Flight Plan: Part I – Geology of the San Rafael Swell

South-facing view of the San Rafael Reef, the monocline of the San Rafael Swell on its eastern limb

TAKING TO THE SKIES

During my most recent trip to the Colorado Plateau in May, my friend, geologist and author Wayne Ranney (WayneRanney.com), suggested an alternative approach to exploring Utah’s backcountry. Our planned itinerary included driving and hiking through the Circle Cliffs Uplift and the San Rafael Swell in east-central Utah in order to study the geology. Both of these uplifted regions are classic Colorado Plateau landforms that display immense monoclinal folds on their eastern limbs. To better appreciate their complex structure and stratigraphy, we decided to take to the air. An aerial view of the landscape provides an unparalleled, big picture-perspective and some beautiful photos as well.

OUR FLIGHT PLAN

Our goal was to fly over the crests of the San Rafael Swell and the Circle Cliffs Uplift, paying special attention to their monoclines, the San Rafael Reef and the Waterpocket Fold, respectively. We mapped out a roughly 500-mile, counterclockwise ellipse beginning and ending in Price, Utah. The plan was to fly south following Castle Valley, eventually turning east over the western limb of the Swell and then follow the San Rafael River across the Swell to its eastern limb. Once at the Swell's monocline, called the San Rafael Reef, we would turn south and trace the strike of the fold to its end. 

After leaving the San Rafael Swell, we planned to investigate the Circle Cliffs Uplift (labeled Capitol Reef on the map below) in the same manner. Our point of return was the Henry Mountains, north of Lake Powell. Along the way we saw many distinctive structures on the landscape such as the Book Cliffs, the Wasatch Plateau, the San Rafael River, Wedge Overlook, Little Grand Canyon, the Black Box, Temple Mountain, Goblin Valley State Park, Cathedral Valley, Upheaval Canyon and the Green River.

The blue line traces the general path of our flight, starting and ending in Price, Utah (top of the diagram).

LIFT OFF

We chartered a Cessna Skyhawk II and an experienced pilot from Redtail Aviation at the Carbon County Airport just outside of Price. We hit the friendly skies bright and early in order to catch the best light on the rocks. As anticipated, the rising sun fully illuminated the topography and the spectacular scenery.

Banner hanging inside Redtail Aviation's hanger

Yours truly, Chief Pilot Larry Newby, and geologist-author Wayne Ranney

Fully fueled, we lifted off with ease from the Cretaceous rocks of Castle Valley’s high desert floor. In less than a minute we were at 1,000-1,500 feet, our cruising altitude for most of the flight. Perfect for observing the geology up close! The visibility was unlimited with deep blue skies. Perfect for those long-range shots!

Lift off

FOLLOWING CASTLE VALLEY TO THE SOUTH
We headed south above the Castle Valley floor in order to enter the San Rafael Swell from its western limb. Buffeted by camera-shaking winds rising off the nearby Manti Mountains to the west, we stayed over the  flatter terrain of the valley for smoother air. The west side of Castle Valley is flanked by the Manti Mountains of the Wasatch Plateau. Across our north, extending eastward, arching to the south, and then into Colorado, are the Book Cliffs. The town of Price lies on flattened Quaternary river-formed gravel deposits eroded into the soft Mancos shale.

Photo below: The Wasatch Plateau and Castle Valley

The line of snow-capped peaks to the west is the Manti Mountains (also referred to as Manti Top) of the Wasatch Plateau. The Wasatch is the northernmost of a series of high plateaus that act as a division between the Colorado Plateau to the east and the Great Basin to the west. Geologically, it is considered to be part of the “Basin and Range-Colorado Plateau Transition” physiographic province. The High Plateaus region extends from the Grand Staircase northward to the Book Cliffs and is divided into three north-south trending segments separated by structural troughs. The Wasatch Plateau is the most easterly of the high plateaus.

The Wasatch rises to 11,300 feet at its high point. On the east, it ascends to a dramatic shear escarpment 1,000 to 2,000 feet above Castle Valley. On the west, it falls by steep slopes into the trough of the Sanpete Valley. The region is an erosional remnant of the strata that once covered Canyonlands and the Grand Canyon to the south. Many of the plateaus are capped with 10-30 million year old lava, but the Wasatch is capped by Paleocene Flagstaff Limestone. The bulk of the plateau consists of Cretaceous Seaway-derived sedimentary deposits such as the Mancos Shale and the overlying Mesaverde Group.

The snowcapped Manti Mountains of the Wasatch Plateau frame the skyline to the west.
Castle Valley is in the foreground. Note the raised river terraces.

Photo below: The Wasatch Plateau, the Book Cliffs and Pinnacle Peak

Sharing the horizon with the Manti’s of the Wasatch Plateau (left in photo) are the Book Cliffs (beginning right in photo). Famous for their sequence stratigraphy, their name is derived from the fact that they appear as an open book seen on edge. The cliffs delineate the southern edge of the Uinta Basin. They are located to the north and east of Price, and run as an unbroken escarpment for over 200 miles from Utah into Colorado. The steep eastern, erosional-escarpment of the Wasatch Plateau is a continuation of the Book Cliffs. The region is known for its Native American-petroglyph and energy-rich reserves. Under BLM jurisdiction and management, the province is isolated, permanently-roadless, and qualifies as being called a wilderness (although not officially designated as such).

Looking northwest, the Wasatch Plateau is on the horizon-left (west), and the Book Cliffs are partially visible on the horizon-right. In the foreground is Pinnacle Peak (elevation 6,337 feet) in the Mancos Shale. Chief Pilot Larry informed us that an eagle's roost is located at the Pinnacle's spired-summit.

Photo below: The Book Cliffs
The cliffs are formed on near-shore and shallow marine, sandstones during the insurgence of the Western Interior Seaway. They consist of a series of Cretaceous Mesaverde sandstone units (Castlegate and Blackhawk), which rise vertically above the eroded gray slopes of the soft Mancos Shale. They are very much the geological equivalent of the Gray Cliffs in the Grand Staircase (http://written-in-stone-seen-through-my-lens.blogspot.com/2011/01/grand-staircase.html). 

A closer look at the Book Cliffs taken on our return flight to Price

Photos below: Oil, gas and coal reserves 

Although this particular coalbed along the Manti front was spent, surface mining for Cretaceous coal is part of the rich reserves on the Wasatch Plateau that also includes oil and gas. Coal in the subsurface underlies about 180,000 acres of the Wasatch Plateau. The best-defined frontier coalbed-gas play consists of the thick, gassy coals of the Blackhawk Formation in the nearby Book Cliffs coalfield. Coal beds in the Emery Sandstone Member of the Mancos Shale occur under the Wasatch Plateau.

Spent coalbed along the Manti front to the west

Oil rigs by the hundreds dot the landscape around Price in the Castle Valley

TRACING HUNTINGTON CREEK TO THE SAN RAFAEL RIVER

Still within Castle Valley, we followed Huntington Creek through Fuller Bottom as it meandered southward until it joined other headwater streams to form the San Rafael River. The river crosses the  Swell from west to east and eventually empties into the Green River (which later joins the Colorado River in Canyonlands).  

Photo below: Huntington Creek leads to the San Rafael River
Huntington Creek, part of the San Rafael River's watershed, flows through Fuller Bottom as it meanders across the Castle Valley floor on its way to the western limb of the San Rafael Swell.

Huntington Creek looking south in the Castle Valley

THE SAN RAFAEL SWELL

The San Rafael Swell, with roughly 1,800 square miles of public land, is one of the Colorado Plateau’s classic Laramide-age uplifts. It is a broad, elongate, structural dome trending mostly northeast. Originally horizontal, the rock layers comprising this structure were compressed during the Laramide Orogeny into a convex, upward-arching anticline measuring about 75 miles long and 30 miles wide.

Top: The kidney-shaped San Rafael Swell can be seen to topographically "rise" from the plateau. The oldest formations are visible at the crest of the Swell, while progressively younger formations are visible on its down-turned periphery. Line A-A' represents a west-to-east cross-section (about 50 miles). Bottom: Note the fold of the monocline over a deep basement fault. The A-A' cross-section is the approximate path of our flight transecting the swell. The vertical scale is exaggerated 80 times.

(Modified from Doelling and Hylland, 2002)



A VERY ABBREVIATED GEOLOGICAL HISTORY OF THE UTAHAN REGION OF THE SWELL

The geological history of the San Rafael Swell essentially encompasses that of central and southeastern Utah, as well as much of the Colorado Plateau. Regional stratal differences in the vicinity of the Swell certainly exist, many of which will be identified during the flight.  

PRECAMBRIAN FOUNDATIONS, ANCIENT PALEOZOIC SEAS AND ANCESTRAL MOUNTAIN RANGES
Precambrian continental accretion assembled the basement rocks on which subsequent early and middle Paleozoic marine sedimentary units were deposited in western North America, on the Colorado Plateau and the region of the San Rafael Swell. In the late Paleozoic during Pennsylvanian time, the Ancestral Rocky Mountains were responsible for both marine (Hermosa) and alluvial to shallow-marine (Cutler) sediments in a large area, including eastern Utah. These structures are deeply buried beneath Mesozoic deposits (http://written-in-stone-seen-through-my-lens.blogspot.com/2011/02/ancestral-rocky-mountains-and-their.html).

TRIASSIC FLOODPLAINS AND BRAIDED RIVERS

During the Early Mesozoic, Triassic-age floodplain-fluvial-shallow marine and floodplain-lacustrine-fluvial sediments (Moenkopi and Chinle Formations, respectively) blanketed the region with siltstones, sandstones, shales and mudstones.

JURASSIC SAND SEAS, A MARINE INCURSION AND CONTINENTAL DEPOSITS

Early Jurassic-age windblown sandstones (Glen Canyon Group’s Wingate and Navajo) became the prevailing sedimentation with interposed and interfingering fluvial and eolian deposits (Kayenta Formation). During the Middle Jurassic, a narrow, marine incursion called the Sundance Sea (or the Curtis-Summerville Sea) advanced into Utah from Wyoming and left alternating deposits of sandstones, siltstones and mudstones (San Rafael Group) in the accommodation space of the Utah-Idaho trough (whose subsidence was driven by mountain-building to the west). The San Rafael Group has no designated type locality, being named for the superb exposures existing within the Swell. As the fingerlike, marine incursion waxed and waned, the deposits of the San Rafael Group's deposits likewise interfingered and varied (eolian Page Sandstone, Carmel Formation mudstones and sandstones, eolian Entrada Sandstone, and Curtis and Summerville Formations' siltstones and sandstones). In the Late Jurassic, generated by mountains of the Nevadan Orogen to the west, the multi-colored Morrison Formation spread its fluvial and floodplain deposits to the east over a widespread area. Continental deposition remained dominant for 100 million years thereafter.

A TECTONIC COLLISION, CRETACEOUS MOUNTAINS AND A VAST INLAND SEA

Beginning in the latest Jurassic and continuing throughout the Cretaceous into the Eocene, the oceanic Farallon Plate subducted beneath the continental North American Plate at its western margin. That tectonic collision  initiated two continuous and overlapping, but one-prolonged mountain-building event (Sevier and Laramide Orogenies). Although both orogens possessed east-directed compression, they are distinguished by their differing modes of deformation. The driving force of Laramide deformation stretched eastward 1,500 km. The most severe zone of deformation formed the backbone of the modern Rocky Mountains and further imprinted the Sevier downwarping (foreland basin) of the craton. Thus, the expansive Western Interior Seaway formed which received sediments from the mountainous front to the west. The Seaway extended throughout the  contemporary Great Plains and many western states including eastern Utah, and from the Arctic to the Gulf of Mexico. Coevally, mid-ocean ridge activity in the Pacific, with its hot, buoyant crust, elevated the ocean basin's floor and displaced huge volumes of seawater. Subsequently, global sealevel rose which spilled onto lowlying  worldwide landmasses. That served to further expand the confines of the epicontinental seaway. Numerous transgressions and regressions of the inland sea left marine shales and sandstones such as the Mancos Shale, Dakota Sandstone and the Mesaverde Group.

As we shall see, east-directed Laramide compression was accommodated by folding that uplifted the plateau as a whole, and formed numerous broad anticlines and synclines. The San Rafael Swell is a classic example of the unique expression of Laramide deformation on the Colorado Plateau.  



Stratigraphic section for central and southeastern Utah (not shown to scale).

(From http://www.abdn.ac.uk/~wpg008/Draut2005.pdf



CRUSTAL CONTORTIONS OF THE COLORADO PLATEAU
The collision of the two aforementioned tectonic plates during the Mesozoic, far to the west of the Colorado Plateau and well before its uplift, eventually identified it as a physiographic region. The Laramide phase of that collision not only uplifted the plateau but created the landforms that came to be the region's signature expression: its uplifts, basins, folds and monoclines.

LARAMIDE OROGENY
The Laramide Orogeny was one of monumental uplift that thrust the Rocky Mountains skyward and elevated the Colorado Plateau, but in so doing, only moderately affecting it. Near-horizontal movement along deep-seated, Precambrian faults in basement rocks served to elevate the plateau while essentially preserving the integrity and relationships of its Paleozoic and Mesozoic sedimentary structures.



A schematic illustration of the subducting oceanic Farallon Plate beneath the continental North American Plate. That induced one continuous, mountain-building event with differing modes of deformation: the steep-angled, “thin-skinned” Sevier Orogeny in the latest Jurassic or Early Cretaceous (top) and the shallow-angled, basement-cored, “thick-skinned” Laramide Orogeny in the latest Cretaceous (bottom).

(Original source of diagram unknown)

Subduction of the Farallon Plate at a shallow angle uplifted the Colorado Plateau and produced the characteristic monoclinal folds that we see in association with the many uplifts, upwarps and swells.

(Original source of diagram unknown) 

TRADEMARK UPLIFTS AND MONOCLINES
Laramide forces of compression deformed and warped the plateau into long, broad, domal uplifts which are essentially large-scale anticlines with a north-south trend. These uplifts (also called upwarps and swells) have a western and an eastern limb that bends downward from the crest of the anticline. West to east dimensions of the uplifts are generally not symmetrical with the eastern flanks, marked by a steep monocline (also called a fold or reef), the trademark structures of the Colorado Plateau province. Monoclines fold downward like a carpet draped over a stair step, where sedimentary layers cover a deep, underlying fault (or fault system).

Diagram of a monoclinal fold showing a basement fault at depth and overlying sedimentary layers draped over the fault. Both the uplift and its monocline possess distinctive patterns of erosion of the surficial landscape. Monoclines characteristically erode into strike valleys, ridges, flatirons and hogbacks.

(Modified from Web: http://hays.outcrop.org/schott)

Deformational structures such as the uplifts on the Colorado Plateau (there are also basins that go "hand in hand" with the uplifts) were not solely formed in response to Laramide forces but were merely modified and finalized by them. Uplift of the San Rafael Swell actually began in Paleozoic time, as rocks of the Permian age rest directly on strata of Carboniferous age. Deep pre-existing faults and folds have been present since the Precambrian, over a billion years before Laramide initiation of compression.

Typical of uplifts and most other large monoclines across the Plateau, the Swell is cored at depth by a regional-scale reverse fault. This thrusted the western block of basement rock upward and eastward with a down-dropped section to the east. Undoubtedly, Laramide faults formed initially during Late Precambrian rifting that occurred along the western margin of proto-North America (Rodinia), thus forming in an extensional tectonic regime. Laramide reactivation, along long-lived Precambrian zones of weakness, was driven by a regime of intense compression.

UPLIFTS AND MONOCLINES ON THE PLATEAU
Examples of uplifts and their monoclines abound on the Colorado Plateau such as the San Rafael Swell (and its San Rafael Reef), the Circle Cliffs Uplift (and its Waterpocket Fold), the Kaibab Uplift (and the East Kaibab Monocline) and Monument Upwarp (and the Comb Ridge monocline).



Colorado Plateau uplifts: Circle Cliffs (CC), Defiance (D), Echo Cliffs (EC), Kaibab (K), Lucero (L), Monument (M), San Juan (SJ), San Rafael (SR), Uncompahgre (U), White River (WR), and Zuni (Z).

DISTINCTIVE PATTERNS OF EROSION
The patterns of erosion of uplifts and their monoclines are dictated by the underlying stratigraphy, structural geology and elevation. Their anticlinal structure, as well as the relative resistance to weathering and erosion of the rocks, controls the topographic form of the Swell; hard layers form the steep cliffs and tops of benches, mesas and buttes, as well as flatirons or hogbacks (tilted surfaces) around the perimeter of the Swell, while  intervening softer formations weather into slopes in the interior of the Swell and strike valleys at its perimeter.

The most elevated parts of the uplift are the most severely attacked by erosion especially at its crest. Thus, older rocks are exposed the most along the axis or center of the uplift leaving the uppermost rock layers discontinuous (see diagram below). By extending a line of trajectory from one limb to the other, one can obtain a sense of the massive amount of rock material that has been lost. Erosion also attacks the strata at the uplift's monocline creating its distinctive hogbacks, flatirons and strike valleys. On our flight, we saw Middle Permian rocks in some of its deeply dissected canyons. Our goal was to identify and photograph these erosive structures across the Swell.

This west-to-east cross-section through the Swell is somewhat parallel to Interstate-70, which roughly divides the uplift in two. The Swell's landform is an asymmetrical anticline with its western flank dipping less severely than its eastern flank, which is draped over a basement fault system at depth. The oldest layers are exposed along the crest of the dome. Our flight transected the Swell from Castle Valley on the west (at left diagram) to the San Rafael Reef on the east (at right diagram)

(From http://www.castlecountry.com/i/whattosee/GeologySanRafael.pdf

EXPOSED STRATIGRAPHY ON THE WESTERN LIMB OF THE SWELL
As the river enters the uplifted strata of the Swell from the west, it dissects into progressively deeper (and older) layers, flowing through the Mancos Shale, the Dakota Sandstone, the Morrison Formation, the San Rafael Group, the Glen Canyon Group, the Chinle and Moenkopi Formations, and even the Kaibab Limestone within the deeply cut canyons at the crest of the Swell. As the river emerges from the Swell at its eastern limb, the reverse erosional architecture is encountered.

This diagram depicts the Swell's western limb as seen from the north. The San Rafael River flows from Castle Valley (upper right) eastward through the Swell (to the left), dissecting into deeper and older strata as it flows east toward the axis of the Swell.
(Modified from Michael Kelsey, Hiking Utah's San Rafael Swell, 1986, Kelsey Publishing)

CROSSING THE SWELL

Our flight roughly followed the San Rafael River from west to east, beginning at the Swell's western limb. That initiated a procession downsection into older strata. Erosion has stripped away the Mesozoic rocks that once arched across the anticline leaving Paleozoic rocks exposed at its core. Typically, the crests of folds are more highly fractured than the flanks and the energy of running water is greater with elevation, contributing to the erosion of the Swell at its crest. Like so many other uplifts on the plateau, its western limb is gently inclined, while the Swell’s eastern limb, the San Rafael Reef, is steeply-inclined. Triassic and Jurassic-age rocks of the Glen Canyon and San Rafael Groups are exposed to the forces of erosion where the monocline drapes over a deep basement fault.

Photo below: The Cleveland-Lloyd Quarry

As the river initially enters the uplifted strata of the Swell, it begins to dissect into deeper layers flowing from the Mancos Shale to the Morrison Formation. Within the confines of the photo is the world famous Cleveland-Lloyd Quarry, a designated National Natural Landmark located in the Morrison Formation’s Brushy Basin Member. The quarry is located in the northern portion of the San Rafael Swell and is one of the world’s foremost dinosaur fossil sources. More than 12,000 individual dinosaur bones, many juvenile, and one dinosaur egg have been excavated from the 147 million year old mudstone of this active quarry since 1929.


The dinosaur-rich Brushy Basin Member of the Morrision Formation. 
This is the region of the Cleveland-Lloyd Quarry.

It is thought that many of the disarticulated remains might represent complete skeletons that have been scattered about the site. Analysis has confirmed an enigmatic preponderance (66%) of predatory dinosaurs (such as Allosaurus fragilis). This is an unnatural, or at least, inexplicable distribution in any known dinosaurian ecosystem. In addition, determining the ancient environment of the quarry’s deposits has remained somewhat of a mystery, possibly being riverine, lacustrine or even a boggy locale. A recent scenario evokes a watering-hole in a drought-stricken land, which might explain the presence of opportunistic predators, and the trampling and crushing of underlying osseous remains. There are many explanations that exist, but none explains all the facts convincingly.

Photo below: The Morrison Formation

The Morrison Formation’s mudstone, sandstone, siltstone, and even limestone appear in a rainbow of colors: vivid red, brown, green, gray and purple. The Morrison has three members (in ascending upsection): the red to brown fluvial Tidwell; white and grayish sandy Salt Wash (dinosaur and uranium-rich) and the color-banded bright-green, lavender and reddish siltstones and shales of the Brushy Basin Member (dinosaur-rich). The Morrison is frequently exposed in strike valleys and low ridges.


The multi-colored Morrison Formation

Photo below: The Little Grand Canyon

The San Rafael River and its tributaries flow across the structural axis of the Swell from west to east and have carved the deep gorge of the Little Grand Canyon. The canyon is a textbook example of a superimposed stream cutting through an anticline. The walls of the gorge expose maroon-stepped ledges of the Moss Back Member of the Late Triassic Chinle Formation at the base. The varnish-streaked, eolian Jurassic Wingate Sandstone overlies the Chinle, and the fluvial Kayenta above that. The canyon’s walls are capped by massive cliffs of Navajo Sandstone.

The Little Grand Canyon is dissected into the Swell by the San Rafael River

Photo below: The Black Box

We’re nearing the crest of the Swell where erosion is greatest. At the Black Box (Upper and Lower), the San Rafael River has dissected into the limestone of the Middle Permian Kaibab Formation, the formation that holds up the rim of the Grand Canyon! Here, the Kaibab is close to its northern and easternmost extent before pinching out. Below the rim of Kaibab is the White Rim Sandstone, the eolian terrestrial equivalent of the marine Toroweap Formation also seen in the Grand Canyon. The Box gets its name from the black desert varnish that has stained its walls. Brachiopod dating of the Kaibab in the region of the Swell appears to be slightly younger than the true Kaibab to the west. Subsequently, the "Kaibab" of the Swell has been named the Black Box Dolomite. This region of the Swell is a popular tubing and hiking area but carries a great risk of flash flooding due to the enormity of the river’s watershed and the narrow, steep-walled gorge.

The Black Box cut into the black varnished Kaibab Formation by the San Rafael River



This spectacular mesa and amphitheater is in Sinbad Country near the crest of the Swell. Wingate Sandstone cliffs hold up the underlying brown slopes of the Late Triassic Chinle Formation. The escalloped ledges on the floor are formed by the Moenkopi's Sinbad Limestone Member with the Black Dragon Member below. Once again, older layers are erosionally exposed at the crest of the anticline.

Here's the Moenkopi up close. Hauntingly beautiful!

An isolated, nipple-like butte capped with Sinbad Limestone holds up Moenkopi slopes just south of the Black Box. The surrounding bedrock is Kaibab Limestone.

THE SAN RAFAEL REEF

Having crossed the axis of the San Rafael Swell, we emerged at its steeply-dipping, eastern flank, the San Rafael Reef. It is here where the Swell drapes over a deep Precambrian fault causing the rock layers to turn skyward. A tremendous volume of rock has eroded both from body of the Swell and at the monocline. The hogbacks and strike valleys that are exposed at the monocline formed in response to their differential capacity to resistant or submit to the forces of erosion.

EXPOSED STRATIGRAPHY ON THE EASTERN LIMB OF THE SWELL AND AT ITS MONOCLINE


This diagram illustrates the east limb of the San Rafael Swell as might be seen looking south. Its plunging, monoclinal Reef drapes over a deep Precambrian fault. On the surface, hogbacks of resistant sandstone, and strike valleys of erodible mudstones and shales form.

(Modified from Michael Kelsey, Hiking Utah’s San Rafael Swell, 1986, Kelsey Publishing)

As mentioned, the distinctive pattern that forms the Reef’s spectacular scenery is related to its upturned hard and soft rock layers that have eroded into strike valleys and hogbacks. The Navajo Sandstone is tilted upward at a steep angle and is eroded into massive flatirons with the mudstone of reddish-brown Kayenta Formation underlying it. The Carmel Formation has likewise been eroded into smaller, escalloped shapes against the Navajo flatirons.

My first impressions upon seeing the monocline from the air were how abruptly it seemed to terminate on the landscape and that “plunge” was the perfect word to describe it. I found this dramatic tectonic expression of the monocline to be visually stunning but mentally challenging to decipher its intricate stratigraphy.

Wayne

reminded me, “Don’t look at a particular formation to figure out what it is.

Instead, look at the relationship of one formation to another.” 

Photo below: The San Rafael Reef

The hogback ridge that defines the Swell's eastern flank is formed by the Navajo and Wingate Sandstones. Younger Mesozoic strata have been stripped from the crest of the Swell but can be seen as prominent hogbacks on its eastern (left) flank. The nearly horizontal beds on the crest of the fold begin to dip gradually at its eastern flank and then suddenly plunge steeply downward.

 We have completed our traverse of the Swell from the west and are about to turn south to follow its monocline. The perspective of the San Rafael Reef plunging into the subsurface is very dramatic. Flatirons of the Navajo Sandstone appear to rise from the crust, which they literally do. To the left (east) is a broad strike valley containing smaller resistant ridges of the various formations of the San Rafael Group's mudstones and sandstones. The Henry Mountains are perched on the horizon. 

Photo below: Hogbacks, flatirons and strike valleys of the Reef

In this dramatic view we can see the Swell diving into the subsurface at the monocline. The Navajo Sandstone, the uppermost formation of the Glen Canyon triad, arches upward almost suspended in space and dominates the skyline. These flatirons prevented pioneer wagons from heading west in their “prairie schooners” which they nautically called “reefs” and do not connote carbonate composition. The small, escalloped layers of purple, red and tan sandstones and shales are of the Carmel Formation. Just outside of the Carmel is the Entrada Sandstone, less resistant to erosion here than in Arches National Park. Further to the east (left) are the Morrison (red, gray and green sandstones and shales), and the Cedar Mountain Formation (yellow and brown sandstones), as we ascend the stratigraphic column. Beyond view is a sandstone ridge of the Dakota Sandstone, and then the flat terrain of the Mancos Shale.



Photo below: The plunging monocline of the Swell

This west-facing view of the Swell was taken during our northward return to Price. We’re looking directly at the plunging fold of the monocline on the Swell’s east limb and the almost vertical “reefs” of Navajo Sandstone. This photo provides a dramatic sense of perspective and conveys the immense proportions of the Swell's eastern limb. To the east (left) beyond the monocline is the San Rafael Desert formed in the strike valley of the Carmel Formation. Further to the south, the desert is surfaced largely with Quaternary eolian deposits, and to the north it’s largely Mancos Shale.


Photo below: The upturned Glen Canyon Group
Through an eroded window within the Navajo flatirons, we caught a glimpse of the underlying Kayenta Formation, and beyond it, the Wingate Sandstone, all members of the Glen Canyon Group.

On the return leg of our flight while heading north over the San Rafael Desert, we caught this all- encompassing view of the monocline to the west with the Swell doming upward behind it. From this angle,
we get a perspective on the volume of rock that has been removed through erosion.

Continuing our flight southward along the strike of the monocline, we briefly crossed back into the Swell to the west to view Temple Mountain up close.

Photo below: Temple Mountain

Temple Mountain is the highest point on the San Rafael Reef. It was once the site of intense uranium and vanadium mining during the boom-days of the Cold War during the 1950’s. The mines are long closed leaving a network of roads and abandoned rigs as testimony to the once productive region. The principal ore hosts are the Moss Back Member of the Chinle Formation and the Salt Wash Member of the Morrison Formation. Much of the ore is found in association with “collapse” features (such as located to the left of the summit in the photo), thought to have arisen through dissolution in the underlying Pennsylvanian Paradox Formation (assisted by carbonate dissolution in the Kaibab and Moenkopi). From the air, we could see a few stone buildings, spoil heaps, rusty iron equipment and a large winch tower.

The Temple's summit consists of cliffs of Wingate Sandstone, followed by multi-colored slopes of the Chinle Formation, a small Moss Back Member cliff, and brown slopes of Moenkopi at its base.

We then flew back outside the monocline again just to the east and continued southward to Goblin Valley (south of Wild Horse Butte).

Photo below: Goblin Valley State Park
The bizarrely-shaped hoodoos are the big attraction at Goblin Valley which is located about a mile or so to the southeast of the San Rafael Swell. Four formations are exposed regionally here (from oldest to youngest): the Entrada Sandstone, the Curtis Formation, the Summerville Formation and the Morrison Formation.

The cliff face at the eastern edge of the park is largely composed of the reddish-brown Entrada Sandstone. In Arches, the Entrada was deposited as dunes, but here, the formation has changed laterally into an interbedded mix of shale, siltstone and sandstone (deposited on Sundance tidal flats). The greenish-gray (an iron-containing glauconitic clay) Curtis Formation, also a member of the San Rafael Group, caps the butte. Grotesque, little goblins or hoodoos can be seen speckling the landscape beyond the butte, which is giving birth to more creatures as it erodes away.

Photo below: Muddy Creek

After flowing southeasterly through the southernmost section of the San Rafael Swell, Muddy Creek has crossed the monocline and is meandering through a colorful section of the Morrison Formation. It will eventually join the Dirty River which flows into the Colorado above Lake Powell. All these rivers and streams are part of the Colorado River drainage system.

Photo below: Factory Butte

We then flew to the southwest still following the monocline which had become dramatically subdued as  Factory Butte came into view. The butte's base is eroding into an intricate network of stream channels in the soft Blue Gate Shale Member of the Mancos Shale forming a badlands topography with the underlying Ferron Sandstone Member. It is capped with the resistant Muley Canyon Member (formerly Emery Sandstone Member) of the Mancos.

 

Photo below: Factory Butte, North Caineville Mesa, Caineville Reef and the Henry Mountains

The view is to the south at the southern end of the San Rafael Reef overlooking Caineville Reef, the southern extension of the San Rafael Reef. The isolated mesa to the left is again Factory Butte

. The large mesa to the right is North Caineville Mesa. Sandwiched in between in the distance are the snow-capped, laccolithic Henry Mountains.

At this location, the hogbacks, linear ridges and flatirons of the monocline have become rather subdued in their relief. To the south the fold merges with Waterpocket Fold, our next destination, which is a northwest-trending monocline that defines Capitol Reef National Park. The eastern, downwarped sides of these two massive folds have been instrumental in preserving vast tracts of easily eroded Cretaceous rocks in the Book Cliffs to the north and the Henry basin to the south.  

Typical of uplifts and their associated monoclines, a tremendous amount of rock has been eroded away in the formation of this immense landform. The sequence of inclined strata is reflective of the monocline’s plunging fold into the Earth’s crust. The variegated blue-gray shales of the Mancos Formation are in the badlands at the base of Factory Butte. Toward us lie progressively deeper (and older) strata, a ridge of Dakota Sandstone and Cedar Mountain Formation, a strike valley of Morrison Formation, and the San Rafael Group with a ridge of Entrada Sandstone and a Carmel strike valley. Out of view to the right, are the submerging and now diminutive Navajo Sandstone flatirons.

Looking to the south, Factory Butte (left) and North Caineville Mesa (right) frame the Henry Mountains (in the distance). The monocline in the foreground is an extension of the San Rafael Reef called the Caineville Reef. It eventually merges with the Waterpocket Fold to the south which continues to Glen Canyon.

I was astounded by the incredible beauty of the San Rafael Swell and its monocline. Taking to the sky was definitely a great idea that not only gave me a better appreciation and understanding of the forces that created this spectacular Laramide landform but the tremendous loss of material that has occurred by erosion.

After leaving the Swell and following the San Rafael Reef to the south, we continued on our flight to the Circle Cliffs Uplift and its monocline, the Waterpocket Fold at Capitol Reef National Park. A future post will be entitled “Flight Plan: Part II – The Geology of the Circle Cliffs Uplift and the Waterpocket Fold at Capitol Reef National Park."