Geological Survey Bulletin 1393 The Geologic Story of Arches National Park

AMONG THE QUESTIONS commonly asked by visitors are, "How do arches form?", "Why are some openings called windows, others arches?", "What is the difference, if any, between arches or windows and natural bridges, such as those at Natural Bridges National Monument?", and "How many arches are there in Arches National Park?" Before taking up the origin and development of arches, I shall attempt to explain the differences between the three types of natural rock openings named above and comment upon the number of arches.

I believe most geologists and geographers are in general agreement with Cleland (1910, p. 314) that "a 'natural bridge' is a natural stone arch that spans a valley of erosion. A 'natural arch' is a similar structure which, however, does not span an erosion valley." According to this definition, Natural Bridges National Monument includes three true bridges, whereas all the larger rock openings in Arches National Park with which I am familiar are properly termed "arches," but some are called windows. If we were to distinguish between arches and windows, we might say that arches occur at or near the base of a rock wall, as do the doors of a house or building, whereas windows are found well above ground level. This distinction was not followed in naming the rock openings in the park, however; for example, Tunnel Arch (fig. 14) is considerably higher above the ground than North Window (figs. 37, 38) or South Window (fig. 39).

As to the number of arches in the park, I might begin by saying that there is no universal agreement as to how large a rock opening must be to qualify as an arch. The pamphlet formerly handed to visitors entering the park proclaimed that "Nearly 90 arches have been discovered, and others are probably hidden away in remote and rugged parts of the area," but the average visitor probably sees less than a third of this number.

David May, Assistant Chief of Interpretation and Resource Management, Moab office of National Park Service (oral commun., Oct. 1973), believes that if only those in the park having a minimum dimension of 10 feet in any one direction were considered to be arches, the number would boil down to about 56 or 57. The most complete count of arches and other openings in all of southeastern Utah was made by Dale J. Stevens, Professor of Geography at Brigham Young University, during the period February through April 1973. He considered those with openings of 3 feet or larger and found more than 300 in southeastern Utah, of which 124 are in Arches National Park, although he stated that several areas of the park were not intensively searched because of time limitations (written commun., July and Sept. 1973). The 124 arches and openings are distributed among the several named areas of the park, as follows: Courthouse Towers, 13; Herdina Park, 11; The Windows section, 25; Delicate Arch area, 3; Fiery Furnace, 19; Devils Garden, 25; upper Devils Garden (northwest of Devils Garden), 14; Eagle Park, 2; and Klondike Bluffs, 12.

Professor Stevens generally used a range finder or a steel tape to measure the width and height of the openings and the width and thickness of the spans, but estimated a few of the dimensions. In the text descriptions of arches or captions of figures that follow, I am including all or part of these measurements, without further acknowledgment.

All the arches in the park were formed in the Entrada Sandstone, mainly in the Slick Rock Member but partly in the Slick Rock and Dewey Bridge Members, and a few in the Slick Rock Member occur not far beneath the base of the overlying Moab Member. The sandstone of the three members is composed mainly of quartz sand cemented together by calcium carbonate (CaCO₃), which also forms the mineral calcite and the rock known as limestone, but the Dewey Bridge Member also contains beds of sandy mudstone. Limestone and calcite are soluble in acid, even in weak acid such as carbonic acid, HHCO₃, also written H₂CO₃, formed by the solution of carbon dioxide (CO₂) in water. Ground water, found everywhere in rock openings at different depths beneath the land surface, contains dissolved carbon dioxide derived from decaying organic matter in soil, from the atmosphere, and from other sources. Even rainwater and snow contain a little carbon dioxide absorbed from the atmosphere—enough to dissolve small amounts of limestone or of calcite cement from sandstone. The calcite cement in the Entrada and in many other sandstones is unevenly distributed, however, so that all the cement is removed first from places that contain the least amounts, and, once the cement is dissolved away, the loose sand is carried away by gravity, wind, or water.

Both nearly flat but slightly irregular beds of sandstone and relatively thin walls or fins of sandstone are prime targets for this differential erosion. Potholes, as shown in figure 18A, may be formed in relatively flat beds by the dissolving action of repeated accumulations of rainwater or snowmelt, even in arid regions like the Plateau.

Relatively thin walls, or fins as they are called in parts of the Plateau including Arches, are targets for the formation of alcoves and caves by solution of cement and removal of sand by gravity, wind, and water, aided by the prying action of frost in joints, bedding planes, or other openings. Once a breakthrough of a wall or fin occurs, weakened chunks from the ceiling tend to fall, and natural arches of various shapes and sizes are produced. Arches form the strongest shapes for supporting overlying rock loads, as the rock in the arch is compressed toward each abutment by the heavy loads. Blocks of compressed rock beneath a relatively flat ceiling tend to be dislodged also by expansion due to release of pent-up pressure, until a strong self-supporting arch is formed. Release of pent-up pressure in rock walls may help also in initiating the formation of alcoves or caves in cliff faces. Man, including the ancient Greeks, Romans, Egyptians, and others, has long made use of arches in building bridges, aqueducts, temples, cathedrals, and other enduring edifices.

As vividly shown in figure 12, the Entrada Sandstone on the northeast flank of the Salt Valley anticline has been broken by Earth forces into thin slabs mostly 10 to 20 feet thick between nearly parallel joints, but, as will be noted in the descriptions of individual arches, some rock walls are only 1 or 2 feet thick, whereas others are 50 feet thick or more. Some weak or thin slabs have weathered away, leaving the stronger or thicker ones as towering fins, particularly in the Fiery Furnace and Devils Garden areas. Jointing on a less spectacular scale also has broken the Entrada in areas south of Salt Valley, leaving walls or fins of rock.

JOINTED NORTHEAST FLANK OF SALT VALLEY ANTICLINE, viewed westward from an airplane. Light-colored wedge in middle background is Salt Valley bordered on extreme left by Klondike Bluffs. Dark-colored fins and pinnacles on left, of Slick Rock Member of the Entrada Sandstone, form Devils Garden. Sharp pinnacle above valley is the Dark Angel. (See fig. 57.) White bands of sandstone extending to foreground are composed of Moab Member of the Entrada. Note vegetation in the joints. Photograph by National Park Service. (Fig. 12)

Although all the arches in the park were carved from the Entrada Sandstone, slight differences in their mode of origin or placement within the Entrada allow them to be grouped into three classes: (1) vertical arches formed in the Slick Rock Member alone or in the Slick Rock and Moab Members, (2) vertical arches formed mainly in the Slick Rock Member but partly in, and with the aid of, the incompetent underlying Dewey Bridge Member, and (3) horizontal arches, or so-called pothole arches, formed from the union of a vertical pothole and a horizontal cave. Hereinafter, the three members will be referred to alone, without reference to the Entrada.

Before giving examples of arches in each of the three classes, it is appropriate to remark that the arches and other erosion forms in the park represent but a fleeting instant in geologic time. Many of the pinnacles or piles of rock may be the broken remains of former arches, and many of the arches we see may be gone tomorrow, next year, or a few hundreds of years and, certainly, before many thousands of years. On the other hand, many new arches will form by the processes described above as the geologic clock ticks on.

Examples of Arches

Tunnel Arch (fig. 14) is a good example of an arch eroded entirely within the massive Slick Rock Member. Just southwest of Sheep Rock (fig. 31) is an unnamed opening in the lower part of the Slick Rock Member which I call "Baby Arch," because it is one of the newest ones visible from the park road (fig. 15). It is only 25-1/2 feet wide and 14 feet high and penetrates a wall 14 feet thick. Note that the breakthrough probably began along the prominent recessed bedding plane at the base of the arch. Its youthfulness is also indicated by the sharp, angular breaks in the ceiling and by the pile of freshly fallen rocks. Some visitors have asked park personnel why they have not cleared away such debris! Despite its youthfulness, the ceiling has already taken on the shape of an arch.

TUNNEL ARCH, reached by short trail north of main trail through Devils Garden. Opening is 26-1/2 feet wide and 22 feet high; span is about 14 feet thick. (Fig. 14)

"BABY ARCH," just southwest of Sheep Rock in Courthouse Towers area. For details, see text. (Fig. 15)

Broken Arch (fig. 16) was formed near the top of the Slick Rock Member and is strengthened and protected by the more resistant overlying Moab Member, which forms the upper half of the span. The crest is only 6 feet thick at the thinnest point and is not broken as the name seems to imply.

BROKEN ARCH, reached by a 1/2-mile trail leading northward across field that separates Fiery Furnace from Devils Garden. White thin-bedded unit at top is the Moab Member, which rests upon the massive salmon-colored Slick Rock Member. Opening is 59 feet wide and 43 feet high. (Fig. 16)

Double Arch (fig. 17), "one" of the most beautiful in the park, is in The Windows section near the east end of the road. The southeast arch, which is 160 feet wide and 105 feet high, is the second largest in the park, but the west arch measures only 60 feet wide and 61 feet high. In common with most arches in The Windows section, these two arches of the Slick Rock Member rest upon bases of the weak, easily eroded Dewey Bridge Member. More rapid erosion of the Dewey Bridge undercut the arches and hastened their development.

The cause of the wavy bedding in the Dewey Bridge Member, as shown in figure 17 but as better shown in the frontispiece, is not known for sure but generally is regarded to be the result of irregular slumping during or just after deposition of the sediments in a body of water, caused by the weight of overlying sediments.

DOUBLE ARCH, in The Windows section. (Fig. 17)

The last example I shall take up is Pothole Arch (fig. 18), which differs from all the other examples in that this arch is roughly horizontal rather than vertical. Most park visitors, including me, were not aware of this interesting feature until after publication of the pamphlet "The Guide to an Auto Tour of Arches National Park," which, as previously noted, may be purchased at the Visitor Center. Pothole Arch caps a ridge high above the road half a mile northwest of Garden of Eden, so only those who happened to look up at the right place were aware of its existence.

PROBABLE STEPS IN FORMATION OF POTHOLE ARCH. A, Original pothole probably formed in relatively level bed of sandstone, such as this one, which is in an older rock unit—the White Rim Sandstone Member of the Cutler Formation, a unit not present in Arches. This pothole, which contains 4 feet of water, is in nearby Canyonlands National Park (Lohman, 1974, fig. 17), just north of the edge of the White Rim, about 4-1/2 miles north of the confluence of the Green and Colorado Rivers. Photograph by E. N. Hinrichs. B, Pothole is being deepened by solution while cliff is receding toward pothole by weathering. C, As erosion continues, pothole and cave in cliff face are growing deeper. D, Pothole Arch formed by union of vertical pothole and horizontal cave. E, Telephoto view of Pothole Arch from park road near stop 14. Visible span is 90 feet across and 30 feet high. (Fig. 18)

A different mode of origin than that given in the caption for figure 18 is depicted on a poster in the Visitor Center, which shows the pothole being formed by a waterfall having an apparent flow rate of several cubic feet per second. Potholes can be formed in this manner in places where sufficient streamflow is available, either continuously or following rainstorms, but I believe the process depicted in figure 18 is a more likely mode of origin for Pothole Arch.