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.
[Illustration: INDEX MAP, showing localities where most of the
photographs were taken. Arrows point to distant views. Numbers refer
to figure numbers. (Fig. 13)]
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.
[Illustration: 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)]
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.
[Illustration: “BABY ARCH,” just southwest of Sheep Rock in
Courthouse Towers area. For details, see text. (Fig. 15)]
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.
[Illustration: BROKEN ARCH, reached by a ½-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)]