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Basic, sometimes oversimplified, answers to the questions below are found by clicking on them. Hyperlinks embedded in these top-level answers will lead serious researchers to lower levels of detail and more complete and technically precise answers.

Q:  What is a natural arch?

Definition: A natural arch is a rock exposure that has a hole completely through it formed by the natural, selective removal of rock, leaving a relatively intact frame.

This seems simple enough, but there are some subtleties in this definition that should be examined further.

First, a natural arch must be made of rock. A feature made of compacted soil, ice, or organic matter (e.g., a tree trunk, unless it has turned into rock via petrification) may exhibit all the other attributes of the definition, but is still not a natural arch.

Second, the rock must be exposed. It must be substantially surrounded by air. It may be partially embedded in soil or water, but must not be completely encased in either. The rock must be sufficiently exposed to observe that it exhibits the other attributes of the definition.

Third, the hole through the rock must conform to the mathematical, or topological, definition of a hole. In the terminology of topology, a surface with a single hole has a genus of 1. This means that it is possible to draw a nonintersecting simple closed curve on the surface without separating the surface into different regions. A torus, or do-nut shaped surface, has a genus of 1 and has a hole by this definition. A closed curve drawn through or around the hole does not divide the surface. There is still only one region. By contrast, you cannot draw a closed curve on a sheet of paper or a sphere without dividing it into two regions, one inside the curve, and one outside the curve. A sheet of paper and a sphere both have a genus of 0. A natural arch with a single hole is topologically equivalent to a torus. This means that caves, alcoves, and other recesses or concavities in a rock do not qualify as natural arches, even if they are arch shaped. In non-mathematical terms, the hole must go completely through the rock.

Fourth, the hole must have formed from natural, selective removal of rock. Typically this removal is the result of erosional processes, but other natural processes of removal (e.g., lava flow) may have contributed to hole formation. However, features constructed by man do not qualify. Note that a feature is not automatically disqualified just because man modified the hole after it formed naturally. But if the modification has obliterated any convincing evidence of a previous natural origin, then it must be disqualified. Features that result from the build up or movement of rock are also disqualified. For example, a boulder that has created a hole by falling against or between other rock does not qualify. Nor does a rock column created when a stalagmite and a stalactite join.

Fifth, the frame of rock that remains to surround the hole must still be relatively intact. Fractures and joints may be present. Even some slippage along these may have occurred, as long as it is clear that this has happened subsequent to hole formation. Of course, no air gaps can exist in the frame of rock.

Finally, note that size is not a factor in the definition. Some features not normally considered natural arches, because of their size, still qualify as such. For example, consider a large cavern with two small openings connected by miles of underground passages. In this case, the hole is completely through rock and formed by natural selective removal of rock. Further, the remaining rock frame is intact. Although it is debatable whether the hole of a typical cavern occurs through a rock exposure, it is certainly likely that this is true in some instances. At the other extreme of size, a very small peephole through rock also meets all the attributes of the definition.

While there may be no fundamental difference between a cavern, a peephole, and Rainbow Bridge, human perceptions clearly make a distinction. Calling the first two of these natural arches would certainly confuse most people. Size and shape do matter and are factors in how natural arches are classified. Although a cavern might technically be a natural arch, it is more appropriately called a cavern. Size and shape determine when and where this label is to be preferred. Similarly, size determines whether a natural arch is significant. A peephole one inch in diameter might technically be a natural arch, but it is also an insignificant one.

Q: What is the difference between a natural arch and a natural bridge?

Definition: A natural bridge is a type of natural arch. In general, a natural bridge is distinguished from other types of natural arches by having one or more of the following attributes:

• a current of water, such as a stream, clearly was a major agent in the formation of the opening (hole) [genetic]
• a current of water, such as a stream, flows through the opening (hole) [contextual]
• it is being, or has been, used by man as a bridge supporting a portion of a road [anthropomorphic]
• it has the general appearance of a man-made bridge, e.g., a flat, level top over an arched opening [anthropomorphic].

This definition tries to combine several different uses of the term 'natural bridge' that are commonly found. These different uses are the result of various authors using different sets of attributes to classify natural arches. The attributes used in classification schemes for natural arches fall into seven broad categories (see Natural Arch Classification and Taxonomy). Each of the four bullets in the definition above reflects an attribute of a natural arch that might be used to classify it as a natural bridge. The category that each of these attributes falls into is listed in square brackets at the end of the bullet. For example, having the appearance of a man-made bridge is considered to be an anthropomorphic attribute.

Regardless of the attributes used, however, it is important to note that there is no fundamental difference between a natural arch and a natural bridge. A natural bridge is just one of many types of natural arch. The definition of what a natural arch is applies to natural bridges in the same way it applies to all other types of natural arch. The set of attributes that distinguish a natural bridge from other types of natural arch depends on the classification scheme being used.

The preferred use of the term 'natural bridge' is as a type label based primarily on genetic attributes (see Natural Arch Classification and Taxonomy). Therefore, it is recommended that the following definition be used in any serious description or classification of natural arches.

Definition: A natural bridge is a type of natural arch where a current of water, such as a stream, clearly was a major agent in the formation of the opening (hole).

Q: Are natural windows or natural tunnels anything different?

The terms 'natural window' and 'natural tunnel' have been used in some classification schemes to describe certain natural arches. These classifications are based on comparing a natural arch to a man-made object. Thus, a 'natural window' is a natural arch that reminds an observer of a window, and a 'natural tunnel' is a natural arch that reminds an observer of a tunnel. Neither usage is based on any quantifiable criteria or any set of unambiguous attributes. Not surprisingly, neither term has been applied in any consistent way. Although it might be possible to develop unambiguous definitions for these terms, and some attempts have been made, no real utility in doing so has been identified. Therefore, the use of these terms is not recommended in any serious description or classification of natural arches.

Q: What other types of natural arches are there?

There are many different types of natural arches. The following is a list of some of the more commonly occurring ones. To see a complete list of types with definitions and photographs of examples, link to Natural Arch Taxonomy.

To gain an understanding of how natural arches are classified into types, the reader should link to Natural Arch Classification and Taxonomy.

Q: How big does something have to be to qualify as a natural arch?

Size is not a factor in determining if a feature is a natural arch or not. It may be a surprise that some features qualify. For example, consider a large cavern with two small openings connected by miles of underground passages. All of the criteria in the definition of a natural arch are met. The hole is completely through rock and formed by natural selective removal of rock. Further, the remaining rock frame is intact. Although it is debatable whether the hole of a typical cavern occurs through a rock exposure, it is certainly likely that this is true in some instances. At the other extreme of size, a very small peephole through rock also satisfies all the requirements of the definition.

Although there is no fundamental difference between a cavern, a peephole, and Rainbow Bridge, human perceptions clearly make a distinction. Size and shape are factors in how natural arches are classified (see Natural Arch Classification and Taxonomy). Although a cavern might technically be a natural arch, it is more appropriately called a cavern. Size and shape determine when and where this label is to be preferred. Similarly, size determines whether a natural arch is significant. A peephole one inch in diameter might technically be a natural arch, but it is not a significant one.

Q: What is the largest natural arch?

The answer to this question depends on two other questions that are rather tricky. First, what is meant by "largest," and second, how does one go about determining the relative sizes of natural arches? The complete (and fairly technical) answers to these questions are provided at Natural Arch Dimensions. For a simplified answer, most visitors to this site will be satisfied by refering to our page on Giant Arches. This page lists in rank order all the arches known to us that have spans greater than 200 feet. It provides information and photos for each.

Q: How should natural arches be measured?

There are many different ways to measure the dimensions of natural arches. Some are accurate and some are not. Measurement techniques in common use today include steel tape, range finder, differential GPS, photo estimates, pacing, triangulation, comparison to other objects of known size, and visual estimates. Field investigators will make their choice based on many factors including what they are experienced with, what equipment they have, and how easy it is to physically access the arch, i.e., those points on the arch that must be measured.

Measurements of a natural arch are only useful if they are measurements of the standard dimensions defined in these pages. Otherwise, it is not possible to interpret what the measurements represent, i.e., what dimensions were measured. An investigator that reports a measurement that is not consistent with the standard set of defined natural arch dimensions is not communicating information of value. Their readers will not be able to unambiguously understand what they were trying to convey. Misconceptions and misinterpretations are inevitable unless the standard, defined dimensions are used.

Defining a standard set of dimensions for the purpose of describing and studying natural arches is a very complex subject that can't be addressed in a short answer. It is covered in depth at Natural Arch Dimensions. Readers already familiar with this subject who just want to reference the definitions of the standard set can link to the synopsis. Also presented is a more introductory article, Finding the Span of Arches with Simple Shapes.

Q: How are natural arches formed?

As stated in the definition of a natural arch, a natural arch is formed by the natural, selective removal of rock from a rock exposure. There are many different processes of erosion that can contribute to the natural, selective removal of rock. These processes usually only cause a natural arch to form when certain combinations of them act on a rock exposure of a specific shape. For a detailed discussion of the various process of erosion that can contribute to natural arch formation, link to Natural Arch Formation. A review of those pages will reveal that water, gravity, and temperature fluctuation are the principle forces involved in carving natural arches out of rock.

An important point to understand, even for those only interested in a basic understanding of natural arches, is that wind is not a significant factor in their formation. Although 19th century geologists began this myth, some claiming that wind formed natural arches while water caused natural bridges, subsequent study has shown that wind never has more than a polishing effect after a natural arch has formed. Furthermore, a natural bridge is just one type of natural arch, not a distinct landform. There is no fundamental difference between how a natural arch and a natural bridge form.

Q: How old are natural arches?

Except for the handful of natural arches whose formation has been observed and recorded, there is currently no way to determine the age of a specific arch. However, it is also clear that all of these features are very short-lived on geologic time scales, and are quite recent phenomena. Most of them formed as a result of the rapid uplift and erosion that certain areas of the world have experienced since the last ice age. Therefore, it is safe to say that no natural arch is older than about 30 thousand years. Most are probably between 5 and 15 thousand years old, i.e., not incomparable to the span of recorded history.

Certain types of natural arch, however, are much younger than this on average. These are either relatively weak structurally (e.g., caprock natural arches) or are subject to much higher rates of erosion (e.g., sea natural arches). Sea natural arches have typical ages of a few centuries, or even decades, as opposed to millennia. For a discussion of the various types of natural arch link to Natural Arch Classification and Taxonomy.

On the other hand, natural arches exist in rock that is very much older. The age of the rock itself is determined by observing what geologic formation and member it is part of. Ages can range from a few million years to several hundred million years, three to six orders of magnitude older than the arch!

Q: How many natural arches are there?

The glib answer would be billions and billions, since even miniscule pinholes through rock technically qualify as natural arches (see How big does something have to be to qualify as a natural arch?). However, it is more interesting to consider the question of how many significant natural arches there are. (As listed in Standard Attributes, a significant natural arch has two orthogonal opening dimensions with a product of 10 square meters or more.)

The natural arches in the continental United States have been fairly thoroughly catalogued, in part by Vreeland (reference 1) and subsequently by The Natural Arch and Bridge Society. Extrapolating from this data, and making an assumption about how many might still be undocumented, there are about 2000 significant natural arches in the US (now including Alaska and Hawaii.) Assuming this density is typical, one can then estimate that there are about 30,000 significant natural arches in the world.

Through a similar analysis, one can estimate that there are about 400 major natural arches in the world. (Major natural arches have a span of at least 50 meters.) However, less than 100 have been documented.

Q: How do natural arches get named?

The answer to this question is rather long because, unfortunately, there just isn't a simple answer. Indeed, what follows is very oversimplified. My apologies to the reader.

The primary reason a natural arch gets named is to create a simple label that can be used to refer to it conveniently. Another fairly common reason is to memorialize a person or event by naming a natural arch for that person or event. Virtually every time a natural arch is named, it is to satisfy one or both of these two purposes. Consequently, most named arches are large, impressive, easily and frequently seen, or have some other distinguishing characteristic. The vast majority of natural arches are not named. They remain unnamed because they are either small, unimpressive, remote and difficult to visit, or all of the above. Most people do not feel a great need to refer to such arches and would not choose them as memorials. A name for such an arch would be of little value.

Some people argue that this is a good thing. They believe a name detracts from a person's sense of discovery when they first visit a remote or little-documented natural arch. Others argue that this state of affairs just makes cataloging and reference difficult. They would like to have a unique name for every natural arch in existence.

Of course, unique names don't get applied very often, even to the world's most famous natural arches. Different people usually invent different names for the same natural arch. Most famous, well-documented natural arches get multiple names attached to them after becoming an object of public scrutiny. Over time, common usage usually converges on one of these names and the others are dropped and forgotten. Nevertheless, although it might seem a "tempest in a teapot," bitter battles have been fought over what name is the "right" one for several natural arches.

It is also the case that different natural arches have wound up having the same name. Some generic names, e.g., "Window Rock," have been applied to large numbers of natural arches. Even specific names have been attached to different arches. For example, there are at least four natural arches in the US named "Royal Arch". Three of them are in the same state (Arizona). Worse, there is a famous landform in Yosemite National Park named "Royal Arches" that is not even a natural arch! Many similar examples exist.

In part to address such issues, the US Board on Geographic Names was established to definitively determine the names to be used by the US Federal Government for all geographic features, including natural arches. Several other countries have created similar boards. The names decided upon by such boards should be considered the official name of any land feature such as a natural arch. However, since these boards generally do not publicize their decisions, many examples exist where the most commonly used name differs from the official name. Examples can also be found where a map produced by the government shows an arch with a different name than the official name. And, of course, even if all other names were dropped except the official name, that would only be half the battle. There would still be lots of instances where more than one arch had the same official name.

The conclusion that must be reached by most people seriously interested in cataloging natural arches is that names, even official names, are not useful for uniquely identifying arches. At best, names are a convenience for quick reference to a few well known and well documented arches. As a result, various natural arch catalogers have invented alternative systems for uniquely identifying and labeling the arches they catalog. Most catalogs also include any commonly used names and official names that might exist. However, such names are provided as interesting or historical data about the arch rather than as a reference label.

Unfortunately, no standard system has emerged for uniquely identifying and labeling natural arches. Every catalog that has been published to date uses its own system. Fortunately, some catalogs do provide cross-references to other catalogs that have an overlap in coverage. In hopes of eventually correcting this unfortunate state of affairs, a proposed standard system for identifying natural arches is described in these pages.

Finally, even if names do not serve well as reference labels, the author strongly supports the very human need to memorialize certain individuals by naming a natural arch for them. In general, such commemoration should be done judiciously and should be reserved for individuals who have contributed to the study, appreciation, or preservation of natural arches, either directly or indirectly. For example, the author encourages the use of "Will Minor Arch," "Cleland Arch," "Vreeland Arch," and "Horowitz Arch," although only the first of these has become official as of this writing.

Q: Where is the closest natural arch to me?

Answering this question definitively would require a list of all the natural arches in the world and their locations. Because no complete list of arches and arch locations exists, the best answer that can be given to this question is a list of arches that are located in a specified area. Depending upon the area chosen, the completeness of the list of arches that could be provided would vary considerably. In certain areas, e.g., Arches National Park in Utah, the cataloging of natural arches has advanced to a point where any reasonable person would view the list as complete. For other areas of the world, no cataloging has been done so no list could be made available. These are the extremes. For many, if not most, areas of the world, the list that could be produced would be rather incomplete, but would contain at least some natural arches.

The other problem associated with answering this question is that the vast majority of the data on arches and arch locations exists only on paper, e.g., books and pamphlets, or in highly proprietary electronic formats. Furthermore, much of this material is difficult to obtain for most people. In an effort to correct this situation and make information about natural arches readily available to the general public, members of the Natural Arch and Bridge Society, including the author of these pages, are working to create an Internet-based catalog/database about natural arches. However, this effort has just gotten underway. The information available through this budding resource is still very incomplete and preliminary. Only a very small percentage of the information that is available in paper catalogs has been posted on the Internet.

Visitors interested in this effort may wish to read the material in Selected Natural Arches. Those who just want to try to find a natural arch that is reasonably close by can try the following procedure:

1. Access the Internet and go to one of the big search engine sites. Google is recommended.

2. In the search box enter a two word search. The first word you enter should be NABSQNO. The second word should either be a country, e.g., France; a state if the country of interest is the US, e.g., Arizona; or a UTM zone, e.g., 16S.

3. This search will likely return some Internet pages that document natural arches in the area you specified. If not, try a country, state, or zone that is near the first one you tried.

Good hunting!