Pickett Lake Natural Bridge Earthcache EarthCache
Pickett Lake Natural Bridge Earthcache
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Pickett Lake Natural Bridge is located in Pickett State
Park in Pickett County Tennessee.
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 type of natural arch is always associated with an active
stream or streambed. The opening is a semicircular aperture and the
lintel is arched. Most examples have a flat lintel, but there are
some notable exceptions to this. Nevertheless, this characteristic
shape of the lintel, flat on top and arched underneath, is so
reminiscent of a man-made bridge that these features have
consistently been labeled natural bridges in all previous
taxonomies. Retaining this concession to anthropomorphic
description in this taxonomy was deemed both necessary (to avoid
the confusion that would certainly arise from any new type label)
and tolerable (because of the consistent previous usage).
Although flowing water in the streambed plays a crucial role in the
formation of this type of natural arch, it is not the only erosion
process involved. Wall collapse must also occur. Indeed, this is
the reason the opening is always a semicircular aperture. Flowing
water is the cause of the wall in the first place and lateral
stream piracy completes the picture after the opening has formed.
Nevertheless, because there must also be wall collapse, it can not
be said that flowing water is the primary cause of this type of
natural arch.
Formation begins when either a stream meander or two parallel
tributaries are incised into rock as a result of rapid uplift. The
flow may be either permanent or occasional. This results in a thin
neck or wall of rock that separates two streams of flowing water.
In the case of a meander, the flow is due to one stream that has
doubled back on itself. In the case of parallel tributaries, two
separate but adjacent streams cooperate to create the wall.
Once the wall has reached a sufficient height, i.e., once the
stream action has sufficiently incised the rock, wall collapse may
occur. The initial opening may be oval, but soon expands downward
to the level of the stream and takes on the characteristic shape of
a semicircular aperture. At that point, occasional floods trigger
lateral stream piracy so that the stream eventually flows through
the opening. Subsequent development is due to further wall
collapse, weathering, and compression strengthening. In rare cases,
lateral stream piracy can occur upstream of the opening, leaving
the floor of the opening dry.
However, there must be evidence that a stream once flowed through
the opening. It is possible for wall collapse to only proceed down
to a harder layer of rock that is still above the level of the
stream. Continued uplift and deepening of the incised streambed may
leave the opening isolated above the flow. In this case, the
feature is a shelter natural arch, not a meander natural bridge. It
is also possible that an opening might form from some other process
than wall collapse and again not enlarge to the point where stream
piracy occurs. In this case, the feature is a fin natural
arch.
A significant amount of subsequent development due to wall collapse
can occur after the initial formation of a meander natural bridge,
whether or not a stream continues to flow through the opening. In
some cases, the stream flows at the foot of one of the abutments,
preferentially enlarging that end of the opening. But it is usual
for the opening to expand in all directions away from the stream
flowing through it. This process continues until either the lintel
can no longer assume a catenary shape (becomes unable to support
its own weight) or one of the abutments becomes too thin to support
the weight of the lintel.
These are the observable indicators of maturity. A meander natural
bridge is young if the stream only flows through the opening when
in flood, or if the stream fills the floor of the opening. If at
least one of the abutments is significantly removed from the
streambed, it can be considered adult. If the lintel has become
delicate or has lost its catenary-shaped underside, or if one of
the abutments has narrowed to the point that much further expansion
of the opening will result in collapse, the natural arch is
considered old.
To log this earthcache please post a picture of you, not just
your GPS in your hand, 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)In your own opinion is this a Natural Bridge or a Natural
Arch?
Any found logs without the required picture
of you with the arch in the background posted with the log and the
correct answers emailed to me will be deleted. No GPS hand shots
will be accepted, Your face must be in the picture or don't log the
cache!
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