Donnelly's Window Earthcache EarthCache
Donnelly's Window Earthcache
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Donnelly's Window is located on the Cumberland Trail in
Cumberland County Tennessee.
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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.
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)Is this "window" really a Natural Arch?
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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