 Middle Creek Arch #1 Earthcache
A cache by BackBrakeBilly Hidden: 8/10/2007
Size:  (Not chosen) Difficulty:  Terrain:  (1 is easiest, 5 is hardest)
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Middle Creek Arch #1 is located on the Cumberland Trail in
Hamilton 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.
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)What is so unique about this arch and the surrounding area?
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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