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Natural
Bridges National Monument
Before the Bridges
If you had visited this area 260
million years ago, you would be
standing on the dazzling white beach
of a sea which covered eastern Utah
during the Permian geologic period.
You may have noticed the sweeping
lines, known as crossbedding, that
pattern the white sandstone. Crossbedding
represents the down-current face
of a sand dune, down which sand
slips as the dune advances under
the force of wind or water. Geologists
debate whether the Cedar Mesa Sandstone
formed under water or along the
shore as windblown dunes. You can
see ripple marks forming today in
the mud left in the canyon bottoms
by receding flood waters.
Although
the waters of the warm Permian Sea
supported abundant life, fossils
are rare at Natural Bridges. If
you have ever stood on the ocean
shore, you may know why. A beach
is classified as a high energy environment,
where grains of sand continually
grind back and forth with each sweep
of the tide. Few organisms can survive
such rough treatment; thus, few
make it their home. Any plant or
animal remains swept ashore soon
wear away.
If
you examine the Cedar Mesa Sandstone
with a hand lens, you may see that
some of the sand grains are actually
fragments of fossils. One type of
fossil is abundant in the streambeds
of White and Armstrong canyons:
petrified wood. This wood washes
out of the Chinle Formation, found
high above the Cedar Mesa Formation.
When the trees died, they fell into
stagnant swamp water which prevented
their decay. Eventually, silica
derived from volcanic ash replaced
the wood, preserving its grain in
stone.
Bridges
and Arches
You may have noticed that arches
stand on the skyline whereas bridges
form in the bottoms of deep canyons.
Once water dissolves the cement
between the grains of sand in a
narrow fin of sandstone, frost wedging
and gravity begin to work. While
seeping moisture and frost shape
arches, running water carves natural
bridges. As the curving meanders
of streams carved down into the
sandstone, they undercut the canyon
walls and bent back upon themselves
until only a thin fin of stone separated
them. Flash floods periodically
pounded against weak spots formed
by the soft siltstone layers in
the sandstone. Eventually, the water
cut through the narrow neck of the
meander, forming a natural bridge.
At first each bridge is thick and
massive, as is Kachina Bridge, but
as erosion attacks them on all sides,
the bridges become more delicate
(as with Owachomo Bridge) and eventually
collapse.
How
Old Is Old?
We know that compared to the other
bridges in the monument, Owachomo
is the oldest bridge, but how old
is old? Geologically speaking, the
bridges themselves are relatively
recent and short-lived occurrences.
Since sandstone erodes at different
rates (more weathering occurs when
the climate is wet than during times
of aridity), the exact age of the
bridges is difficult to determine.
We do know that ten million years
ago the Colorado Plateau was flat
and featureless. When the last glacial
period ended 18,000 years ago, glacial
melt and increased rainfall speeded
the erosion of canyon country. A
wet climate between 900 and 4,000
years ago probably began the erosion
of most spans; the largest spans
are believed to be over 5,000 years
old.
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