The Natural Bridge of Lee County is
an arch which is actually used as a bridge. It is crossed by SR 662
and can be very easy to miss when driving over it. You must stop
and walk down a short path to the creek to see it. According to
Tony Scales, a geologist with the Virginia Department of Mines and
Minerals, "It is a classic remnant karst arch due to
dissolution of Ordovician limestone. Geologically, it is in the
core of the breached, dissolved, and eroded Powell Valley
Anticline. "
Ordovician period
The Ordovician period began about 490
million years ago and ended about 440 million years ago. Conditions
during the Ordovician period helped to shape the geology of Lee
County. In North America it was a time of shallow seas rich with
life. This life was mainly marine invertebrates. Corals and
bivalves (animals with two shells) were very common. When these
marine organisms died their shells were deposited on the sea floor.
This became important in the formation of limestone.
Limestone
Limestone is a very common type of
sedimentary rock. It is biochemical in origin. Calcite is its main
component and comes from the remains of many types of marine
organisms. Some of these organisms were clams, brachiopods,
bryozoans, crinoids, and corals. The remains of these organisms
accumulated at the bottom of the seas and became compressed into
limestone rock. Many times limestone may contain large amounts of
sand and silt.
Karst
The Natural Bridge is located a short
distance west of Jonesville, VA. This area is a classic karst
environment. Karst is a landscape made by the dissolution of
limestone. When rain falls through the atmosphere it picks up
carbon dioxide and forms mildly acidic carbonic acid. Over time
this acidic water dissolves the limestone along fractures and
fissures in the rock.
Summary
At the posted coordinates all this
information comes together. The limestone rock you see was formed
over 400 million years ago from the bodies of dead marine
organisms. Batie Creek found a fracture or fissure in this
limestone and dissolved the tunnel you see. If you look around you
see a classic example of a karst environment formed by the action
of mildly acidic rain on limestone.
Important Information About Your
Visit
Please follow
the path to the creek on the upstream side of the bridge. The path
goes from the road down to the right side of the creek.
Permission is not given to enter the private property on the
downstream side of the bridge.
To claim a find
for this EarthCache you must post a picture (optional) of yourself
with the bridge in the background.
You must also
email me the answers to the following:
1. Estimate the height of the
tunnel from the creek bed to the roof of the
tunnel.
2. Estimate the
width of the tunnel.
3. Estimate the
length of the tunnel.
Thanks to Tony Scales for
information on the geology of the area. Thanks also to Gary Russell
for permission to place this earthcache.
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