Natural Arch Classification and Taxonomy
There are many different ways to classify natural arches. Classification
schemes can serve multiple and different purposes, but a purpose common
to most is to provide a shorthand way of describing natural arches based
on their observable attributes. Saying that a natural arch is of a certain
type indicates that it exhibits some set of attributes.
Many classification schemes also attempt to identify how a natural
arch formed. In addition, some may attempt to identify where a natural
arch is in its evolutionary lifecycle. Identification of the processes
that contributed to the formation and subsequent evolutionary development
of a natural arch must be deduced from its observable attributes. The
assumption is that certain combinations of observable attributes are
the result of, and hence are an indicator of, specific processes. For
example, it is reasonable to assume that, if a stream is flowing through
the opening (hole) of a natural arch, the stream played a role in its
formation and/or subsequent development. Usually, however, the observation
of several attributes in combination is required to draw conclusions
about what processes were involved in the formation and evolution of
a natural arch.
The observable attributes used in the classification of natural arches
fall into five categories:
- Contextual – aspects of the surroundings in which the natural
- Morphologic – the general shape and orientation of various
parts of the natural arch.
- Metric – the size of various parts of the natural arch.
- Geologic – the type(s) of rock and/or geologic formation(s)
in which the natural arch occurs.
- Anthropomorphic – actual or perceived relationships between
the natural arch and man.
Anthropomorphic attributes are only included in this list because the
lay public has used them extensively to describe natural arches. Two
examples of natural arch types that are based primarily on anthropomorphic
attributes are 'natural window' and 'natural tunnel'. Anthropomorphic
attributes are difficult to establish objectively and, to a large degree,
require the subjective judgment of the observer. Use of anthropomorphic
attributes to classify natural arches has only resulted in confusion
and is discouraged for serious descriptions or research. A possible
exception might be historical, psychological, or aesthetic analyses.
They are not considered here any further.
As stated earlier, the presence of certain combinations of observable
attributes may lead to deductions about how a natural arch formed, and
even where it is on its evolutionary lifecycle. These deductions may
be viewed as attributes as well, and may be used to classify a natural
arch. Such 'deduced' attributes fall into two categories:
- Genetic – the primary set or sequence of processes (typically
erosional) that led to the formation and subsequent development of
a natural arch.
- Maturity – a relative assessment of where a natural arch
is in its lifecycle, or how far erosion has progressed from initial
formation toward eventual destruction.
Attributes from these six categories (contextual, morphologic, metric,
geologic, genetic, and maturity) can be combined in various ways to
create a taxonomy of natural arches. As with most taxonomies, natural
arch taxonomies are based on placing any given natural arch in one of
a suite of types. Each type has a label for reference (a type label)
and some set of attributes that distinguish it from other types within
the taxonomy. To be successful, a taxonomy for natural arches should:
- provide a suite of types that permits all natural arches to be classified
as one type or another,
- list the set of attributes that defines each type,
- only use attributes that are observable or deduced from observation,
- and contain no types to which no natural arch can be assigned.
The two most important taxonomies for natural arches published previously
are those authored by Vreeland (reference 1)
and Stevens/McCarrick (reference 2). Both
include type definitions based primarily on contextual and morphologic
attributes. Geologic and metric attributes were not used to any extent.
Genetic attributes were then deduced for most (Vreeland) or some (Stevens/McCarrick)
of the types. These can be thought of as genetic types. The types for
which genetic attributes were not deduced can be thought of as morphologic
types, since they were defined primarily by attributes in that category.
When a natural arch is assigned to a morphologic type in preference
to a genetic type, this is usually because erosion has erased the evidence
of how the natural arch formed. Consequently, natural arches assigned
to morphologic types are usually assumed to be old, i.e., in the later
stages of their lifecycle.
Vreeland also deduced maturity attributes for some types. For example,
he might classify a natural arch as either a young alcove type or an
old alcove type. The maturity attribute, young versus old, does not
change it from being an alcove type natural arch. Thus, attributes can
be used as type modifiers as well as to define the type.
Unfortunately, both the Vreeland and Stevens/McCarrick taxonomies incorporated
anthropomorphic attributes in some of their type definitions, unsuccessfully
attempting to recast these attributes as either morphologic or contextual
based on ambiguous definitions. Another source of confusion in applying
these two earlier taxonomies is that some terms are used to mean different
things. For example, both include the term "free-standing" as a type
label, but the definitions they provide for this type are quite different
from each other.
Despite these shortcomings, these two pioneering taxonomies, especially
Vreeland's, were based on extensive field experience and have proven
to be largely successful. The taxonomy recommended in these pages merges
the best features of the two while removing most of the problems.
Before presenting the recommended taxonomy, we provide a list of standard
attributes by category. These attributes are not only the basis for
the taxonomy, they are useful in describing natural arches independent
of the taxonomy. In addition to defining taxonomy types, these attributes
can be used to modify a type in the taxonomy or in a description that
does not specify a taxonomy type, e.g., an occluded, granite natural
The list of attributes below is sorted by category. For most, a brief
definition is provided. These definitions often use terms that are explained
in Natural Arch Components, Natural
Arch Dimensions, or Natural Arch Formation.
The reader may wish to become familiar with these topics before proceeding.
Contextual Attributes (see Natural
Arch Components for definitions of some terms):
- Coastal – occurring in close proximity to the shore of an
ocean, sea, or major lake.
- Stream – occurring over, or adjacent to, a stream or streambed.
- Waterfall – occurring at, or downstream from, a waterfall.
- Ridge-top – occurring on top of a narrow ridge, neck, or
promontory of land.
- Elevated – having an opening well above the base of the vertical
fin, slab, or wall in which it occurs.
- Isolated – not attached to, or in close proximity to, any
rock other than its base.
- Projecting – occurring in a fin, slab, or wall that projects
outward from (roughly perpendicular to) a cliff, or occurring at one
end of a vertical fin, slab, or wall.
- Occluded – occurring in sufficiently close proximity to a
cliff face such that the opening is mostly obscured.
- Blocked – having large, unattached boulders in its opening.
- Filled – loose or compacted soil covers part of the rock
frame under the opening.
- Flooded – water covers part of the rock frame under the opening.
- Subterranean – exposed to air but occurring under the ground,
as in a cavern.
Morphologic Attributes (see Natural
Arch Components and Natural Arch Dimensions
for definitions of some terms):
- Semicircular aperture – the entrances are roughly vertical
and separated by a distance that is small compared to both the span
and height, there is an arched lintel, and the base is roughly horizontal.
- Oval aperture (upright or prone) – the entrances are roughly
parallel (roughly vertical when upright; roughly horizontal when prone),
roughly oval, and separated by a distance that is small compared to
- Slotted aperture (upright or prone) – the entrances are roughly
parallel (roughly vertical when upright; roughly horizontal when prone),
elongated and pointed at the ends, and separated by a distance that
is small compared to the opening breadth.
- Cylindrical – the entrances are roughly vertical, are separated
by a distance that is comparable to or larger than the span, and are
connected by a hole that does not bend more than about 60°.
- L-shaped (upright or prone) – the entrances are roughly perpendicular
to each other (both roughly vertical when prone; the uppermost entrance
roughly horizontal and the lower entrance roughly vertical when upright)
and are connected by a hole that bends at an angle between about 60°
- C-shaped (upright or prone) – the entrances are roughly co-planar
and vertical, are connected by a hole that bends at an angle greater
than 120°, and are side-by-side (if prone) or one atop the other (if
- U-shaped – the entrances are roughly co-planar and horizontal,
and are connected by a hole that bends at about a 180 angle.
- Complex – the entrances are connected by a hole that has
more than one distinct bend.
- Cavernous - light entering the opening, including diffused
and reflected light, does not reach all parts of it, i.e., an observer
can be positioned in the opening such that they are in total darkness
during broad daylight.
- Arched – the underside of the lintel has an overall upward
convex curvature such as a catenary or arch.
- Flat – the top of the lintel is roughly horizontal and planar.
- Vertical lintel – the lintel is roughly aligned with the
- Massive lintel – the lintel is very large compared to the
- Specific measurements of standard dimensions (see Natural
- Miniature – all opening dimensions are smaller than 1 meter.
- Minor – one or more opening dimensions are at least 1 meter.
- Significant – the product of any two orthogonal opening dimensions
is at least 10 square meters.
- Major – having a span of 50 meters or more.
- Rock type – type of rock (sandstone, limestone, granite,
etc.) in which the natural arch occurs.
- Formation – name(s) of the geologic formation(s) in which
the natural arch occurs.
- Member – name(s) of the geologic member(s) in which the natural
Genetic Attributes (see Natural
- Incised meander
- Lateral stream piracy
- Subterranean stream piracy
- Vertical joint expansion
- Bedding plane expansion
- Cavity merger
- Roof collapse
- Wall collapse
- Wave action
- Lava flow
- Compression strengthening
- Stress relief exfoliation
- Chemical exfoliation
- Differential erosion in one member
- Differential erosion in adjacent members
- High gradient of erosion
- Thermal exfoliation
- Flowing water
- Seeping water
- Freeze expansion
- Young – clear evidence of formation mode is present, but
little or no evidence is present that subsequent development has occurred.
- Adult – sufficient evidence of formation mode is present
along with evidence of subsequent development.
- Old evidence of formation mode is absent or inconclusive,
but there is clear evidence of extensive subsequent development.
Natural Arch Taxonomy
Of course, not every possible combination of attributes in the list
above is found in nature. Only a few special combinations are found
with any frequency. These are the types in the taxonomy. Indeed, that
is what makes the taxonomy useful. Classifying together (as a single
type) all the natural arches that share a combination of attributes
not only makes description (perception) easier, it also facilitates
the study of natural arches on a conceptual level (e.g., deducing how
they usually form and evolve). Those natural arches that have combinations
of attributes that are found infrequently may not easily fit into any
of the taxonomy types, but can still be described using the standard
attributes. Such "odd balls" are labeled as irregular in the taxonomy.
Click here then, for the recommended natural
arch taxonomy. Each type is described using the attributes that
define it. Photographs of examples of each type are also provided.