Drama at the Canyon Mouth

Clarks Fork Canyon and southeast Beartooth front; photo courtesy Tim Schoessler.

In northwest Wyoming, roughly 30 miles northwest of Powell, the Clarks Fork of the Yellowstone River leaves the Beartooth Mountains with great drama—by way of a canyon mouth so geologically spectacular that it’s included in field trips for everyone from high school teachers to professionals.  Unfortunately, the rocks are a bit hard to explain.  But the scenery’s great, even on cloudy days.

Looking downstream at south side of canyon mouth and basin beyond.

North side of canyon mouth.

Steeply-tilted Paleozoic sedimentary rocks rise abruptly from the lowlands, and are in contact with Precambrian rocks to the west.  Between them is a 2-billion-year gap in the rock record, a Great Unconformity.

Arrow points to Great Unconformity between brownish Cambrian Flathead sandstone (500 million years old) and Archean metamorphic rocks (2.7 billion years) on north side of canyon.  See the magpie? (lower right)

Most mountain ranges in Wyoming were uplifted roughly 70-40 million years ago during the Laramide orogeny, the event that created the Rocky Mountains.  They have a distinctive style:  broad blocks of crust shoved up along reverse faults over younger rocks.

Laramide-style uplift; from Regional Geomorphology, University of Wyoming, 1984.

This diagram shows a typical Laramide range.  On the east, Precambrian (ancient) igneous and metamorphic rocks (brown) have been shoved up and over younger sedimentary rocks along high-angle reverse faults; Precambrian rocks make up the east flank and crest.  On the opposite side, the rocks were folded rather than faulted, and there are outcrops of tilted sedimentary strata (in this case eroded to form hogbacks and strike valleys).

At the canyon mouth, the Clarks Fork cuts through steeply-tilted sedimentary strata.  Here the Beartooth front is steep and abrupt, and decorated with contorted sculpted rock.

Can you imagine rocks being folded like this?

In less than a quarter mile, the tilted strata start to flatten out.

Immediately west, the strata are nearly horizontal—note limestone bed on skyline.  Further west sedimentary strata are gone, having been eroded off the crest of the range.

When I first saw the dramatic exposures of eroded sedimentary rocks on the Beartooth front, I presumed I was looking at a steeply-tilted, fault-free flank.  Wrong.  Here the Beartooth uplift was shoved east over younger rocks, via high-angle reverse faults (like the east side of the "typical" Laramide uplift above).  So what’s with the sedimentary strata?

Cross-section through Beartooth front in vicinity of Clarks Fork Canyon (unlabeled diagram from Lageson & Spearing 1991, no source given).

The story is complicated and controversial.  Popular geology guides wisely keep explanations short.  In his great guide to geology east of Yellowstone Park, Bob Carson quickly passes by deformed sedimentary strata, and focuses on Precambrian rocks, the Great Unconformity and glacial features.  In Roadside Geology of Wyoming, Lageson and Spearing include a cross-section showing the Beartooth reverse fault cutting through Precambrian and sedimentary rocks (basis for diagram above).  Sedimentary strata are draped over faulted Precambrian rock.  But discussion is brief:  “Beds of Paleozoic limestone were folded over the edge of the Beartooth uplift, then eroded”

Precambrian metamorphic rocks dominate the Clarks Fork Canyon and Beartooth Plateau; view southwest from inside canyon mouth, at end of paved road.

I defer to experts for further discussion (see Sources below).  Here’s some of what they’re thinking:

In the “spectacular fold at the mouth of the canyon” sedimentary strata are draped over a faulted basement (Precambrian) block.  This structure is slightly older than the Beartooth uplift and therefore was deformed by those faults (various authors; summarized in Heasler et al. 1996; PDF).

Laramide models used to incorporate vertical uplift.  But with seismic and drill-hole data, it has fallen out of favor, replaced by uplift via high-angle reverse faults, accommodating crustal shortening.  However in situations where faults are very steep (e.g. Beartooth fault at Clarks Fork Canyon), the vertical uplift model may be valid, and sedimentary strata may well be draped over faulted basement rock (Brown 1995). 

O’Connell (1996) argued for multiple stages of faulting, and invoked zones of preexisting weakness in basement rocks.  He argued against drape-folding. 

The southeast Beartooth front is in a complex transition zone involving the western Bighorn Basin, with its own faults and Laramide history.  Thus the front has been affected by multiple episodes of deformation (Neely 2006; PDF).

North side of canyon mouth (labeled based on Heasler et al. 1996).

Looking downstream at south side of canyon mouth; arrow points to rotated Bighorn dolomite (based Heasler et al. 1996, Wise 1983).

“We gather at the base of the crag to study the rocks.  The instructor assigns them to type and age, and proclaims the cause of their deformation—all the while waving his arms in the air while we frantically write everything down in our field books.  Later, we read that others think differently.”  (anonymous geology student ca. 1985)

Sources (in addition to links in post)

Brown, WG.  1995.  Structural style of Laramide basement-cored uplifts and associated folds in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds.  Geology of Wyoming.  Wyoming State Geological Survey Memoir 5:312-371.

Carson, RJ.  2010.  East of Yellowstone; geology of Clark’s Fork Valley and the nearby Beartooth and Absaroka Mountains.  Sandpoint, ID: Keokee Books.

Heasler, HP, Jaworowski, C, Jones, RW, De Bruin, RH, and Ver Ploeg, AJ.  1996.  A self-guided geologic tour of the Chief Joseph Scenic Highway and surrounding area, northwestern Wyoming.  Laramie, WY:  Wyoming State Geological Survey Public Information Circular No. 35.  PDF

Lageson, DR and Spearing, DR.  1991.  Roadside geology of Wyoming, rev. 2nd ed.  Missoula: Mountain Press Publishing.

Neely, TG.  2006.  Three-dimensional strain at foreland arch transitions: structural modeling of the southern Beartooth arch transition zone, northwest Wyoming.  MS thesis, Colorado State University, Fort Collins.  PDF

O’Connell, PJ.  1996.  Kinematics of the eastern flank of the Beartooth Mountains, Montana and Wyoming, in Wyoming Geological Society 47th Annual field Conference Guidebook.

Wise, DU.  1983.  Overprinting of Laramide structural gains in the Clarks Fork Canyon area and eastern Beartooth Mountains of Wyoming, in Wyoming Geological Society 34th Annual field Conference Guidebook.

bold or striking would at first seem only grotesque

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 there

US 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.

Walking the Devil’s Backbone

Driving home from Rocky Mountain National Park we stopped just a few miles west of Loveland, Colorado, to examine the Devil’s Backbone.  How could we not?!  It’s an eye-catching landform and there’s a trailhead at the south end, right off Highway 34.

Demonic geologic features are common in Colorado -- there are at least twenty-one (Colorado Geological Society 2009).  Curiously, all are composed of either igneous rock or Mesozoic sandstone (Mesozoic means 252-66 million years old).  Some early geologists assumed the Devil's Backbone was igneous, a dike.  It has that look -- long, linear and narrow.  But then someone investigated and found it’s Mesozoic sandstone, specifically the Dakota sandstone -- sediments deposited by rivers flowing into an interior seaway to the east roughly 100 million years ago.

Here the Dakota is a mix of fine and coarse river deposits.  Note nearly vertical orientation of beds.

Along the Front Range of the Rocky Mountains, the Dakota forms hogbacks -- ridges of steeply-tilted sedimentary strata.  In the Devil’s Backbone the sandstone is really steep, basically vertical.  It's part of the west flank of a small anticline (uplift) east of the main Rocky Mountain uplift.  The more gentle east flank of the anticline is visible from the trail.  In the next photo, it's marked by rimrock with trees on the horizon, above boulder-strewn slopes.  The Triassic red Chugwater Formation is exposed in the valley below.  The valley also contains the “crest” of the breached anticline (cut through by erosion.)

Looking from the Backbone toward the hogback on the east flank of the anticline. 

The Wild Loop trail along the Devil’s Backbone is about two miles roundtrip, and is an easy stroll.  A short spur leads to the “Keyhole” where you can stand among the Devil’s vertebrae!

A demonic vertebra.

The Keyhole.

Long's Peak (Rocky Mt NP) from inside the Keyhole; Dave's finger rests on summit (click on image to view).

For more information, download the trail brochure provided by Larimer County’s Parks and Open Spaces.  This is the southern end of a network of trails extending north along the Front Range to Fort Collins.  Consider a weekday visit.  It’s popular, and the parking lot often fills on weekends.

NOTE:  The Devil has multiple backbones -- at least eight in the USA.

The Most Mysterious Mountains in Wyoming

This is the final post in a series about a recent trip through the “Heartland of Laramide Tectonics” in south central Wyoming.  By definition, this a folded land.  It was deformed during the Laramide Orogeny, the period of mountain-building that created the Rocky Mountains.  But when I left Alcova Reservoir and drove south, the landscape quickly changed from folded to flat.

Alcova Reservoir is in a land of folds and faults.

The scenery along the county road to the south is very different.

Occasionally there were curious granite hills and mountains sticking up above the rolling land.  They didn’t look much like Laramide mountains, which are elongate ranges with upturned sedimentary rocks on the flanks.  These were bare rounded granite outcrops of various shapes and sizes, with no obvious geologic structure.

“massive pink granite ... crops out in bald stark knobs that stand above the light colored, flat-lying sediments” -- D. Blackstone, 1988

This kind of scenery continued for miles and miles until I arrived at the north flank of the Seminoe Mountains -- steep, dark and ominous.  It was hard to believe that the county road continued on, but it did!  Here the North Platte River had cut into the surface I’d been driving on, revealing flat, undeformed rock strata.  Aha! ... Tertiary fill!

 Informative exposure of nearly-flat pale Tertiary sediments, with North Platte River in foreground (bridge) and Seminoe Mountains behind.

“Tertiary fill” refers to sediments that were eroded off Laramide mountain ranges and deposited in adjacent basins.  When the basins were filled, deposition continued onto the flanks of the ranges themselves.  Eventually they were mostly buried, with just the highest peaks exposed (cross-section below).

[Note:  The Tertiary Period has been replaced with two subunits -- Paleogene and Neogene -- but we still use “Tertiary fill” as it’s a useful descriptor.]

Laramide uplifts were eroded and largely buried in Tertiary fill (yellow).  Block diagrams by B Mears, Jr, from originals by SH Knight (Regional Geomorphology class, University of Wyoming, 1984).

Then came the Great Exhumation (Snoke 1993) -- a time of widespread erosion.  Much of the Tertiary fill was removed, uncovering the old landscapes.  Why this happened isn't clear.  Perhaps regional uplift increased erosive power of streams, or maybe climate change brought more rain and more erosion.  Whatever the cause, exhumation was followed by yet another depositional phase (burial), and then another period of erosion (exhumation) to produce today’s great Laramide scenery.

For reasons still debated, the old landscapes were exhumed ... 

... and then buried again ... 

 ... and then re-exhumed to produce today's Wyoming landscapes.

Turns out I had just driven across some of the best preserved Tertiary fill in Wyoming.  But why wasn’t it removed during the Great Exhumations?  And is there something still buried down there?  Here are some clues as to what that might be:

-- Tertiary fill laps directly onto clusters of Precambrian granite outcrops.

-- Boulders of the same Precambrian granite have been found on summits of mountains a short distance south, at higher elevations.

-- Normal faults bound the area on the north and south (click on map to view).

Neogene normal faults in Wyoming (Flanagan & Montagne 1993), with Granite Mountains labeled in red.

Buried in the Tertiary fill are the remains of a Laramide uplift!  These are the Granite Mountains, the most mysterious mountains in Wyoming.  They were once a typical Laramide uplift, but then the core collapsed and the crest ended up below the flanks.  The down-dropped block is bounded on the north and south by normal faults, forming a graben.  Displacement is estimated to be 2000 feet.  Because the block subsided significantly, much of the Tertiary fill has been preserved, with only the highest peaks of the old range exposed.  So this is a remnant of Wyoming landscapes prior to the Great Exhumations (block diagrams 1. and 3. above).

A bit of the Really Old West:  high peaks of the Granite Mountains rise above Tertiary fill.  These are the Pedro Mountains -- the east end of the old range.

Cross-section through the Sweetwater graben (Mears et al. 1986); click on image to view.

Only the core of the Granite Mountains collapsed; the flanks remained in place.  These are now represented by the Rattlesnake Hills on the north side of the graben, and the Seminoe, Ferris, Green and Crooks mountains to the south.  On the summits of Green and Crooks mountains are granite boulders eroded off the high peaks before they were down-dropped.  In other words, the boulders now lie higher than their source.

South side of Seminoe Mountains in distance, with steeply-dipping sedimentary strata.  They were part of the south flank of the Granite Mountains before the core collapsed.

Dark foreboding metamorphic rocks on the north side of the Seminoes, exposed by normal faulting.

The normal faults of the Granite Mountains are fairly recent.  Flanagan and Montagne (1993) dated them to about 11 million years ago, at least 30 million years after Laramide uplift ceased.  The cause is debated.  Normal faults generally are associated with crustal extension, yet there’s no clear link between normal faulting in Wyoming and the major episodes of Tertiary extension in western North America -- Basin and Range province and Rio Grande Rift.  So “anomalous” is what it’s called for now (Snoke 1993).

The Sweetwater River flows though the graben from west to east, through the old high peaks of the Granite Mountains.  This was the route of the Oregon and Mormon Trails, traveled by many thousands of pioneers headed west to better lives.  About 50,000 passed in 1847 alone (Blackstone 1988).  The granite outcrops became landmarks -- Devils Gate, Split Rock, Independence Rock.

"Independence Rock" by WH Jackson.  Split Rock is in distance.  Source.

“Camp of the US Geological Survey" in the Sweetwater graben; by WH Jackson, 1870.  Source.

"The Emigrant's Grave" along the Sweetwater River; by WH Jackson, 1870.  Source.

Other posts about the “Heartland of Laramide Tectonics” include an overview of the Folded Land, a look at the Great Unconformity, a tour of Permo-Triassic redbeds, and a search for Jurassic pterosaur tracks.

Sources

Blackstone, DL, Jr.  1988.  Traveler’s guide to the geology of Wyoming, 2nd ed.  Geological Survey of Wyoming Bulletin 67.  Laramie, WY.

Flanagan, KM and Montagne, J.  1993.  Neogene stratigraphy and tectonics of Wyoming, in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds.  Geology of Wyoming.  Wyoming State Geological Survey Memoir 5:572-607.  Laramie, WY.

Mears, B, Jr, Eckerle, WP, Gilmer, DR, Gubbels, TL, Huckleberry, GA, Marriott, HJ, Schmidt, KJ and Yose, LA.  1986.  A geologic tour of Wyoming from Laramie to Lander, Jackson and Rock Springs.  Geological Survey of Wyoming Public Information Cirucular No. 27.  Laramie, WY.

Snoke, AW  1993.  Geologic history of Wyoming within the tectonic framework of the North American Cordillera, in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds.  Geology of Wyoming.  Wyoming State Geological Survey Memoir 5:2-56.  Laramie, WY.

Wyoming State Geological Survey.  Granite Mountains.  Accessed August 2014.  http://www.wsgs.uwyo.edu/research/stratigraphy/GraniteMts/Default.aspx