon in Natural Bridges National Monument. White Canyon was named for the color of the Cedar Mesa Sandstone exposed in its walls.
Geologic History and Stratigraphy
Rocks exposed in Natural Bridges National Monument were deposited between about 310 and 205 million years ago (late Paleozoic through the Mesozoic eras). The Ancestral Rocky Mountains in Colorado, which were northeast of the Natural Bridges, were formed during the Pennsylvanian and Permian, and were eroding away during the time interval when rocks exposed in Natural Bridges were being deposited.
The geologic history of the Cedar Mesa area changed dramatically with the onset of the Laramide Orogeny, the mountain building event that formed the Rocky Mountains, uplifted the Colorado Plateau, and formed the Monument Uplift. The erosional events that formed the monument’s landforms, including its natural bridges, are very recent geologically. The overall landscape has been developed over the last few million years, and individual features such as the natural bridges, are much younger.
Paleozoic Era
Paleozoic rock units exposed in Natural Bridges National Monument include (from oldest to youngest):
Cutler Group
The stratigraphy of the Cutler Group in Natural Bridges National Monument consists of two units: The Cedar Mesa Sandstone and the overlying Organ Rock Formation.
Regionally, the Cutler Group is the package of Permian sediments that were eroded off the Uncompahgre highlights of the Ancestral Rockies Mountains in western Colorado. The Cutler consists of thick sequences of arkosic sandstone conglomerate closer to the source, but in southeastern Utah, geographic variations in depositional environments allow differentiation of different formations within the group.
In Canyonlands National Park (e.g., north of Natural Bridges), the Cutler Group is divided (from oldest to youngest) into the lower Cutler beds (Halgaito and Elephant Canyon formations), Cedar Mesa Sandstone, Organ Rock Formation, and White Rim Sandstone. These units were deposited between 290 and 275 million years ago and have a total thickness of up to 2,500 feet (760 m) in the park.
The Cedar Mesa Sandstone is the bedrock unit exposed in almost all of Natural Bridges National Monument. The type locality is near Cedar Point near the southern edge of Cedar Mesa approximately 26 miles (43 km) south of Natural Bridges National Monument.
The Cedar Mesa Sandstone is eolian in origin and was deposited in a large erg with its northeastern boundary near the Needles District of Canyonlands National Park. It thickens to the south in the Cedar Mesa region where it around 800 feet (250 m) thick.
In Natural Bridges, the Cedar Mesa Sandstone consists mostly of very-fine-grained white sandstone with high-angle crossbedding. There are rare thin interbeds of red mudstone in the unit.
The Organ Rock Formation is only exposed in two small areas near the boundary of Natural Bridges National Monument. Is consists mostly of red-colored sandstone, siltstone, and mudstone largely deposited in fluvial environments and localized ergs. The Organ Rock Formation is about 300 feet (100 m) thick in the Natural Bridges area.
Mesozoic Era
Small areas of exposure of two Mesozoic rock units are found in Natural Bridges National Monument (from oldest to youngest):
Moenkopi Formation
In Natural Bridges National Monument, the Triassic Moenkopi Formation consists of a series of mostly brick red thinly bedded sandstones, siltstones, and mudstones with abundant ripple marks. These rocks were deposited in shallow tidal flat and river floodplain environments about 240 million years ago in a hot and arid climate.
Chinle Formation
The Chinle Formation was deposited in the Late Triassic, between about 230 and 205 million years ago. In the Natural Bridges area, the Shinarump Conglomerate Member is present at the base of the unit, with overlying colorful mudstones and ledgey sandstones that were deposited in a monsoonal climate in the channels and floodplains of river systems.
Cenozoic Era
The Cenozoic geologic history of the Cedar Mesa region is strikingly different than its previous geologic history. Instead of being part of depositional basins, as it was throughout much of the Late Paleozoic and Mesozoic, southeastern Utah was impacted by the tectonic and erosional events that have shaped the entire Intermountain West, and ultimately led to the formation of the dramatic canyon scenery that the park was established to protect.
Uplift of the Colorado Plateau and Cedar Mesa Region
Rocks exposed in Natural Bridges National Monument were deposited at or near sea level, but now stand as high as 6,500 feet (1,980 m) in elevation. Further, it is estimated that the park’s youngest rock layers were once buried by an additional 4,000 to 6,000 feet (1,200 to 1,800 m) of sedimentary rocks above them and that have been removed by erosion.
The Colorado Plateau has a complex geologic history and experienced multiple periods of uplift.
Most of the uplift of Cedar Mesa and the surrounding Colorado Plateau occurred during the Laramide Orogeny between 70 and 40 million years ago.
The Laramide Orogeny impacted a wide area much broader than what occurs above a typical subduction zone. The unusual tectonics of the Laramide Orogeny was caused by the subduction of a large oceanic plateau within the Farallon plate, where unusually thick oceanic crust caused the subduction angle to shallow so that orogenic forces extended far from the tectonic boundary.
In addition to regional uplift, the Laramide Orogeny formed several major uplifts including the Monument Uplift and other major monoclines including the Waterpocket Fold in Capitol Reef National Park.
The Mid-Cenozoic Uplift was the first of two additional periods of uplift consequent to the tectonics of the Laramide Orogeny. It occurred between 35 and 25 million years ago, an interval during which shallow igneous intrusions formed the three laccolithic mountain ranges (La Sal, Abajo, and Henry) that surround the park.
Hot asthenosphere inflowing above the sinking Farallon slab underneath the Colorado Plateau caused the surface uplift.
The most recent period of tectonic uplift took place within the last 10 million years due to complex mantle processes following the subduction of the Farallon plate.
Incision of the Colorado River and Landscape Evolution
The landscape of Natural Bridges National Monument and the surrounding Cedar Mesa is geologically very young and has undergone very rapid erosion.
In the big picture, canyon incision on the Colorado Plateau was initiated after the integration of the upper Colorado River to the Gulf of California which occurred within the last 5 to 6 million years. Important evidence that constrains the age of the modern Colorado River is present near Grand Junction, Colorado and below Grand Canyon National Park in Arizona.
Landscape evolution in the central Colorado Plateau where Natural Bridges National Monument, Glen Canyon National Recreation Area, and Canyonlands National Park are located appears to have occurred even more recently, with much of the erosion most likely occurring during rapid exhumation during the last two to three million years or so.
The entire Colorado Plateau has undergone extensive erosional stripping with up to 5,000 to 13,000 feet (1,500 to 4,000 m) of erosion centered over the mainstem of the Colorado River, with lesser amounts of erosion progressively towards the edges of the plateau.
Incision by the Colorado River has initiated and driven erosion of the entire Cedar Mesa area. Erosion moves up tributaries like White and Armstrong Canyons, and cliffs retreat through mass wasting, rock fall, and sapping processes.
Erosion rates have been unsteady through time, with pulses of especially rapid erosion and periods of relative stability. Erosion rates in the Canyonlands region, north of Cedar Mesa, over the last several hundred thousand years have averaged 1,300 feet (400 m) per million years or more, which is among the highest rates of erosion on the Colorado Plateau.
Other Geodiversity Values
The geologic significance of Natural Bridges National Monument is clearly identified by its park name. In addition to the park’s geomorphology, its geologic history and exceptionally well exposed rock record is of equal significance. Other geodiversity values of the park include its well-developed biological soil crusts, seeps and springs, and desert varnish and other rock surface coatings.
Biological Soil Crust
The Colorado Plateau, including in Natural Bridges National Monument, has some of the best-developed biological soil crusts in the world. Biological soil crusts consist of communities of microorganisms including cyanobacteria, mosses, lichen, green algae, and microfungi, and bacteria and form in arid and semi-arid areas. They are integral to Natural Bridges’ ecosystems because they fix nitrogen and provide other nutrients as well as inhibiting soil erosion. They develop well on the sandy soils found in the park.
Biological soil crust are very susceptible to crushing from human activities. Footprints, tire tracks from bicycles and vehicles can destroy crusts. The damage may take decades to repair and damaged crusts may never fully recover.
Protect soil crusts by staying on established roads and trails and protecting trailside vegetation and soils. When hiking off-trail, stay on bare slickrock or walk in dry washes where crusts do not form.
Springs and Seeps
Springs and seeps along with the hanging gardens that they support are very important to the ecosystems of Natural Bridges National Monument because they are reliable water sources in a place where they are rare. The eolian Cedar Mesa sandstone has high porosity and permeability. Springs may form where a less permeable lens causes water to flow laterally where it may emerge at the surface as a spring.
Desert Varnish
Stable sandstone surfaces in the Colorado Plateau are usually coated with a variety of thin rock coatings collectively known as desert varnish. Desert varnish consists clay minerals, heavy metal (iron and manganese) skins, silica glaze and other components. It may preferentially form where water streaks down cliff faces. In Natural Bridges National Monument, desert varnish makes brown to black streaks that have a stark visual contrast to the light-colored Cedar Mesa Sandstone.
Paleontological Resources
With almost all of the park underlain by the Cedar Mesa Sandstone, which contains few fossils, fossils are relatively rare in Natural Bridges National Monument.
Root traces have been recorded in the monument in the Cedar Mesa Sandstone. No in situ fossils have been in the monument’s other rock units. Bones and dung of an extinct species of mountain goat have been found in Quaternary sediments in a rock shelter. Plant microfossils have been recovered from a Pleistocene packrat midden in the monument.
All NPS fossil resources are protected under the Paleontological Resources Preservation Act of 2009 (Public Law 111-11, Title VI, Subtitle D; 16 U.S.C. §§ 470aaa - 470aaa-11).
Caves and Karst
Deep alcoves and overhangs may be present in the Cedar Mesa Sandstone.
An evaluation of caves and karst programs and issues in national park sites identified 0.2% of Natural Bridges National Monument as potentially being karst based on the presence of limestone bedrock.
All NPS cave resources are protected under the Federal Cave Resources Protection Act of 1988 (FCRPA)(16 U.S.C. § 4301 et seq.).
Related Links
Geohazards
Geohazards present in Natural Bridges National Monument include those associated with slope stability, rock fall and other mass wasting events, as well as flash flooding during intense rainfall events.
Seismic Hazards
Overall, Natural Bridges National Monument has a moderate seismic hazard. The USGS 2014 Seismic Hazard Map indicates that the Natural Bridges area has a 2% chance that earthquake peak ground acceleration of between 12 and 18 %g (percent of gravity) being exceeded in 50 years. This peak ground acceleration is roughly equivalent to VI on the Modified Mercalli Intensity Scale. The expected number of damaging earthquake shaking in the vicinity of Natural Bridges National Monument in 10,000 years is between 10 and 20.
Abandoned Mineral Lands
There are no documented AML sites in Natural Bridges National Monument.
NPS AML sites can be important cultural resources and habitat, but many pose risks to park visitors and wildlife, and degrade water quality, park landscapes, and physical and biological resources. Be safe near AML sites—Stay Out and Stay Alive!
Related Link
Burghardt, J. E., E. S. Norby, and H. S. Pranger, II. 2014. Abandoned mineral lands in the National Park System—comprehensive inventory and assessment. Natural Resource Technical Report NPS/NRSS/GRD/NRTR—2014/906. National Park Service, Fort Collins, Colorado. [PDF]
The Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks. The products listed below are currently available for this park, check back often for updates as many maps, reports, and posters are still in progress.
- Scoping summaries are records of scoping meetings where NPS staff and local geologists determined the park’s geologic mapping plan and what content should be included in the report.
- Digital geologic maps include files for viewing in GIS software, a guide to using the data, and a document with ancillary map information. Newer products also include data viewable in Google Earth and online map services.
- Reports use the maps to discuss the park’s setting and significance, notable geologic features and processes, geologic resource management issues, and geologic history.
- Posters are a static view of the GIS data in PDF format. Newer posters include aerial imagery or shaded relief and other park information. They are also included with the reports.
- Projects list basic information about the program and all products available for a park.
Source: Data Store Saved Search 3028. To search for additional information, visit the Data Store.
A NPS Soil Resources Inventory project has been completed for Natural Bridges National Monument and can be found on the NPS Data Store.
Source: Data Store Saved Search 3075. To search for additional information, visit the Data Store.
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National Park Service Geodiversity Atlas
The servicewide Geodiversity Atlas provides information on geoheritage and geodiversity resources and values within the National Park System. This information supports science-based geoconservation and interpretation in the NPS, as well as STEM education in schools, museums, and field camps. The NPS Geologic Resources Division and many parks work with National and International geoconservation communities to ensure that NPS abiotic resources are managed using the highest standards and best practices available.