Hwy.154 Natural Bridge Earthcache EarthCache
Hwy.154 Natural Bridge Earthcache
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This Natural Bridge/Arch is located just off Hwy 154 in Pickett 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.
What is the difference between a natural arch and a natural bridge?
Definition: A natural bridge is a type of natural arch. In general, a natural bridge is distinguished from other types of natural arches by having one or more of the following attributes:
1)A current of water, such as a stream, clearly was a major agent in the formation of the opening (hole) [genetic]
2)A current of water, such as a stream, flows through the opening (hole) [contextual]
3)It is being, or has been, used by man as a bridge supporting a portion of a road [anthropomorphic]
4)It has the general appearance of a man-made bridge, e.g., a flat, level top over an arched opening [anthropomorphic].
This definition tries to combine several different uses of the term ‘natural bridge’ that are commonly found. These different uses are the result of various authors using different sets of attributes to classify natural arches. Each of the four bullets in the definition above reflects an attribute of a natural arch that might be used to classify it as a natural bridge. The category that each of these attributes falls into is listed in square brackets at the end of the bullet. For example, having the appearance of a man-made bridge is considered to be an anthropomorphic attribute.
Regardless of the attributes used, however, it is important to note that there is no fundamental difference between a natural arch and a natural bridge. A natural bridge is just one of many types of natural arch. The definition of what a natural arch is applies to natural bridges in the same way it applies to all other types of natural arch. The set of attributes that distinguish a natural bridge from other types of natural arch depends on the classification scheme being used.
The preferred use of the term ‘natural bridge’ is as a type label based primarily on genetic attributes (see Natural Arch Classification and Taxonomy). Therefore, it is recommended that the following definition be used in any serious description or classification of natural arches.
Definition: A natural bridge is a type of natural arch where a current of water, such as a stream, clearly was a major agent in the formation of the opening (hole).
To log this earthcache please 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)In your opinion is this a Natural Bridge or a Natural Arch and why?
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