Well, it was off from Bluff and
Hovenweep heading north on U.S. 163 toward
the community of Blanding where we had camping reservations. The geology along the highway was pretty non-descript
as the road was constructed on White Mesa where resistant sandstone of the Dakota
Formation (Cretaceous) crops out and holds up the Mesa. The only exciting feature was to look west at
mighty Comb Ridge, the upturned package of rocks that define the eastern edge
of the Monument Upwarp (described in Post July 18, 2018).
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| Utah 95 cutting through Comb Ridge. the dip slope to the east has large visible exposures of the Navajo Sandstone. The back side of the Ridge is on the west where a cliff forms. Comb Wash with a small ephemeral stream trends north-south and is floored by the Organ Rock Formation. Utah 95 continues on to NBNM. |
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| Five Kiva Pueblo at Westwater Ruins near Blanding. |
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Nation's Natural Bridge near Blanding.
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Then it was off to visit another of my Bucket List locations—Natural
Bridges National Monument (NBNM). Highway
Utah 95 heads west just south of Blanding and picks up NBNM after about 30
miles. At that junction Utah 276 heads
southwest to Halls Crossing on the Colorado River while Utah 95 continues 85
miles northwest to Hanksville after crossing the Colorado River at Hite. At any rate, it is best to fill the
gas/diesel tank at Blanding!
The road
starts out on the massive sandstones of the Dakota Formation (Cretaceous)
holding up White Mesa; however, in some of the deeper gullies the softer shales
and sands of the underlying Morrison (Jurassic) may be spotted. But there seems
to be some action to the west, and that is our friend from down south, Comb
Ridge, the eastern boundary of the Monument Upwarp.
But before cresting Comb Ridge take a really good short side trip and examine Butler Wash Ruins, well marked with a sign, just as you are starting up the ridge.
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| Butler Wash Pueblo. |
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| Rock art panel at Butler Wash. |
Although Comb Ridge has been noted
in previous Postings (July 18, 2018) the Utah 95 road
cut through the Ridge offers the most impressive views of the entire
stratigraphic section. As Utah 95 leaves
the Blanding area traversing the Dakota Formation it slowly gains elevation and,
the rocks get older as one approaches Comb Ridge in about 14 miles. Rather
suddenly the Jurassic Navajo Sandstone appears and is easily identified by the sparse
vegetation and light orange color complete with massive crossbeds. The highway climbs
on the Navajo until it goes through the roadcut and reaches the summit of the
Ridge. Here the panorama opens with
views to the north and south displaying the geologic section while the view to
the west looks toward the crest of the Monument Upwarp.
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| The west (back) side of Comb Ridge with Comb Wash in the foreground. K = Kayenta Formation below the Navajo. M = Moenkopi Formation below the Chinle. |
Comb Ridge is the eastern boundary
of the Upwarp, and rock layers plunge steeply to the east; geologists refer to
this type of structure as a monocline. At one time the rocks exposed at Comb Ridge
covered the area occupied by the Upwarp.
However, Laramide compression from the west, caused by subduction of
oceanic plates under the continental plate, begin to push/squeeze rocks “up”
into this large anticline we know today as the Monument Upwarp. As the same time as uplift (and part of) a
large deep-seated fault (paralleling what is now Comb Ridge) pushed the rocks on
the west higher than those on the east.
In many/most cases the rocks layers would rupture/break along the
fault. However, this particular fault
never reached the surface (a blind fault) and the rock layers “simply” draped
over the fracture and sloped to the east.
Later in the Eocene the entire Colorado Plateau was uplifted as a single
cohesive unit and erosion started to remove overlying rock layers. By the late Tertiary (~6 Ma.) erosion was in
full swings with local streams carrying huge loads of sediments to the Master
Stream, the Colorado River. Sometime in
the recent past (30 ka?) stream action along Comb Wash eroded away hundreds of
feet of rock and created the steep cliffs along the west side of the current
Comb Ridge.
After going through the road cut
and winding down to Comb Wash it is time to pull over, get out of the vehicle
and simply take in the scenery of the cliffs at Comb Wash. It is an experience not to be forgotten. In just standing there I remembered something
Sylvia Plath (American author and poet) once said. Actually, I only remembered her saying
something about breathing in the fresh air and needed to look up the entire
quote: I felt my lungs inflate with the
onrush of scenery—air, mountains, trees,... I thought, this is what it is to be
happy.
So, then it was off down the road
to continue the journey to NBNM and Bears Ears National Monument. In driving along Utah 95 it was quite
difficult to determine if we were actually in the Monument or had traveled over
the boundary. The creation, but later dismantling
of Bears Ears, is one of the saddest episodes in the history of our revered national
parks and monuments. The Monument was
created in December 2016 and reduced in size by 85%, by Executive Order, a year
later. The original Monument protected one of the most significant cultural landscapes
in the United States with thousands of known archaeological sites. It also protected numerous areas that are
spiritual significant to Native Americans of many cultural tribes. Geologists and other visitors are attracted
to the area due to the ruggedness and spectacular beauty of the canyons, mesas,
cliffs and highlands. History will judge
this embarrassing episode rather poorly as we continue to degrade the environment
and landscape, and allow destruction of our cultural history.
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| The kiva at Mule Canyon roadside ruins. |
Along the way across Cedar Mesa another
roadside stop of interest was approaching—more Ancestral Puebloan ruins at a
locality termed Mule Canyon. There is a
small parking area off the highway where one can visit a kiva and foundation
ruins. However, for those visitors who
like to hike, this entire area of Cedar Mesa has hundreds of
sites to visit—just make certain that you have good maps (talk to the BLM) and
appropriate clothing and plenty of water (cell phones probably will not have
service to call for a rescue).
The Bears Ears seem always in view
along the road and rocks range from the ever present Late Permian Cedar Mesa
Sandstone overlain by the Organ Rock Formation followed by the Triassic
Moenkopi Formation and the Chinle Formation.
The two buttes are capped by another massive and resistant sandstone,
the Jurassic Wingate Sandstone. The paleontological
resources of the area have been briefly studied but appear to be richly endowed.
So, I just look at the geological features and dream.
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| The Bears Ears with the Wingate Sandstone (Jurassic) capping the buttes. Photo courtesy of Utah Geological Survey. |
Utah 275 splits off from Utah 95
and takes a short four-mile ride into NBNM where Quaternary forces have eroded Permian
bedrock and produced three of the largest natural bridges in the world. Many visitors tend to confuse natural bridges
with arches; however, natural bridges are produced by flowing water. At NBNM this flowing water was/is in Armstrong
and White Canyons and the underlying bedrock is the Lower Permian Cedar Mesa Sandstone
~(270 Ma). During this period of time the
western shoreline of North America trended approximately north-south and ran
through central Utah. The Cedar Mesa Sandstone
in the Monument was deposited as large windblown sand dunes that migrated
inland from the shoreline; therefore, visitors will observe large scale
crossbedding in the rocks (Huntoon and others, 2000).
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| The Cedar Mesa Saandsone is exposed throughout the Monument. The black arrow points to Owachoma while one of the "Ears" is in the background. | | |
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| Horsecollar Ruins in an alcove of the Cedar Mesa Sandstone. Photo courtesy of National Park Service. |
NMNM lies near the crest of the
Monument Upwarp where rocks of late Paleozoic and Mesozoic (Cenozoic?) age covered
the Cedar Mesa to a depth of at least 5000 feet (perhaps up to 10,000) but have
since been eroded. The Laramide Orogeny
(Rocky Mountain building event) pushed/squeezed up the Monument Upward in the
Late Cretaceous but the Cedar Mesa was not exposed until sometime in the last
six million years (Huntoon and others, 2000).
It is interesting to note that the
landscape at NBNM (including the bridges) is related to the Grand Canyon! The removal of rocks overlying the Monument
Upwarp “started” in the Eocene (~39-35 Ma) when the Colorado Plateau began to
rise as sort of a cohesive unit. As
erosion was proceeding accompanying streams were flowing and meandering over the area.
The base level stream of the area was the Colorado River and as streams reached
the Cedar Mesa Sandstone, they continued their meandering ways. However, the
major downcutting of the Colorado River, and the Grand Canyon, was in the last
six million years, and was rapid. So,
the meandering streams in NBNM also cut rapidly into the Cedar Mesa bedrock and
soon were entrenched---as readers have seen in Goosenecks of the San Juan (Huntoon
and others, 2000).
The current actual bridges, and
those preceding but now collapsed, probably did not form until perhaps 30-12ka (and
later) when more abundant water was available from runoff due to climate changes
associated with glaciers to the north.
Natural forces, such as frost wedging and root growth following rock joints,
created opportunities for cutoffs that created abandoned meanders and resulted
in the formation of bridges (Huntoon and others, 2000).
REFERENCES CITED
Huntoon, J.E., J.D. Stanesco, R.F.
Dubiel and J. Dougan, 2000, Geology of Natural Bridges National Monument, Utah
in Geology of Utah’s Parks and Monuments, Utah Geological Association,
Publication 28, Salt Lake City.
