West side of Comb Ridge (answer to recent Geo-challenge). The sloping east side covered in Navajo sandstone is more familiar – see photos below.
Around 70 million years ago, something dramatic happened along the west coast of North America that profoundly affected the western interior of the continent. Two of the Earth’s crustal plates collided, and one dove under the other. Maybe “collision” is an exaggeration; it was quite a slow process by our standards. Yet by the time it ended 30 million years later, western North America had been compressed enough to create spectacular mountain ranges.This particular episode of mountain building is known as the Laramide orogeny. The most obvious Laramide structures are the Rocky Mountains. They extend from Canada to northern New Mexico (3000 miles), and include many individual ranges. Some peaks stand 9000 feet above the adjacent plains.
The Rocky Mountains ... whatever the plates were doing out west, the results were huge!
The Front Range of the Rockies above Fort Collins, Colorado. Source.
There also are Laramide structures on the Colorado Plateau, west of the Rocky Mountains. In my opinion, they’re even more spectacular. Mountain ranges are impressive but they’re familiar. The Laramide landforms on the Colorado Plateau are surprising and dramatic, especially in the stark settings.
“outlines which at first seemed harsh and trivial have grace and meaning” (Monument Valley in distance).
“forms which seemed grotesque are full of dignity” (San Rafael Reef, courtesy Jack Share of Written in Stone).
“magnitudes which had added enormity to coarseness have become replete with strength and even majesty” (Comb Ridge; town of Bluff is lower center; source).
We’re taken by landscapes that are a little bit familiar, but mostly new and unexpected. We think they're enchanting, awe-inspiring. These are the kinds of landscapes that make the Colorado Plateau such a wonderland. But just a few days after I wrote these words, a great geologist – in fact an authority on the Colorado Plateau – disagreed with me:“The lover of nature, whose perceptions have been trained in the Alps, in Italy, Germany, or New England, in the Appalachians or Cordilleras, in Scotland or Colorado [i.e. the Rocky Mountains], would enter this strange region with a shock, and dwell there for a time with a sense of oppression, and perhaps with horror.” (Dutton 1882)Clarence Dutton was one of the late 19th-century pioneering geologists of the American West – men that I hold in great regard, and envy in some ways. Not only was the territory unexplored, geology was still a young science, with endless opportunities for discovery. This was good for reputation and career of course, but I think the best thing must have been rounding a corner and finding something brand new, dramatic, and inexplicable.
“whatsoever might be bold or striking would at first seem only grotesque” (entrenched meanders of the San Juan River).
I sometimes experience a little of the delight of discovery if I don’t read in advance about a place I intend to visit. But like many people, I'm too familiar with the Colorado Plateau to react as Dutton described. So I was intrigued that just 60 years ago Wallace Stegner questioned whether these landscapes would ever be widely-appreciated:“Being innovations, and newly discovered [in Dutton’s time], they were powerless until cultivation released them into the aesthetic consciousness. Whether they have yet been so liberated, and whether the forms and colors of the plateau country strike most of us even yet as anything more than bizarre, is an open question.” (Stegner 1954)Now Stegner’s open question has been answered. Every year millions of people visit the Colorado Plateau to enjoy what were once bizarre horrifying landscapes. Books, magazines, movies, television, and the internet have released them “into the aesthetic consciousness” of the masses.
West side of Comb Ridge from Hwy 95. Graceful? ... or harsh and bizarre?
I’m especially taken by the long dramatic rock ridges in southeast Utah. They were folded during the Laramide orogeny, and later uncovered and sculpted by erosion.
It's neat to think about how this amazing ridge was created – by colliding crustal plates and erosion, both extremely slow.
These are hogbacks … or so I used to think. One side is covered and protected by a sloping rock layer that’s resistant to erosion. On the other side, erosion has cut down to form a steep escarpment. However, Wikipedia tells me that a ridge where one side slopes more gently than the other is a cuesta, not a hogback. But since the Wikipedia hogback example looks very much like the subject of this post – Comb Ridge – we’ll stick with hogback.
Comb Ridge on right and Comb Wash center; line is a small normal fault (after Sears 1953)
West side (escarpment) of Comb Ridge hogback, showing teeth of the (cock’s) comb.
Comb Ridge, subject of last week’s geo-challenge, is a spectacular hogback on the east flank of the Monument upwarp. The rocks were tilted when the area was uplifted. There may be a deep sub-surface fault in the basement hard rocks (see diagrams below), but it isn’t expressed at the surface.Below: How uplift and erosion created the Comb Ridge hogback. The first diagram is a theoretical view of the fold in the absence of erosion. The second shows the hogback created by erosion (after Robinson, Utah Geological Survey 2012).
Comb Ridge is long – on the order of 80-110 miles (length varies among sources). Whether viewed from the ground or the air, it’s dramatic.
Most of Comb Ridge.
The sloping east side is covered by the photogenic Navajo sandstone. On the steep west side, erosion has cut down through hundreds of feet of rock underneath.
Lower Jurassic Navajo sandstone on east side of Comb Ridge near Hwy 95.
How do you like this pinyon - juniper woodland?!
Escarpment on west side of Comb Ridge; Hwy 95 passes through the deep narrow roadcut.
Jurassic and Triassic rocks of the escarpment.
It was impossible to capture the magnitude and spectacle of Comb Ridge in photographs. My mind assembled views into stunning panoramas, making my photos pathetic in comparison. They're valuable to me because they bring back impressions and feelings. But you the reader … you must go there yourself!How to get thereUS Highway 163 crosses Comb Ridge west of Bluff, and Utah Highway 95 crosses it southwest of Blanding, via the dramatic road cut shown above. Unfortunately there’s only one pullout near that road cut – immediately west, and only for east-bound traffic. For great views of Comb Ridge and many other features, continue west from the road cut roughly 11 miles to Salvation Knoll. A well-constructed trail leads to the summit.
Morning view of Comb Ridge (right) from pullout just west of road cut on Utah Highway 95 (for east bound traffic). The sunny slope is Cedar Mesa sandstone (Permian), which covers most of Cedar Mesa.
View down to Salvation Knoll trail head.
Between the two paved highways, gravel and dirt roads run through the washes on either side of Comb Ridge: Butler Wash on the east and Comb Wash on the west. I can give only limited reports on conditions: The roads at the north and south ends of Comb Wash were fine. I especially enjoyed the south end of Butler Wash, with many terrific views of the photogenic Navajo sandstone. It looks like it would be easy to explore Comb Ridge from the Butler Wash road.
Comb Ridge escarpment from Comb Wash road north of Hwy 163 (west of Bluff).
One of the many great views of Comb Ridge and the Navajo sandstone from the Butler Wash road.
Sources (in addition to links in post)Dutton, CE. 1882. The Tertiary history of the Grand Canyon District. USGS Monograph II, 26.Fillmore, Robert. 2010. Geological evolution of the Colorado Plateau of eastern Utah and western Colorado. Salt Lake City: University of Utah Press.Robinson, M. 2012. Geosights: Comb Ridge, San Juan County, Utah. Utah Geological Survey.Sears, JD. 1953. Geology of Comb Ridge and vicinity north of San Juan River. Geological Survey Bulletin 1021-E.Stegner, W. 1954. Beyond the hundredth meridian: John Wesley Powell and the second opening of the American West. Houghton Mifflin Co.
