March Arch Earthcache EarthCache
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March Arch is located on the Cumberland Trail in Morgan
County Tennessee.
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.
Natural Arch Formation
Natural Arches are formed by the natural, selective removal of
rock. The natural processes that lead to selective removal of rock
from a rock exposure are almost exclusively processes of erosion.
Erosion can selectively remove rock both macroscopically and
microscopically. Both modes are effective, albeit on different time
scales, because of the basic structure of virtually all types of
rock.
Rock of any type (with the sole exception of a pure crystal) is a
complex matrix of small, interlocking, solid particles. These
particles are mostly microscopic fragments of various mineral
crystals known as grains. Under high temperatures and pressures,
some of the crystalline grains fuse, especially the smaller ones,
and act as a cement between the larger grains.
Macroscopic erosion occurs when joints or fractures are first
induced in this rock matrix through some (usually catastrophic)
process, and then widened through a variety of other processes.
This splits the rock into distinct macroscopic pieces that can then
move relative to each other under the forces of gravity or water
pressure.
Microscopic erosion occurs when certain processes dissolve the
crystalline cement, thus destroying the rock matrix and allowing
other processes to disperse the remaining loose grains.
Both types of erosion occur separately and in combination on all
rock exposures. Only under very special circumstances will a
natural arch form. These circumstances include the type, or types,
of rock that are present, the shape of the rock exposure
(especially in relation to the gravity gradient), and the
combination of erosional processes that act upon it. Usually a very
specific sequence of erosional processes must operate on a specific
shape of rock exposure before a natural arch will form. Since some
erosional processes are more effective on certain types of rock
than others, the type of rock is also an important factor.
Relevant Processes of Erosion
Several processes of erosion can contribute, usually in
combination, to natural arch formation. Each of these process is
described separately in the paragraphs below. Different sequences
or combinations of these individual processes conspire to form
natural arches of different types. Because the type of arch is
critically dependent upon them, these combinations are described as
part of the natural arch taxonomy included on this site rather than
here.
Before delving into the details of these processes, an important
observation should be made to dispel what has been a persistent
myth about natural arches. Every single process relevant to natural
arch formation involves the action of water, gravity, temperature
variation, or tectonic pressure on rock. Wind is not a significant
agent in natural arch formation. Wind does act to disperse the
loose grains that result from microscopic erosion. Further,
sandstorms can scour or polish already existing arches. However,
wind never creates them.
Finally, it must be acknowledged that most of the material in the
paragraphs below is based on more detailed treatments by several
other authors available in the literature on geology and physical
geography. An excellent summary of this material is found in the
chapter in reference 3 on natural arch formation and in its
bibliography.
Tectonic movement and uplift. The earth’s crust consists of plates
that float on a sea of magma. Magma is rock that is liquefied by
the tremendous pressures of the earth’s interior. As these crustal
plates slowly move over the magma, a process known as tectonic
movement, they collide in places. Such collisions cause portions of
the plates to be raised up. This is one example of what is known as
uplift. Tectonic movement can also result in thinner areas of crust
gradually becoming repositioned over hot spots in the magma. When
this happens, these areas also experience a general uplift due to
the increased pressure from below. Uplift generally accelerates
erosion. It is especially important in creating certain land
features that frequently are the precursors to natural arches,
e.g., joints, fins, and incised meanders. As a result, many of the
world’s natural arches are found in areas currently experiencing
uplift.
Glaciation. The advance and retreat of glaciers can result in
significant erosion. Advancing glaciers can carve shear-walled
valleys and highly sculpted terrain. Such features are likely
places for natural arches to form. The run off from retreating
glaciers usually causes a temporary increase in local erosion
rates. This also may contribute to arch formation if other
conditions are right.
Incised meander. A continuous flow of water over rock, e.g., a
stream or river, will erode its path into that rock. If the rock is
highly sloped, the water will generally cut a fairly straight
channel down the slope. However, if the rock is level, the water
will snake its way around any slight bump in the terrain. This
frequently leads to the water course making wide, curling loops
that almost, but not quite, double back on themselves. Such a loop
is called a meander. The point where the water course almost closes
the loop is called the neck of the meander. If there is uplift in
the area, the water will tend to erode its path into the rock to
remain at a constant elevation as the rock around it rises. If the
uplift is rapid, shear-walled cliffs may form along the banks of
the water course. In this way, meanders can become deeply incised
into rock. For many such incised meanders, the neck will become a
tall, thin wall of rock. Other processes of erosion can then create
an opening through the wall to form a natural arch.
Lateral stream piracy. When two water courses, e.g., two streams,
are separated at some point by a relatively thin rock barrier, this
barrier may be breached, allowing one of the streams to shorten its
path. In a sense, the water of one of the streams is ‘stolen’ by
the other. This is known as lateral stream piracy. It can occur in
two similar situations. One is at the neck of an incised meander.
The other is where two tributaries run closely parallel to each
other for a distance upstream of their juncture. The breach in the
separating barrier may be caused by any of several processes, but
most of these do not lead to arch formation. The process of
interest here is wall collapse, which can lead to the formation of
a natural arch. The opening created by wall collapse grows down to
a level where water can flow through the opening when the stream is
in flood. This clears out any debris in the opening and accelerates
the growth of the opening. Eventually, the stream channel is
re-routed through the opening, completing the process of lateral
stream piracy.
Subterranean stream piracy. Water flowing over rock in a channel,
e.g., a stream, will, of course, seep into any cracks or joints in
that rock. In most cases, seeping water will cause chemical
exfoliation and freeze expansion, enlarging the crack or joint.
This allows a greater flow of water into the crack or joint which
accelerates erosion. When cracks and/or joints combine to create a
pathway through the rock through which the water can travel and
rejoin the stream (or a different nearby stream), subterranean
stream piracy can occur. Basically, the pathway is enlarged until
most, if not all the water in the stream flows through it rather
than the original channel. It has ‘stolen’ the water from the
original stream. When this occurs at the lip of a waterfall, a
waterfall natural bridge may form. In other situations,
subterranean stream piracy can create long and extensive
underground passageways. These may become caverns (a type of
natural arch) or, if roof collapse occurs above the passageway, a
variety of waterfall natural bridge.
Vertical joint expansion. Water seeping into a crack or joint in a
rock exposure will, over time, act to enlarge the joint, creating a
gap in the rock. Chemical exfoliation and freeze expansion
frequently combine to cause this to happen. The expansion of joints
that are roughly vertical may contribute to natural arch formation
in several ways. Three examples follow:
When a series of parallel vertical joints are present in a rock
exposure, e.g., as a result of uplift or tectonic movement, some or
all may expand into sizeable gaps. This results in a field of
parallel rock walls or fins. Wall collapse and other mechanisms can
then cause a natural arch to form in one or more of the fins.
When a vertical joint is present near, and parallel to, a cliff,
e.g., as a result of stress relief exfoliation, its expansion may
couple with other processes, e.g., wall collapse or cavity merger,
to form various types of natural arches.
When a vertical joint is present in, and perpendicular to, an
exposed wall, fin, or narrow projection of rock, it may expand
preferentially near the bottom or middle. In certain cases, this
can result in a natural arch being formed. Bedding plane expansion.
Sedimentary rock is deposited in layers. The boundaries between
these layers, known as bedding planes, are similar to joints or
cracks. Water seeping between the bedding planes will cause
chemical exfoliation and freeze expansion. This often leads to the
growth of a horizontal air gap between the layers of rock. In this
way, the expansion of a bedding plane in a rock exposure can
contribute to the formation of a natural arch.
Cavity merger. Differential erosion and chemical exfoliation acting
on the surfaces of a rock exposure frequently cause concave
recesses in the rock. As these grow into cavities, some may become
connected. Cavities can become connected, or merge, by growing into
and expanding a joint that was already present in the rock, or
simply by growing into each other. This can happen in simple and
complex ways. When a lintel is left as a remnant of the barrier
that once separated the cavities, a natural arch is formed.
Roof collapse. When the roof of rock that is over a subterranean
passage or a cave becomes too thin for the tensile strength of the
rock to hold it together against the force of gravity, it will
fracture catastrophically and collapse, i.e., sections of rock will
fall out of the roof. The sections of roof that remain suspended
may be left as the lintels of natural arches.
Wall collapse. Wall collapse is a complex, cyclic process that can
occur as a result of gravity and thermal flexing acting upon a
tall, thin exposure of rock. This process first causes the
formation and growth of an arched shape recess (an alcove) above
the base of the wall. This alcove eventually grows into a
semicircular aperture through the wall. Wall collapse does not
require water to occur, but the presence of water can accelerate
it. It is one of the most important erosion processes that can lead
to the formation of a natural arch. For this reason, and because of
its complexity, the reader may choose to link to this more detailed
description of wall collapse.
Wave action. The waves that batter the shoreline of a large body of
water, such as an ocean, sea, or great lake, are a major force of
erosion on any coastal rock exposures that are present there. Waves
trigger and accelerate several erosional processes, especially
chemical exfoliation, differential erosion, cavity merger, and wall
collapse. In addition, particles carried in the waves (e.g., sand)
act as an abrasive on the rock. As a result, coastal rock exposures
experience erosion rates ten to a thousand times higher than those
inland. Therefore, coastal natural arches are formed and destroyed
relatively quickly and frequently. They are short-lived compared to
most inland natural arches. Furthermore, combinations of erosional
processes occur on coastal rock exposures that are seldom, if ever,
encountered inland. This often results in natural arches of unusual
shape.
Lava flow. Flowing lava cools from the outside in. At first, the
crust of hardened, solid rock that forms on the outer layers of a
lava flow gets carried along with it. But as this crust cools even
more, it eventually thickens and stabilizes. Nevertheless, the lava
inside this stable crust is still hot enough to flow. Indeed, the
crust acts as an insulator, keeping the interior parts of the flow
viscous for a long time. The 'inside' lava may even drain out of
the stable, outer rock crust, emerging 'down-flow' to cool and
become rock as well. This sequence of events frequently leaves
behind long chambers or "tubes" in the interior of the newly cooled
rock - "tubes" that were evacuated by the last of the hot, flowing
lava. If roof collapse subsequently occurs above such a "tube," one
or more natural arch may form.
Compression strengthening. The weight of rock is, of course, due to
the force of gravity. This force acts to compress any rock that
resists it. Normally, this force acts in the vertical direction.
Rock underneath other rock is compressed by the weight of the rock
above it, i.e., the rock it supports. However, when rock is
supported over an opening or hole, the lines of force are diverted
from the vertical into a pattern the shape of an inverted catenary.
A catenary is the shape a rope takes when suspended freely from its
two ends. An inverted catenary is that shape turned upside-down.
It's the shape of an arch. Thus, the weight of rock above an
opening compresses the rock that supports it along force lines that
are arch-shaped. Regardless of whether the compression is vertical
or arch-shaped, it strengthens the rock that gets compressed. This
is because compression acts to fuse more grains, including larger
grains, in the rock matrix. In effect, it adds cementing and
increases the bonding force of the cement that is there. The rock
becomes harder and more resistant to erosion. Natural arch lintels
that take the shape of an inverted catenary often experience
compression strengthening. Compression strengthening makes a lintel
more resistant to erosion and, therefore, increases the lifespan of
a natural arch.
Stress relief exfoliation. Rock is subjected to many forces.
Tectonic movement, uplift, and gravity can each put stress on a
rock exposure. Rock will eventually fracture as more and more
stress is placed upon it. The specific point and pattern of the
fracture is dependent upon a complex set of variables. When
stress-related fracturing leads to macroscopic fragments of rock
separating from a rock exposure, this is called stress relief
exfoliation. Stress relief exfoliation contributes in many
different ways to natural arch formation.
Chemical exfoliation. Water that is in contact with rock will, over
time, dissolve the lattice of fine crystalline grains that cement
the larger grains of the rock together. In effect, the water
dissolves the rock into grains which can then be removed either by
the water itself, gravity, wind, or other mechanisms. This process
of erosion is known as chemical exfoliation. It contributes to
natural arch formation in several ways. One of these ways is the
creation of potholes, caves, and/or smaller depressions wherever
standing, flowing, or seeping water comes in contact with exposed
rock. Another is the expansion of joints into air gaps when seeping
water gains access to a joint.
Differential erosion. When erosion proceeds at two different rates
at the same location, e.g., on adjacent rock surfaces, it is called
differential erosion. This can happen wherever the grain and
cementing properties of rock vary from place to place in a rock
exposure. For example, if the distribution of grain size in the
rock matrix is different in one part of the rock exposure than in
another, these two places will experience different rates of
erosion. Differences in the degree of small-grain fusing, i.e.,
cementing, will also cause different erosion rates. Such
differences commonly occur when a rock exposure comprises more than
one geological formation or member. Each member will erode at its
own pace. However, many geological members form as the result of a
long period of sedimentary deposition. Such a member may consist of
several layers laid down at vastly different times. Differences in
graining and cementing can certainly occur between such layers.
Therefore, differential erosion can occur in a rock exposure that
consists of a single member. Differential erosion contributes to
the formation of natural arches in several ways, e.g., the
undercutting of harder layers of rock that are supported by softer
layers.
High gradient of erosion. A rock exposure with a significant slope
will erode faster, and be susceptible to more types of erosion,
than a similar exposure with a gentler slope. This is simply due to
gravity. Gravity can remove fractured rock fragments or loose rock
grains from surfaces only if it can overcome friction. For any
surface, there is a critical slope at which gravity is able to
overcome friction and pull away the detached fragments of rock.
This then exposes the next layer of the rock to erosion. The
erosion cycle proceeds more efficiently, and hence more rapidly,
when it gets this assist from gravity. An exposure with slopes
greater than the critical value (which depends complexly on several
factors) is said to have a high gradient of erosion. Natural arches
are more likely to form on rock exposures with a high gradient of
erosion.
Thermal exfoliation. Temperature fluctuation causes rock to expand
(as temperature rises) and contract (as temperature falls). This
cycle of alternating expansion and contraction frequently leads to
the rock fracturing. Fractures preferentially occur along stress
patterns in the rock. Fracturing then permits the removal of rock
fragments by gravity or water pressure. Even when the ambient
temperature is relatively constant, sunlight striking the surface
of a rock exposure will create a temperature gradient in the rock.
The surface layer of rock will become hotter than deeper layers.
The hotter temperature of the surface layer forces it to expand
more than the cooler, deeper layers. In effect, the surface tries
to bow outward. This can lead to stress fractures parallel to the
surface. Should these fractures also be parallel to bedding planes
or vertical joints, huge sheets of rock can become detached from
the rock exposure. The macroscopic fracturing and removal of rock
as the result of temperature fluctuation or temperature gradients
is known as thermal exfoliation. This process of erosion
contributes to the formation of natural arches in many ways.
Freeze expansion. When seeping water that has permeated a rock
joint freezes, it expands. This puts stress on the rock and
frequently fractures the rock adjacent to the joint. As the water
thaws and is replenished from whatever source is involved, it gains
access to these fractures. In this way, repeated cycles of freezing
and thawing will break up the rock along a joint into small pieces
that can then be removed by gravity or water pressure. The
expansion of joints into air gaps via this cyclic process
contributes to natural arch formation in many ways. See for example
the paragraph on vertical joint expansion.
Weathering. Weathering is the combined effect of precipitation and
wind on the surfaces of exposed rock. Frozen precipitation, e.g.,
snow, can be a steady source of seeping water that can permeate the
rock and cause localized chemical exfoliation. Steady or frequent
rain may become a similar source. Strong winds can pick up grains
and pummel the surface of a rock exposure with them, in effect
sandblasting the rock. These processes act in combination to smooth
and age the surface of rock. They seldom have sufficient impact to
sculpt the rock to any significant degree. Therefore, although
weathering sometimes plays a roll in how a natural arch ages, it is
not a process that leads to the formation of natural arches.
Natural Arch Classification and Taxonomy Background
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.
To log this earthcache please post a picture of you in front of
the earth feature and email me the answers to the following
questions...
1)What are the inside measurements of the arch?
2)What kind of rock is the arch formed in?
3)What erosional process (Wind, Water, Ice, etc.) formed this
arch?
4)After reading the cache page and seeing the Arch in person, What
classification would you say this Arch best fits in to?
Any found logs without the required picture
posted with the log and the correct answers emailed to me will be
deleted.
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