NATURAL
BRIDGES NATIONAL MONUMENT,
UTAH
Natural Bridges National Monument is
located in San Juan County in southeastern Utah, USA. It has the highest concentration of natural
bridges on Earth. A natural bridge is a
rare weathering-erosional feature that consists of a rock arch over a river or
stream (sometimes the stream is dry).
The park has three intact natural bridges in close proximity and
erosional remnants of former natural bridges.
Sites for potential natural bridges in the future are also present.
White Canyon east of President Bridge, Natural Bridges
National Monument, San Juan County, southeastern Utah, USA (looking ~NNE).
White Canyon is the main southwest-flowing
drainage in the northern and northwestern parts of the park. The deep canyons here formed by the erosive
action of flowing rivers as the region was tectonically uplifted. The Natural Bridges National Monument area is
located in the Monument Upwarp region of the Colorado Plateau. The Colorado Plateau is a large physiographic
province that experienced significant uplift during the late Cenozoic. The same downward cutting by river erosion
produced canyon terrains throughout the Colorado Plateau, such as at Grand
Canyon, Zion Canyon, Canyonlands National Park, etc.
Before uplift, the rivers at Natural
Bridges National Monument were meandering channels on a relatively flat
landscape. Tectonic uplift and downward
erosion resulted in the rivers becoming trapped in their own canyons, and they
still retain their meandering river pattern.
These are called entrenched meandering rivers. This accounts for the curving, looping paths
of the canyons in the park.
Armstrong Canyon south of Senator Bridge (“Kachina
Bridge”), Natural Bridges National Monument, San Juan County, southeastern
Utah, USA (looking ~WSW).
Armstrong Canyon is the main
northwest-flowing drainage in the southwestern portions of the park. Its confluence with White Canyon is at
Senator Bridge. Notice the winding path
of Armstrong Canyon - this is the classic appearance of an entrenched
meandering river.
The light-colored rocks making up the
canyons of Natural Bridges National Monument (see above & below photos) are
Cedar Mesa Sandstone (Permian).
The partially vegetated, deep
reddish-colored slopes in the far distance (see above photo) consist of
interbedded mudshales, siltstones, sandstones, plus minor conglomerate and
limestone. Those distant slopes have, in
ascending order: Organ Rock Formation (Permian), Moenkopi Formation (Triassic),
and Chinle Formation (Triassic).
Armstrong Canyon just west of Congressman Bridge
(“Owachomo Bridge”), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA (looking ~W).
Cedar Mesa Sandstone along trail to Congressman Bridge
(“Owachomo Bridge”), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA.
Almost 100% of the surface bedrock in the
park is Cedar Mesa Sandstone, which is about 1200 feet thick in this
area. All of the rocks surrounding the
park’s three natural bridges are part of this stratigraphic unit. The Cedar Mesa Sandstone (Wolfcampian Series,
lower Lower Permian) consists of light-colored quartzose sandstones. Almost all of the sand grains are composed of
quartz (SiO2), with a small percentage composed of K-feldspar (KAlSi3O8)
or marine fossil fragments.
The Cedar Mesa Sandstone has abundant,
large-scale cross-bedding, formed by unidirectional wind currents
(eolian). The types of cross-bedding
here are mostly tangential cross-bedding and trough cross-bedding, plus some
tabular cross-bedding. Soft-sediment
deformation structures are occasionally present.
This cross-bedded quartzose sandstone
lithofacies represents deposition in an ancient, large sand dune field (such
extensive desert sand dune fields have been called ergs). Sand dune deposits consist of wind-blown sand
grains deposited in a variety of specific ways.
Some are deposited by saltation (wind-blown jumping) on the windward
side of the dune. Some are deposited as
grainfalls from the air on the leeward side of the dune. Some are deposited in small sediment slides
(sand avalanches) on the leeward side of the dune. Slumping of sand masses forms soft-sediment
deformation.
Some interbeds of horizontally-bedded
mudshales, siltstones, and fine-grained sandstones occur in Cedar Mesa
Sandstone. These represent interdune
deposits - many of them have fossil root structures.
Regional studies have shown that the Cedar
Mesa Sandstone dune field, during the Permian, was located near the Uncompahgre
Uplift, an ancient mountain range. Those
mountains were, at times, bordered by the ocean. The portions of the ocean proximal to the
Uncompahgre Uplift received siliciclastic sediments derived from erosion of the
mountains. At times of lower sea level,
nearshore, shallow marine sand deposits became exposed and were subject to wind
transport. Cross-bedding geometries in
the Cedar Mesa Sandstone indicate that winds were predominantly blowing from
the northwest (modern coordinates).
Cedar Mesa Sandstone along trail to Congressman Bridge,
Natural Bridges National Monument, San Juan County, southeastern Utah, USA.
The small and large hollows shown above
are called honeycomb weathering.
It forms by differential surface weathering and erosion.
Cedar Mesa Sandstone with honeycomb weathering at Senator
Bridge (“Kachina Bridge”) overlook, Natural Bridges National Monument, San Juan
County, southeastern Utah, USA.
Cedar Mesa Sandstone with trough cross-bedding at Senator
Bridge overlook, Natural Bridges National Monument, San Juan County,
southeastern Utah, USA.
Cedar Mesa Sandstone with tangential cross-bedding and trough
cross-bedding near Senator Bridge overlook, Natural Bridges National Monument,
San Juan County, southeastern Utah, USA.
The thin, reddish-colored interval about
two-thirds of the way up on the above photo consists of horizontally-bedded
siliciclastics, likely consisting of siltstone and sandstone. Such horizontally-bedded intervals have been
interpreted as interdunal deposits.
Notice that black staining becomes common
starting at the reddish, horizontally-bedded siltstone-sandstone interval and
downward. This thin reddish interval is
less porous and less permeable than the overlying sandstone. Downward-percolating water from rain events
moves through the sandstone, encounters the siltstone interval, and then moves
laterally (sideways) to emerge at outcrop surfaces at seeping springs. Seepage sites invariably get stained by
various minerals and algal/bacterial growth.
Cedar Mesa Sandstone with small erosional windows (above &
below) at Senator Bridge overlook, Natural Bridges National Monument, San Juan
County, southeastern Utah, USA.
Windows are not small versions of natural
bridges, because they don’t form by river or stream erosion. Windows are precursors to natural arches,
which are large rock arches that lack underlying streams or rivers.
President Bridge (“Sipapu Bridge”), Natural Bridges
National Monument, San Juan County, southeastern Utah, USA (photo from National
Park Service trailside signage).
The height and span measurements indicated
in the above photo are inaccurate.
President Bridge (“Sipapu Bridge”), White Canyon, Natural
Bridges National Monument, San Juan County, southeastern Utah, USA.
The three natural bridges in the park have
multiple names. Their “official” names
are Indian names, but were not named by Indian tribes. The natural bridge shown above and below has
the rude-sounding modern name of “Sipapu”, but it’s also been called President
Bridge and Augusta Bridge.
President Bridge is the largest example in
the park, and the ninth largest in the world.
It formed by meander cutoff - the narrow sandstone wall forming the neck
of a meander loop in White Canyon was undercut and punched through by hydraulic
action of White Canyon Creek and by abrasive erosion from transported river
sediments. Based on the estimated ages
of the other two natural bridges in the park, President Bridge is probably
between 10,000 and 100,000 years old (Late Pleistocene). The opening of President Bridge is
subcircular, spanning 225 feet and 144 feet high. At its thinnest, the top of President
Bridge’s span is about 50 feet thick.
Fluvial-induced erosion no longer enlarges
President Bridge. Non-fluvial weathering
and erosion processes will modify this structure in the future.
President Bridge (“Sipapu Bridge”), Natural Bridges
National Monument, San Juan County, southeastern Utah, USA.
President Bridge (“Sipapu Bridge”), Natural Bridges
National Monument, San Juan County, southeastern Utah, USA.
Map of Senator Bridge (“Kachina Bridge”), Natural Bridges
National Monument, San Juan County, southeastern Utah, USA (map from National
Park Service trailside signage).
Senator Bridge (“Kachina Bridge”), Natural Bridges
National Monument, San Juan County, southeastern Utah, USA (looking ~S) (photo
from National Park Service trailside signage).
The span measurement shown above (204’) is
slightly inaccurate.
Senator Bridge (“Kachina Bridge”), Natural Bridges
National Monument, San Juan County, southeastern Utah, USA (looking ~NW).
This natural bridge has been known under
three different names: Senator Bridge, Caroline Bridge, and the silly-sounding,
“official” name “Kachina”. It occurs at
the confluence of White Canyon and Armstrong Canyon, the two main drainage
systems in the park. It formed by
meander cutoff. Before the bridge
formed, the White-Armstrong junction was located a little upstream from here. Fluvial erosion caused meander cutoff and the
intersection of the two canyon streams shifted slightly downstream. Senator Bridge is considered the youngest of
the three bridges in the park, possibly less than 10,000 years old (= early
Holocene).
Senator Bridge has a 192 feet span and is
210 feet high. At its thinnest, the top
of Senator Bridge’s span is about 93 feet thick. In June 1992, a significant rockfall event
occurred at the western underside of the bridge. Non-fluvial weathering and erosion, plus fluvial
erosion by White Canyon Creek will continue to enlarge the bridge opening in
the future.
Senator Bridge (“Kachina Bridge”), Natural Bridges
National Monument, San Juan County, southeastern Utah, USA (looking ~NW).
Map of Congressman Bridge (“Owachomo Bridge”), Natural Bridges
National Monument, San Juan County, southeastern Utah, USA (map from National
Park Service trailside signage).
Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin
Natural Bridge), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA (looking ~N) (photo from National Park Service trailside
signage).
Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin
Natural Bridge), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA (looking ~SSE) (vintage photo from 1907).
Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin
Natural Bridge), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA (looking ~SSW).
Congressman Bridge is also known as Edwin
Bridge and the awkward, barely-pronounceable name “Owachomo Bridge”. This is the oldest, thinnest natural bridge
in the park and will collapse soon, in a geologic sense. Congressman Bridge has a 180 feet span and is
106 feet high. At its thinnest, the top
of this natural bridge’s span is 9 feet thick.
It formed the same way that Senator Bridge
did (see above), but subsequent erosion events have occurred. Congressman Bridge was formerly at the
confluence of two canyon creeks - Armstrong Canyon Creek and Tuwa Canyon
Creek. This happened by meander cutoff
and fluvial erosive undercutting of a thin sandstone wall. The present-day Armstrong-Tuwa intersection
is now upstream a bit from Congressman Bridge.
The former confluence and upstream Tuwa channel has been abandoned. Abandoned confluence natural bridges are very
rare. Because the confluence has been
abandoned, some researchers identify this feature as a natural arch, not a
natural bridge. Well, the stream channel
under Congressman Bridge does occasionally have flowing water, during storms
(see 5th and 6th photos below), so this feature is
still a natural bridge.
Congressman Bridge is estimated to be more
than 100,000 years old (early Late Pleistocene or Middle Pleistocene).
Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin
Natural Bridge), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA (looking ~SSE).
Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin
Natural Bridge), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA (looking ~SE).
Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin
Natural Bridge), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA (looking ~SSE).
Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin
Natural Bridge), Natural Bridges National Monument, San Juan County,
southeastern Utah, USA (looking up at the bridge’s underside).
Dry tributary under Congressman Bridge (“Owachomo Bridge”; a.k.a. Edwin
Natural Bridge) (above & below), Natural Bridges National Monument, San
Juan County, southeastern Utah, USA (Above: looking upstream to the
~NNE; Below: looking upstream to the ~NNW).
This normally-dry creek bed, mostly
floored by hard sandstone, was formerly part of Tuwa Canyon, which now
intersects Armstrong Canyon upstream from here.
Flowing water occurs here during storms
and runs underneath Congressman Bridge.
The dark-stained sandstone shown below shows water flow paths under the
natural bridge.
Goblet of Venus, Natural Bridges National Monument, San
Juan County, southeastern Utah, USA (vintage photo from the early 20th
century).
This remarkable feature was the result of
differential weathering and erosion of Cedar Mesa Sandstone. Evil scumbags from the nearby town of
Blanding purposefully tore this unique structure down in the mid-20th
century using a chain around the stem and pulling with a jeep.
Info. mostly synthesized from:
Baldridge, W.S. 2004. Geology of the American Southwest, a Journey
through Two Billion Years of Plate-Tectonic History. Cambridge.
Cambridge University Press. 280
pp.
Chronic, H. 1990. Roadside Geology of Utah. Missoula, Montana. Mountain Press Publishing Company. 325 pp.
Fillmore, R. 2000. The Geology of the Parks, Monuments, and
Wildlands of Southern Utah. Salt
Lake City. University of Utah
Press. 268 pp.
Hopkins, R.L. 2002. Hiking the Southwest’s Geology, Four Corners
Region. Seattle. Mountaineers Books. 287 pp.
Huntoon, J.E., J.D. Stanesco, R.F. Dubiel
& J. Dougan. 2010. Geology of Natural Bridges National Monument,
Utah. in Geology of Utah’s parks and monuments, third
edition. Utah Geological Association Publication 28: 236-253.
Kiver, E.P. & D.V. Harris. 1999. Geology of the U.S. Parklands, Fifth Edition. New York.
John Wiley & Sons. 902 pp.
Natural Arch and Bridge Society.