natural bridge - physical geology
DESCRIPTION
Presentation on the formation of the Natural Bridge in Virginia, by Amanda Schwartz.TRANSCRIPT
Natural Bridge, Virginia
By Amanda Schwartz
Drawing and Etching by Rob Hershelwood
LOCATION
is located in the
southern portion of
Rockbridge County,
Virginia, 14 miles
southwest of
Lexington and 39
miles northeast of
Roanoke. It is
situated in the
Shenandoah
Valley of Virginia .
Natural Bridge
The floor of
the
Shenando
ah Valley forms an
undulating,
but fairly level,
lowland
that contrasts
strongly with the
ridges that
bound it on two
sides.
Shenandoah National Park
A widespread
erosion
surface, known as the Valley- floor (or Harrisburg)
is well developed
in the
Natural
Bridge region,
where its
remnants stand
between 1,200
and 1,400 ‘ feet
above tide. Potomac River and level topped ridges near Magnolia, from Sideling Hill; the ridges preserve remnants of the Harrisburg peneplain. Town Hill in the distance. Morgan County, West Virginia. 1911. Plate 4 in U.S. Geological Survey. Folio 179. 1912.
peneplain
Some of the
on this peneplain
have developed
deep valleys, among
them,
which flows beneath
the bridge.
streams
Cedar Creek
Part of this
process of
erosion, the
bridge is a
massive span of
approximately
90 feet long
and 1oo feet in
average width.
limestone
A large fissure in the earth, served as a conduit for Cedar Creek to become a subterraneous stream, or “sinking creek” beneath the arch of the present bridge, then only the continuation of the transverse ridge of hills. The stream has gradually widened, and deepened this ravine to its present situation. Fragments of its sides also yielding to the expansion and contraction of heat and cold, have also tumbled down .
How It Formed…
Frederic Edwin Church. Oil on canvas. 1852
The rocks of the
bridge itself, which
are limestones and
dolomites of the
Chepultepec
formation of late
Cambrian age, are
substantially
horizontal. Their
stratification is
readily visible both
in the walls of the
gorge and in the
bridge, where there
are many vertical
joint planes. A broad,
open, synclinal
structure, called the
"Natural Bridge
syncline," is
present in this part
of the Valley of
Virginia.
Thomas Jefferson extolled the
Natural Bridge in his Notes on
the State of Virginia, 1782,
consequently drawing national
and international attention
from travelers, artists,
philosophers and writers
alike, and turning this rock
formation into an early
American icon.
Thomas Jefferson
From Notes on the State of Virginia, by Thomas Jefferson, 1782.
Gilmer, Francis William . “On the Geological Formation of the Natural Bridge of Virginia.” Transactions of the American Philosophical Society, New Series. 1 (American Philosophical Society, 1818), 187-192.
Woodward, Herbert P. “Natural Bridge and Natural Tunnel.” The Journal of Geology 44:5 (Chicago: University of Chicago Press, 1936), 604-616.
References