Rainbow Bridge
Administrative History
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When the World Was Young: The Colorado Plateau and the Formation of Rainbow Bridge

Comprehending the region that surrounds Rainbow Bridge is like looking through a telescope backwards: the picture is complete but it is a great distance away. The geologic history of the area currently referred to as Rainbow Bridge National Monument (NM) is long and complex. Comprehending the formation of the bridge is not as difficult when viewed in the larger context of the region known as the Colorado Plateau. The same series of forces that shaped Glen Canyon worked on a smaller scale in Bridge Canyon and gave the world Rainbow Bridge. It is that larger story that puts into perspective the relative place of humans at the bridge.

In the early 1880s, Clarence Dutton, led a team of surveyors from the United States Geological Survey into the heart of the Aquarius Plateau, just north of present day Boulder, Utah. Standing on a high point in the Henry Mountains, Dutton stared south into the expanse of Utah's canyon country. In the distance he could see Navajo Mountain. Dutton later wrote, "it is a maze of cliffs and terraces lined off with stratification, of rambling buttes, red and white domes, rock platforms gashed with profound canons, burning plains barren even of sage—all glowing with bright colors and flooded with sunlight." [1] Dutton's prose conveyed the complexity of the Colorado Plateau but not the accurate sequence of its formation. In recent years a number of excellent monographs have been written that capsulize both the history of the Colorado Plateau and the formation of Navajo Mountain. The effect of these events on the development of Rainbow Bridge is a story flooded with sunlight.

Figure 1 Monocline Faults and Normal Faults (Courtesy of Annabelle Foos, University of Akron)

In one of its earliest forms the Colorado Plateau was covered by an enormous sea. A billion years ago, in the Precambrian era, enormous horizontal fault lines emerged to form the border of the plateau. In the process of geologic and atmospheric evolution, the plateau emerged from that sea approximately 250 million years ago. This period comprised the latter part of the Permian era. The dominant features of neighboring provinces such as the Great Basin were extensive mountain ranges; this feature was noticeably lacking on the Colorado Plateau. Geologists speculate that being bounded by enormous fault lines hundreds of miles in length, the plateau moved in a single block, precluding it from the massive seismic upshifts necessary to form mountain ranges. This is not to say that the plateau lacks mountains; on the contrary, several peaks emerged on the plateau but not from the same causes as larger mountain ranges to the north and west. [2]

The region of the plateau that holds the Colorado River is known as a basin. Basins comprise the area between unique geologic features called monoclines. As large sections of rock rose or dropped vertically along fault lines, forming high and low plains, they created monoclines. Geologist Donald L. Baars describes the monoclines of the Colorado Plateau as "a carpet draping across a stair step." The higher rock is generally flat and forms a graceful slope down to the basin. To the east of Navajo Mountain is the Monument Upwarp monocline and to west lies the Kaibab Uplift. Rainbow Bridge sits just outside the northern boundary of the Black Mesa Basin, the basin formed from these two monoclines. Rainbow Bridge is located in a strange nexus of geologic designations. Technically it lies in the Paiute Folds, but this does not paint a complete picture: The bridge is also at the southern end of both the White Canyon Slope and the Kaiparowits Basin. The magma activity that formed Navajo Mountain (discussed later in this chapter) also contributed to the geologic character of the present day monument. All these geologic structures formed a southerly drainage system that provided the hydrologic outlet known as the Colorado River system. [3] But a cursory look at the structural composition of the landscape near Rainbow Bridge reveals layers upon layers of rock. These layers, referred to as formations, represent the geologic passing of time and the history of how the Rainbow Bridge region came to be. [4]

Figure 2 Stratigraphic Diagram of Formation Layers (Courtesy of Annabelle Foos, University of Akron)

As the great sea receded, the Colorado Plateau was shifting from the Triassic period to the Jurassic period. One of the oldest layers observed near Glen Canyon is the Moenkopi Formation, a reddish brown layer deposited during the early Triassic period. Moenkopi formations tend to be so old that they are generally hidden by younger rocks. Because of the coincidence of time and events, Moenkopi formations are most often found encircling great uplifts such as the Kaibab Uplift and Monument Upwarp. Canyonlands National Park contains excellent displays of the Moenkopi Formation. In the latter Triassic period, the continent was in a calm climatological state. Land-bound remnants of the great sea coursed south from great lakes and the earliest of rivers flowed over the southern Colorado Plateau.

Between 200 and 250 million years ago, still during the Triassic Period, the Chinle Formation spread over the Moenkopi. The rivers that flowed across the breadth of the Colorado Plateau left what are known as the basal members of the Chinle Formation. Especially vivid near Natural Bridges National Monument (NM), the basal units are referred to as Shinarump Conglomerate and Moss Back Members. They are characterized by coarse, compacted sandstone that flows along various vectors in and out of the Moenkopi. These stream deposits formed the light colored cliffs that occur above and below many Moenkopi formations. It is also in these Chinle members that much of Utah's uranium load is located. The main part of the upper Chinle Formation is made up of multicolored shales laced by thin beds of fluvial sandstone and dense limestone. On the Colorado Plateau, the Chinle Formation is also known for its numerous depositions of petrified wood. The close of the Triassic Period did much to change the environment of the Colorado Plateau. The temperate weather conditions that created the seeds of the Colorado River System were replaced by a dryer, hotter climate that turned the plateau into a desert of sand dunes. It was these conditions, at the dawn of the Jurassic Period, that brought about much of the modern character of the larger Glen Canyon region. [5]

The Jurassic Period began approximately 200 million years ago and progressed for about 70 million years. Many of the formations that comprise the national parks and monuments of the Southwest developed during this period. The climate changes that took place from the Triassic to the Jurassic periods were extreme. Soaring temperatures and high winds carried sand across every square inch of the Colorado Plateau. Geologists compare the Colorado Plateau of that time to the Sahara Desert. Wingate Sandstone, Kayenta Sandstone, and Navajo Sandstone all formed during the Jurassic Period. These three closely related sandstones comprise what is called the Glen Canyon Group. The oldest and lowest of these formations is Wingate Sandstone. It was named for the magnificent red cliffs close to Fort Wingate near Gallup, New Mexico. It is generally composed of thin-bedded, reddish-orange siltstone and sandstone. Its combination of cross-bedded and parallel-bedded structure helps Wingate sandstone form massive, vertical cliffs. The highly bonded nature of the sand causes Wingate Sandstone to break off in large blocks rather than the particulate-level erosion of less hardened units of the Jurassic Period. Wingate cliffs tend to directly overlay the Chinle formations. The distinct reddish color of Wingate Sandstone is due to the iron oxide that coats each coarse grain of sand. Wingate formations make up the bulk of Utah's most spectacular cliff sections. [6]

In the middle Jurassic Period, many millennia after the creation of the Wingate Formation, water and streams returned briefly to the Colorado Plateau. These streams deposited the second layer of the Glen Canyon Group called Kayenta Sandstone, which was named for exposures just north of Kayenta, Arizona. The Kayenta Sandstone is a ledge-forming, thin-bedded sandstone that tends to erode in gentle ledges and slopes rather than forming hardened vertical walls. This is typical of stream depositions throughout geologic history. The Kayenta Formation dissects the Glen Canyon Group by forming a ledge-like slope between two massive cliff-forming sandstones (Wingate and Navajo). Kayenta Sandstone is a firmly bonded stone that is perfect for supporting the massive Navajo cliffs on the plateau. The relatively soft nature of the upper bedding surfaces of the Kayenta Formation, coupled with excellent environmental conditions, make this stone perfect for preserving dinosaur tracks. Numerous tracks have been located near Rainbow Bridge NM in the upper layers of Kayenta Sandstone. This formation also makes up the base of Rainbow Bridge, the layer that underpins the bridge's abutments. This fact becomes significant later in the discussion of how the bridge was formed. [7]

The third prominent member of the Glen Canyon Group is Navajo Sandstone. In the region of Rainbow Bridge, the Navajo Sandstone is a distinctive element. It was designated "Navajo" by Herbert E. Gregory in a U.S.G.S. publication in 1917. Gregory spent large amounts of time exploring in the Southwest, and his surveys figure prominently into the story of how Rainbow Bridge was located in 1909 (see Chapter 3). Navajo Sandstone forms steep (sometimes vertical) walls among the canyons of the Glen. Rainbow Bridge was formed from one of these Navajo Sandstone walls. It is usually white or light gray in color, but occasionally it varies into light pink or light red. The formation consists of highly bonded remains from sand dunes that built up after the middle Jurassic period. In many locations Navajo Sandstone is interspersed with thin beds of dolomite or chert, adding a touch of variety to the appearance. [8]

The latter part of the Jurassic Period contributed numerous other formations. One of the more significant formations is the San Rafael Group which includes the Carmel Formation and Entrada Sandstone. The Carmel Formation is famous for the scenic beauty of the mesas outside Zion National Park. Entrada Sandstone does not form into massive cliffs and deep slot canyons but is responsible for the visual delights of places such as Goblin Valley and many of the arches in Arches National Park. The Jurassic Period came to a close approximately 135 million years ago. Towards the end of the period the Colorado Plateau became a lowland once more. The landscape was dominated by streams and feeder lakes that carried material along the channels that became Glen Canyon. Toward the end of the Jurassic Period the great sea returned to the Colorado Plateau, generating the enormous compression needed to form much of the Glen Canyon Group. But that sea receded once again, and three more important eras of deposition ensued. The periods following Jurassic time—the Cretaceous, Tertiary, and Quaternary Periods—did much to shape the landscape referred to as modern. [9]

display case
Figure 3 Rainbow Bridge (Courtesy of Glen Canyon NRA, Interpretation. Photo by Russell I. Alley)

In the West, the recession of the various inland seas was coupled with widespread folding and thrust faulting. These forces produced upward-shooting mountain ranges where seas had once gathered, forcing the seas to drain along new outlets. Erosion processes besieged the freshly made Glen Canyon Group, depositing thousands of feet of collected sand and boulders on the Colorado Plateau. This was the beginning to middle Cretaceous Period. By the late Cretaceous Period, the seas made their way east, cut off from western exit by new mountain ranges. As the seas moved eastward they ran into westward migrating shorelines, creating mud flats and aggressive barriers which prevented exit. As a result, material flowing from the western slopes of new mountains met material traveling from the eastern flats to deposit much of the composition of the basins of the Colorado Plateau. The San Juan Basin, which lies east of present day Rainbow Bridge NM, contains many of the younger formations of this late Cretaceous Period such as Dakota Formation, Mancos Shale, and the well known Mesa Verde Group. [10]

In Black Mesa Basin, just south of Rainbow Bridge, the formations generated in the Cretaceous Period are similar to those found in the San Juan Basin but vary in terms of age and depositional equivalence. For example, deposition of the Dakota Sandstone began later in Black Mesa because it took longer for the eastern shoreline to migrate that far southwest. This also explains why Mancos Shale occurs higher in the stratigraphic map because it took longer for the mud beds to thicken and form the shale in Black Mesa Basin than it did in younger areas to the northeast. Effectively Black Mesa Basin formed the meeting place and exit route of eastward/westward geologic and hydrologic forces that shaped the end of the Cretaceous Period. Similarly, to the north these forces deposited many of the stratigraphic layers that form the Kaiparowits Basin and the Grand Staircase. The latter Tertiary and Quaternary Periods deposited little compressed material. Sand, gravel, terrace material, and igneous intrusions all scattered across the lower Colorado Plateau as a result of the exodus of water that ended the Cretaceous Period. Since no inland sea returned to the lower Colorado Plateau during these last two periods, no massive compression took place. The permanent recession of water from this point on did not allow these periods to leave a lasting geologic impression. [11]

Much of the geologic material formed in these later periods is not present in the modern monument, because of the volume of water present during the end of the Cretaceous Period and the force with which it exited the Rainbow Bridge region. The complex of waterways that are referred to as the Colorado River system began to cut through some 5,000 feet of sedimentary rock 30 million years ago in the middle of the Tertiary Period. Rainbow Bridge is situated in a unique geologic spot. As the Cretaceous and Tertiary periods wore on, more and more drainages from the surrounding basins formed around present day Rainbow Bridge. Consequently more and more water made its way through the region, flowing in a southwesterly direction. Obviously these waterways flowed for a very long time. But at one point they were the conduit for oceanic amounts of water, amounts that could not be measured in cubic feet per second with any realistic point of reference. This is why little compressed material remains in the Rainbow Bridge region from either the Tertiary or Quaternary periods; water simply carried it away. But the Tertiary Period was critical for its seismic contributions to the modern character of the Colorado Plateau and Rainbow Bridge. [12]

Times of extreme folding and faulting, which characterized both the late Cretaceous and entire Tertiary Period, are referred to by geologists as "orogenies." Caused by upward surges from an immense pool of subterranean molten lava, the orogeny that helped shape the Colorado Plateau began on the western coastline of North America and moved east across the plateau. The surging magma searched for release in every available horizontal fissure. When it could not escape horizontally it pushed up and formed mountainous ranges: This specific period of folding and faulting, known as the Laramide Orogeny, came to a climax in the middle of the Tertiary Period. By the close of this orogeny the entire Colorado Plateau rose approximately 5,000 feet in elevation. Navajo Mountain was formed during this tumultuous time. The mountain is referred to by geologists as a "laccolith," which means it is the product of a unified source of magma displacement that did not actually break through the earth's surface.

Figure 4 Laccolith (left) and salt anticline (right) (Courtesy of Annabelle Foos, University of Akron)

Geologists speculate that a massive tube of lava moved horizontally through the earth's deeper layers and after meeting resistance turned upward in a mushrooming emergence. At Navajo Mountain, as with other laccoliths, there was no eruption at the top of the lava's journey. This is evidenced by the lack of cinder cones, lava beds, or volcanic debris. This explains the nearly uniform dome shape of the mountain, since constant pressure moved ever more vertically but never found a fissure to escape through. That pressure folded the sedimentary layers it encountered rather than breaking them. There is evidence of stress fracturing at the top of Navajo Mountain, like the splintering that occurs on the outside part of a bent branch that is about to snap. But that splintering never yielded a volcanic release. It was this aspect of the Tertiary Period that was so critical to the formation of Rainbow Bridge. [13]

As the Laramide Orogeny continued to shake up the Tertiary Period and the last era of inland seas receded to the south, the Colorado River system was beginning to form. While the hydrologic forces that shaped modern Glen Canyon may have been infantile 30 million years ago, they were sculpting the landscape. The depositions left by the Tertiary and subsequent Quaternary Periods were mostly uncompressed particulate in composition. These younger layers did not have a chance to be melded by the enormous pressure of oceanic bodies of water; consequently, the waters of the early Colorado River system made a different use of those sedimentary materials. As the waters receded, they carried tremendous quantities of gravel and sand and even massive chunks of segregated sandstone along their course to the south. These forces acted like a sandblaster on the surrounding landscape. Water alone would probably have shaped the canyons as they are viewed today, but the speed with which those erosional processes completed their task was enhanced by all the large-gauge particulate present in the water. This is why so little geologic evidence (save erosion) remains from the Tertiary and Quaternary periods—it was simply washed away. This was the first factor in how Rainbow Bridge evolved into its current form. [14]

The rudiments of Bridge Canyon were likely born in the aftermath of Navajo Mountain's laccolithic construction. Geologist Donald L. Baars contends that the great drainage patterns of the Colorado Plateau were already well established by the late Tertiary Period, less than 10 million years ago. After the great dome pushed skyward to over 10,000 feet above modern sea level, between 30 and 50 million years ago, the normal work of erosion continued but with greater water flow. The presence of Navajo Mountain near Bridge Canyon intensified climatic activity, as most mountains tend to attract storms. The increased rainfall added to the ever flowing drainage system that was forming deeper and wider canyons. In addition to the increased flow caused by Navajo Mountain, increased precipitation also modified the climate of the Colorado Plateau. Long periods of torrential rain, known as "pluvials," blanketed the Southwest. High volume water flows tended to tear away large chunks of strata from canyon walls as the hydrologic flow intensified, causing canyons to widen as they deepened. To make matters more complicated, much of the Colorado Plateau rose again during an orogeny that took place less than seven million years ago. This increased the velocity of the drainage and lowered the temperatures at the higher elevations, especially on Navajo Mountain. [15]

Near the middle of the Quaternary Period, also known as the Pleistocene Epoch, glaciers from the northern part of the continent moved south. While those glaciers did not make it across the length of the Colorado Plateau, they did help form the modern pale of the La Sal and San Juan Mountains. This Pleistocene Epoch also ushered in periods of snow accumulation on Navajo Mountain. As these glaciers expanded and contracted, melted and thickened, the flow of water continued to intensify through the ever evolving Colorado River drainage system. [16] It was the combination of all these geologic and climatic forces—uplift, laccoliths, pluvials, and glaciation—that made it possible for Bridge Canyon to give birth to Rainbow Bridge.

Figure 5 Formation of Rainbow Bridge (Courtesy of NPS Cartographic Division)

There is little rationale for why Bridge Creek followed the course that it did. The present-day topography reveals significant evidence of how the creek looked before Rainbow Bridge formed. As seen in Figure 5, the stream flowed across the Navajo Sandstone plain following the path of least resistance. As more water flowed during the Pleistocene Epoch, the erosive power of the creek intensified, cutting into the sandstone an ever wider and deeper trench. Like all streams or rivers, there were wide points in the flow. Water tended to swirl back on itself in those wide spots, forming eddies. The higher the flow, the stronger the eddy. The erosive power of Bridge Creek, with all its abrasive material carried down stream from above, intensified the effects of these eddies on the newly forming canyon walls. The result was a series of great ox-bow loops that held immense swirls of abrasive-laden water. The amphitheater-like alcoves that sit opposite the bridge today are all that is left of those ox-bows. As the water pounded into the downstream portion of the walls, the walls thinned, producing elongated fins that would not tolerate extended abrasion. Today one can view the remnants of Late Pleistocene fins cropping out from the alcoves directly opposite Rainbow Bridge.

The number of alcoves created by the meandering course of Bridge Creek is difficult to ascertain. It is probable that the creek flowed from side to side in many curves in the span of only a few miles. As the base of an alcove eroded to progressively thinner dimensions, the overhanging roof of the alcove collapsed and sediment built up along the lower section. What is sure is that at the fin that became Rainbow Bridge, the water encountered a thick bed of Kayenta Sandstone. The base of the fin was much harder than the upper portion and Bridge Creek could not erode any further down the wall of the fin. At this point, some 500,000 to one million years ago, the erosional process focused on thinning the fin on both the upstream and downstream sides above the Kayenta Sandstone base, since eddies would have formed at both locations. Eventually the Navajo Sandstone could no longer withstand the force of Bridge Creek and a hole formed in the fin.

aerial view of Rainbow Bridge
Figure 6 Rainbow Bridge and alcoves (Courtesy of Glen Canyon NRA, Interpretation Files. Photo by Russell I. Alley)

Following the path of least resistance, Bridge Creek plummeted through the widening hole in the fin and abandoned the alcoves in immediate proximity to the bridge. This is why the alcoves near the bridge are still standing today. Large scale flooding, rain, and wind were the reason that the hole in the fin eroded from bottom to top. As the hole expanded, the flow of Bridge Creek moved in a northerly direction, and a trench formed below the bridge. Even the Kayenta Sandstone could not withstand prolonged unidirectional erosion. Slowly, the empty space beneath the bridge expanded as pluvials and wind took their toll. Had the Pleistocene climate pattern not subsided, the bridge might very well have thinned to the point of either snapping under its own weight or being unable to tolerate seismic activity. Fortunately for contemporary humans, weather and seismology favored the bridge and left the most spectacular stone edifice of Southwest. [17]

The history of the Colorado Plateau, as briefly presented in this administrative history, is a complex and dynamic story. While the forces that created the plateau are currently at rest, the plateau's history suggests that calm is never a permanent state of affairs in the Southwest. Regardless, humans have been privileged to witness one of the great masterpieces of erosion in the form of Rainbow Bridge. It is apparent that a number of elements were necessary to produce the bridge. Had Navajo Mountain formed further south in the heart of Black Mesa Basin, the bridge might never have come to be. Whether the creation of the bridge was design or chance is idiosyncratic to the fact that contemporary humans have benefitted from the result.

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Last Updated: 07-Feb-2003