natural bridge - physical geology

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