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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 arch occurs.

  • 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.

Standard Attributes

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 arch.

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 the span.
  • 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° and 120°.
  • 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 upright).
  • 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 vertical.
  • Massive lintel – the lintel is very large compared to the hole.

Metric Attributes:

  • Specific measurements of standard dimensions (see Natural Arch Dimensions)
  • 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.

Geologic Attributes:

  • 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 arch occurs.

Genetic Attributes (see Natural Arch Formation):

  • 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
  • Weathering

Maturity Attributes:

  • 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.