Frequently Asked Questions
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
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.
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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
- 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).
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Q: Are natural windows or natural tunnels
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.
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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.
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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.
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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.
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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.
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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.
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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!
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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.
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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
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
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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.
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Q: How can I learn more about natural arches?
If you have read all the material in these pages and still want to
know more about natural arches, you are clearly a candidate to join
the Natural Arch and Bridge Society. Joining
will also bring you their quarterly newsletter, SPAN,
which is a major source of information about natural arches.
Three other important sources of information about natural arches are
the references that were used extensively
by the author in preparing these pages. In addition to being an excellent
survey of the subject, reference 3 has a very extensive bibliography,
which will lead the reader to many other sources of information. Unfortunately,
these books are hard to find. Inter-library loan is the most likely
way to examine them.
Of course, if you have a specific question you would like to ask, or
a topic you want to discuss, you are always welcome to contact me via email.
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