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 18]A, 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.
TUNNEL ARCH, reached by short trail north of main trail through Devils Garden. Opening is 26½ feet wide and 22 feet high; span is about 14 feet thick. (Fig. 14)