Natural Bridge
Canyon and Bridge Formation
In spite of Death Valley's arid climate, infrequent storms
have produced erosive flash flooding. Over the course of
thousands of years, these floods have carved canyons and
produced features such as Natural Bridge.
The process by which Natura Bridge Canyon
formed was slow and not very steady.
Although carving of the canyon has taken
many thousands of years, the actual work of
erosion and removal of rock, accomplished
during infrequent flash floods, has taken only
a fraction of that time. During most of its
history, Natural Bridge Canyon has been as
you normally see it, dry and quiet. It is hard
to imagine flood-borne torrents, each lasting
only a few hours or days, periodically
scouring a channel through the rock of this
ancient alluvial fan. Meanwhile up canyon,
the Natural Bridge was also being sculpted
by the erosive force of water. At the location
of the bridge, but before the bridge was
actually formed, the eroding stream channel
changed curse, bending slightly to the north.
Although the exact cause of this change is
not fully understood, several possibilities
exist. The stream may have encountered an
erosion-resistant portion of rock (the main
span of the bridge). Unable to erode through
this obstacle, the path of the stream was
diverted around it to the north. Another
possibility is that rivers often bend or
"meander" in response to changes in their
flow pattern. In this way the stream may have
changed its own path. Having changed
course, the stream occupied the new channel
for some time, only to abandon it at a later
date, in favor of a straighter path. (As you
walk up the canyon towards Natural bridge,
this abandoned channel can be seen on the
north or left side of the canyon.) Renewed
erosion along the straighter channel
eventually undercut more rock, eventually
deepening the channel, and forming Natural Bridge.
Geologic History
Death Valley's dramatic relief is the result of fracturing
and tilting of the Earth's surface, which transformed the
regions' once featureless terrain to the spectacular
landscape present today.
Long before the formation of Death Valley's
rugged landscape, low hills and plains
characerized the region's terrain. Explosive
volcanic eruptions, originating from the
northwest in Nevada, periodically covered the
Death Valley region with ash and pumice.
These layers eventually totalled more than
4,000 feet in thickness during their period of
accumulation 27 to 7 million years ago.
(These colorful deposits, known today as the
Artist Drive Formation, can be seen in the
steep slopes of the Black Mountains north of
this canyon.) Following deposition of the
Artist Drive volcanic material, folding and
fracturing of the earth's brittle crust produced
mountains and valleys. Rock fragments
eroding off mountains, collected at the
valley's edge as low angle slopes or "alluvial
fans". Natural Bridge was carved in one
such alluvial fan of the Furnace Creek
Formation, deposited during the late-Piocene
epoch (5-3 million years ago) This fan is
composed largely of erosional fragments of
Artist Drive Formation. Continued fracturing
or "faulting" of the earth's crust allowed
"blocks" of crust to slip, or "downdrop" past
one another. As blocks of crust were
downdropping to form Death Valley, about 3
million years ago, rocks within the blocks
were being moved from their original
locations of deposition. In this manner,
Natural Bridge alluvial fan was moved or
"displaced" down towards the slowly
depening valley floor, towards what was to
become Death Valley. This "valley forming"
process is active today. During the last
several thousand years, periodic
flashflooding eroded an ever-deepening
channel into the ancient alluvial fan. This
resulted in the formation of this canyon, and
its splendid centerpiece -- Natural Bridge.
