This natural bridge span is one of two that I know of in Roane
County. This one has even been incorporated into the existing farm
road and is actually used as a bridge for farm activity.
Over hundreds of millions of years after the Pennsylvanian
period, the sediments turned to rock, and the whole area of western
West Virginia became a plateau because of North America and Africa
colliding. The mountains and valleys in the area were formed by
erosion from rivers and streams. The rocks underneath the bridge
were likely made up of a shaley sandstone and could erode easily,
making the layer that had eroded away the opening under the bridge.
The Saltsburg sandstone that formed the bridge was stronger and was
more resistant to erosion than the shaley sandstone that was
beneath it.
The definition of a Natural Arch is a generally
horizontal remnant of bedrock supported at two opposing points and
spanning an opening created by erosive processes, the sum of whose
span (horizontal dimension of the opening) and clearance (vertical
dimension of the opening) is equal to or greater than 6 feet (1.8
meters) with neither dimension measuring less than 1 foot (.3
meters). A Natural Bridge is a natural arch which spans a
valley of erosion. A natural bridge is always a natural arch, but a
natural arch may not be a natural bridge.
For a natural bridge to form, there are 4 required elements.
First, a natural arch can form only in rock which is solid enough
to hold together against the force of gravity. The strength of the
rock determines how big a natural arch may become and how long it
will survive.This is referred to as "Consolidated Rock".
Second is Erosive Agency, which is almost always water which
is capable of eroding rock through solution (dissolving minerals in
the rock), abrasion (by carrying sand and grit against it) and
fracture (most often by the pressure it exerts when it enters
cracks and freezes). Third is Relief. Relief is defined as
the variation in height of the Earth's surface. Relief is what
gives water its erosive force by providing a gradient through which
it can fall, thus enabling it to move. The greater the relief, the
more potential erosive force exists. The amount of local relief
also determines how big a natural arch may become, since it
obviously cannot be any bigger than the cliff in which it forms.
Lastly you must have a pathway(s) for erosive activity. This
can take the form of pores, vertical fractures and/or horizontal
bedding planes in the body of the rock. The enlargement and
consolidation of these pathways by erosion is a major factor in
forming the openings of natural arches.
In order to log a find at this earthcache you just perform the
following tasks:
1. Take a photo of yourself holding your GPSr with the bridge
in the background.(now optional)
2. In an email, tell me the four elements required to form a
natural bridge.
3. In an email, estimate the length of the span from one bank
to the opposite, estimate the height of the span at its highest
point and estimate the average width of the top surface (road) of
the span.
You can click on my name in my profile to send an email or you
can manually enter it in: wvangler1@yahoo.com Please do not post
your answers in your online log.
Bonus: According to the landowners, there are several "pot hole"
sized formations in the stream trickle upstream of the bridge. They
have been told that these formations are "Indian Wells" and were
hewn from the rock by native Americans. If you have any theory
about the truth of this or if it was a natural geological
phenomenon let me know either in email or in your log.
Sources: Arkle, Thomas, Jr. Sandstones of West Virginia.
Morgantown, West Virginia: West Virginia Geological Survey,
1957.
Hennen, Ray V., and Roane Wirt. Calhoun Counties. Morgantown,
West Virginia: West Virginia Geological Survey, 1911.
Hansen, Michael C.
1988 Natural Bridges in Ohio, Ohio Geology.