Geology of the Grand Canyon: Crossing the Mighty Colorado River at Lees Ferry, and the Silver and Black Bridges

In these very modern times we take many things for granted, only realizing our dependence in their absence. One circumstance comes to mind that I no longer take for granted having seen the mighty Colorado. It's crossing a river, something that the early pioneers and explorers of our country never took for granted and had to repeatedly deal with on their treks west. 

Crossing rivers in the early days was difficult and dangerous. If a river was shallow and lazy, fording was an option, but always at the safest place to avoid getting stuck or sinking into the mire. If a river was deep, they would swim the animals across, and then float the wagons. Children actually helped their parents make a wax paste to waterproof the open spaces in the wagons. You can almost hear the sounds of snapping harnesses and creaking wagon wheels. Deeper and more dangerous rivers meant building a large, flat boat called a scow. Drowning was commonplace. Fast and furious rivers such as the mighty Colorado in the vicinity of the Grand Canyon were virtually uncrossable, without even an opportunity of descent to the river. 

This view looks downriver from the granaries at Nankoweap, 53 miles from Lees Ferry. Here, the easily erodable Bright Angel Shale has encouraged the development of a wide canyon, a lazy Colorado River and Ancestral Native American habitation. In most of the Grand Canyon, shear walls make access to the river from the rims extremely difficult if not impossible. Notice our raft parked along the shore far below.

There have been only three places to cross the Colorado between Glen Canyon Dam and Hoover Dam that are separated by a distance of over 300 miles downriver. Let's investigate these river crossings, both old and new, and their relevant geology on a journey downriver into John Wesley Powell's "Great Unknown."

THE CROSSING AT LEES FERRY

Originally called the "Paria Crossing" for the Paria River that joins the Colorado just upstream, Lees Ferry serves as a natural river-crossing. In fact, it's a natural corridor between Utah and Arizona and figured prominently in the exploration and settlement of Northern Arizona. There, the canyons of the Colorado River surrender their height by allowing travelers to cross the river to the other side. You can still see the old ramps and road leading down to Lees Ferry, used by the pioneers and their wagons, carved into the bedrock opposite from the boat landing. This is the same location where modern-day river-runners launch their vessels into the abyss downriver into the Marble and Grand Canyons.

Lees Ferry is on the map, but it's not a town and it has a population of zero. Usually written without the apostrophe, it bears the namesake of John D. Lee, a Mormon who settled in the region in 1871 with 2 of his 18 wives, and established the first ferry service there. An interesting character, Lee was excommunicated and eventually executed for his part in the massacre of an emigrant group from Arkansas, supposedly being blinded by religious fanaticism and faith in his corrupt church leaders. 

Archived photo of scow with horses and wagon making a crossing at Lees Ferry

Historically, Lee's hand-operated barge in the 1870’s, and later, a steam-operated ferry in 1912 ran Mormons, pioneers, colonists, explorers, Indians, gold rush prospectors, and their horses and supplies across the river. In 1928 the ferry sank drowning three men. Six months later the first Navajo Bridge was completed 4.2 miles downstream, revolutionizing the way people traveled around the region.



These days, rafts, dories and kayaks prepare to shove off on their journey downriver from Lees Ferry (above), the traditional starting point for river runners. Lees Ferry can be a busy place, especially in the morning when everyone wants an early start. 

Check out the local geology! The view is from the boat landing at Lees Ferry, looking downriver. On the opposite bank (above) is an eroded, upturned eastern flank of the Echo Cliffs monocline rising from the river with its sloping, reddish-brown mudstone of the Early Triassic Moenkopi Formation and the overlying resistant, pebbly Shinarump Conglomerate as caprock, the basal member of the Late Triassic Chinle Formation. Further off to the left the Echo Cliffs assumes its full height, being capped by remnants of sharply folded Early Jurassic Navajo Sandstone. My young rafter-daughter is standing in the vicinity of pre- and post-Glen Canyon Dam terraces and deposits, not too far above the hard Permian Kaibab Limestone, the resistant rocks that support the rim of the Grand Canyon. Towering off to the right in the distance are the largely Jurassic age Vermilion Cliffs. As with the Echo Cliffs, its base begins with the Moenkopi and then the overlying Chinle Formation. In ascending order, the Moenave and Kayenta Formations are above, with the cliffs capped by the resistant Navajo Sandstone. A short distance above the boat-launching area the Paria River valley intersects the Colorado River, having emerged from Glen Canyon. Carved into the weaker Chinle Formation, the Paria is a strike valley. 



This is a Google Earth image of Lees Ferry (red arrow) looking north. Notice the Triassic and Jurassic strata of the Echo Cliffs on the right and that of the Vermilion Cliffs on the left as they converge. The Paria River Canyon at the top can also be seen to converge upon Lees Ferry, a natural "funnel" where the Colorado River is conveniently crossable. Lake Powell narrowing at the Glen Canyon Dam

 can be seen at the upper right. The Navajo Bridge is where Route 89A crosses Marble Canyon. The "barbed tributaries" of Marble Canyon can be seen on the floor of the canyon. For additional information on that subject, read Carving Grand Canyon by Wayne Ranney. He paints a fascinating picture of the formation of this ancient landscape.  
Lees Ferry's rock units have funneled human activity into the region for thousands of years. It sits at the geological juncture of the Precambrian and Paleozoic world of the Grand Canyon below and the Mesozoic world of the Colorado Plateau above. Beginning with his prehistoric kin, next with the Spanish and the early explorers and colonists, and right up through today's tourists and river-runners, man has made good use of the geology in travelling through the region.    


 

THE NAVAJO BRIDGES(S)
Floating downstream from Lees Ferry, the river appears to drop quickly as it carves its way into the Marble Canyon. The allusion is enhanced by the Laramide age, 1-2 degree northeasterly tilt of the entire Colorado Plateau. Soon the Navajo Bridge, both old and new comes into view. The Kaibab Limestone now resides at the rim, supported by a cliff of the limestone of the Toroweap Formation and the Coconino Sandstone, the latter just rising from the river. The slope at river level is talus from above. The Hermit Shale will appear about a mile past the bridge. The rim in the distance, still Marble Canyon, clearly shows the Kaibab at its rim.

Marble Canyon, named by Major John Wesley Powell, is a misnomer, the canyon actually being sedimentary-limestone not metamorphic-marble. Powell and his motley crew in three small boats passed through here on August 5, 1869. Technically, we’re not in the Grand Canyon yet, having not reached the confluence of the Little Colorado River, 57 miles downriver. Rafting trips are in the Paleozoic and the Precambrian strata (specifically the Meso- and Neoproterozoic). The trip downriver begins within the Kaibab Formation, a typically thinner stratum in the eastern Marble and Grand Canyons.

The still-narrow gorge doesn't permit these Permian formations to separate into their classic stairstep topography. These formations are far thinner here than to the west and actually pinch out to the east of Lees Ferry, since the Permian Kaibab and Toroweap Formations originated in vast seas to the west of the primitive North American continent. River guides will remind you to say goodbye to these formations, since you won't see these rock layers again near the river as you raft through the Grand Canyon. You'll only see them high up at the rim.

Realizing the importance as both a crossing and boat-launching location, immediately below Lees Ferry the canyon of the Colorado River becomes impenetrable, where the resistant limestones of the Kaibab and Toroweap Formations rise quickly and form the vertical walls of Marble Canyon. It is here where the Navajo Bridge, actually there are two of them side by side, spans the narrow gorge. In 1995 a second bridge was built to accommodate the demands of increased traffic flow and a greater load capacity. The Navajo Bridge is listed in the National Register of Historic Places. 

The Navajo Bridge's abutments are anchored into the Kaibab Limestone 470 feet above the Colorado River, the same rock layer that we launched our trip from at Lees Ferry! The bridges span Marble Canyon, the section of the Colorado River’s canyon from Lees Ferry to the confluence with the Little Colorado River, where the Grand Canyon officially begins.

As you can see, the design of the new bridge was made to be visually compatible with the old. The original Navajo Bridge is now open as a footbridge for pedestrian and equestrian use. The new bridge remains as the only vehicular crossing of the Colorado River from the Glen Canyon Dam above to the Hoover Dam below on Route 89A.

During construction, a crucial piece of equipment for the completion of the Navajo Bridge was stranded on the wrong side of the river. The only way to get it across was to take it through Las Vegas, a detour of over 400 hundred miles. This illustrates the significance of the bridge in facilitating transportation through the region. On a humorous note, because of Prohibition, the Navajo Bridge was baptized with ginger ale rather than champagne.

THE SILVER AND BLACK BRIDGES

These foot-bridges are the third location one can cross the Colorado some 87 miles downriver from Lees Ferry, located at the bottom of the canyon within the Granite or Inner Gorge. There, one will encounter two river-crossings via metal suspension-bridges, one silver and solely for foot traffic, and the other black for both hiker and mule-train traffic.

The Black Bridge spans the gorge east of the Silver Bridge

For hikers, at least for me, crossing the bridge is a symbolic gesture of arrival at the bottom of the Grand Canyon. As one would assume, the crossing would also designate the half-way point. But, looking at a map of the region, one can see that the hike out to the north is far longer than the hike out to the south. That is attributable, in part, to the inclination of the Colorado Plateau to the north.


In 1903 David Rust, a Stanford educated schoolteacher turned trail builder, began developing a layover point for hunters and sightseers at the present day location of Phantom Ranch known as Rust's Camp. In 1907 he built a cable- tram across the Colorado River, which was essentially a harrowing ride in a suspended metal cage. On his way to hunt mountain lions on the North Rim, Teddy Roosevelt used the tram in 1913. In 1921 a wooden suspension bridge was built to replace the tram. It allowed mules to cross the river but was narrow and swayed violently in the wind.

In 1928, the National Park Service began construction of the 440 foot long Kaibab Suspension Bridge, also known as the Black Bridge. Each of the eight 550 foot cables that suspend the bridge weighs about one ton. The spooled cables were far too heavy to be brought to the bottom of the canyon by mules. So, each cable was unrolled from its spool, and its length was supported by 42 men walking the cable down to the bottom of the canyon over 9 miles of trails and 4,000 feet down from the South Rim. Eight times! The width of the bridge span (5 feet) was chosen with consideration to the movement of mules. It was felt that a narrower walkway would interfere with the pack loads and that a wider one would allow the animals to potentially maneuver enough to turn around and cause much confusion. Today, the bridge is used by both hikers and mule-trains, but the latter have the right-of-way.

About a mile downstream from Black Bridge is the Bright Angel Suspension Bridge, also known as the Silver Bridge, completed in 1970. While the Black Bridge has a solid deck, the deck of Silver Bridge is open mesh, allowing a view of the river below, and used only by hikers. Unbeknownst to many hikers, the Silver Bridge also carries the transcanyon pipeline for water from Roaring Springs near the North Rim. Pump houses send the water through Phantom Ranch, across the Colorado River and up to the tourist area on the South Rim, 500,000 gallons of water a day! The Silver Bridge was built in the late 1960s connecting the Bright Angel Trail from the South Rim to Phantom Ranch and the North Rim.

The Black Bridge marks the end of the River Trail connecting the South Kaibab Trail on the southside to the North Kaibab Trail on the northside, which leads to the Bright Angel Campground, Phantom Ranch and eventually the North Rim. 

 


From this lofty perch one can see both bridges spanning the Colorado River, as it makes a sweeping turn flowing to the west. The Black Bridge is in the foreground and the Silver Bridge is beyond. The South Rim is in the distance with the lower bench representing the Cambrian age Tonto Platform. The cliff below it comprises the Inner Gorge and consists of the Grand Canyon Metamorphic Suite, made up of the Early Proterozoic, dark Vishnu Schist and the lighter-colored, pinkish-intrusive rocks of the Zoroaster Granite.

The gorge exposes an extraordinary cross-section of the Proterozoic orogen between the central Arizona Transition Zone and the Rocky Mountain region. The gorge consists of several lithotectonic blocks that were primarily active during the 1.7-1.6 Ga accretion and stabilization of the Yavapai and Mojave crustal provinces.

Standing on the Black Bridge, we're looking into the tunnel at its south end, drilled into the Precambrian crystalline rock of the Metamorphic Suite. Note the wooden walkway created especially for the mules.

Below the Silver and Black Bridges, there are additional river crossings, at least man-made, until we emerge from the Grand Canyon at Grand Wash Cliffs and travel to the Hoover Dam.



Bryce Canyon: Visit It Before It's Too Late

Bryce

Canyon in southern Utah is eroding quickly, almost right before our eyes at 2-4 feet every 100 years, give or take. The agent of erosion is water, mechanically scouring and abrading with its load of rocks, and chemically dissolving the bedrock, aided by the very steep gradient at Bryce. But water acts in another way to pry rocks apart, one that we seldom think of, by freezing.

In a wet environment, and especially one that has numerous freezing and thawing cycles like Bryce (over 275 per year) with its high elevation, water penetrates every available crevice, both big and small. When water freezes, it expands its volume by about 9%, and in so doing, it becomes a powerful force in the process of erosion by literally breaking rock apart, grain by grain. This is called frost wedging.

The spires and hoodoos, ridges, columns and pedestals, shear walls, and labyrinth of box canyons of Bryce have weathered into a distinctive landscape by the combined action of its rock type, geography and climate. The badlands topography of Bryce is carved into the pink, red, orange, tan and white sedimentary rocks of the Eocene Claron Formation, mainly the Pink Limestone Member. This interbedded, fine-grained limestone, calcareous mudstone and fine-grained sandstone member was deposited in a low-energy fluvial (river and stream) and lacustrine (lake) environment.

The Claron Formation is a rock structure that is weakly held together and especially susceptible to erosion. Geologists say it’s poorly lithified. As the canyon continues to erode to the west, it will eventually capture the watershed of the East Fork of the Sevier River. Once this river flows through Bryce, it will dominate the erosional pattern, replacing Bryce’s amphitheatre with a "V" shaped canyon and steep cliff walls typical of the weathering and erosional patterns created by flowing water. That will likely further serve to enhance Bryce's erosive demise! 

Interestingly, the source waters that deposited the Claron Formation originated from headwaters located in the Sevier and Mogollon Highlands to the south and west, formed during the Late Cretaceous to early Paleogene. These mountain ranges have since eroded away but could only have reached their destination at the inland system of adjoining basins in Utah had the Grand Canyon not yet formed, which would have acted as an impediment to northeast flow. This implies, assuming the correctness of this scenario, a later period of “drainage reversal” for this portion of the Colorado River system. It also begs the question of “how much of the modern drainage configuration could be inherited from these Paleogene drainages?” (please read Carving Grand Canyon by Wayne Ranney for an excellent presentation of this concept).

For all these reasons, Bryce may be gone before you know it, in perhaps 3,000,000 years, give or take. A very short time, geologically speaking.

The Grand Staircase section of the Grand Staircase-Escalante National Monument

Good viewing spots of the entire staircase are hard to come by. The curvature of the Earth also obstructs distant areas from view. One of the best is south of Kanab, Utah along Route 89A in Arizona, somewhat north of the Grand Canyon, which is where this photo was taken.

 

 

 

President Clinton created the 1.9-million-acre Grand Staircase-Escalante National Monumenton September 18, 1996. Placed under the management of the U.S. Bureau of Land Management, it is located in south-central Utah. The monument is located within the Colorado Plateau physiographic province, near its western margin. It contains an astounding array of paleontological, geological, archaeological, biological and historical resources. Largely located in a remote area, it is comprised of mesas and cliffs, and canyons and plateaus, and distinguished by colorful geologic formations.  

 

The three sections of the Grand Staircase-Escalante National Monument:
the Grand Staircase section in blue; the Kaiparowits Basin section in yellow;
and the Escalante Canyons section in green.
From Geology of Grand Staircase-Escalante National Monument, Utah by Doelling et al

 

Cross-sectional Diagram of the Grand Staircase

from Geology of Grand Staircase-Escalante National Monument by Doelling et al 

RISERS AND CLIFFS, STEPS AND BENCHES 

The monument is divided into three geographical sections from west to east; the Grand Staircase, the Kaiparowits Basin and Escalante Canyons. More than 275 million years of Earth’s history are exposed in its rock formations extending from the Permian to the Cretaceous. The Grand Staircase section encompasses the western third of the monument. It refers to an immense sequence of sedimentary rock layers that form a series of topographic benches and cliffs that steps up in elevation progressively from south to north.

The staircase's risers correspond to cliffs, each rising as much as 2,000 feet, and the steps correspond to the benches, terraces, or plateaus in the staircase. The staircase generally includes the region from the Kaibab Uplift and Plateau, which forms the north rim of the Grand Canyon at the bottom of the staircase, to the Pink Cliffs of Bryce Canyon, at the top. 

GENESIS OF THE STAIRCASE 

The Grand Staircase began to form when vast Cambrian oceans lapped onto the broad continental shelves of the still-forming North American continent. Later, Triassic rivers and streams in their flood stages left silty and muddy deposits on the ancient landscape. Then, sandy “seas” the size of the Sahara deposited windblown sand dunes during the Jurassic. Next an enormous, shallow inland, Cretaceous sea left mud, silt and sand on its fluctuating shoreline and sea floor. Lastly, a Paleogene system of lakes deposited layer after layer of their limey deposits.

All these ancient geological events occurred in succession, one after the other, depositing a legacy of their presence that was written in stone. Laramide uplift elevated and tilted the staircase's Paleozoic and largely Mesozoic rocks to their existing location, along with the Colorado Plateau as a whole and the Grand Canyon to the south. As erosion attacked the staircase, scarps formed where several soft layers were capped by harder ones. The scarps are aligned approximately  northwest, parallel to the strike of the strata, given the northeast dip of the staircase. Subsequently, erosion retreats the strata to the northeast, down the dip of the strata. 

The Stratigrpahic Units of the Grand Staircase
from Bryce Canyon National Park and the Grand Staircase by Foos

 

COLORED CLIFFS

The cliffs are named by their colors. Starting from the bottom, they are the low cuestas of the Chocolate Cliffs capped by the resistant, Triassic sandstone of the Shinarump Conglomerate, the grand Vermilion and White Cliffs held up by the Wingate and Navajo Sandstones respectively, the Gray Cliffs of Cretaceous shales and sandstones, and at the top, the Paleogene Claron Formation comprising the Pink Cliffs. 

 

The Stratigraphic Units of the Grand Staircase

from Bryce Canyon National Park and the Grand Staircase by Foos

This diagram shows the Grand Staircase from the bottom of the Grand Canyon to the very top at Bryce Canyon.
The vertical perspective is greatly exaggerated. The lower diagram, drawn to more realistic proportions,
is more realistic of the actual topography. Click the diagram for a larger version. Diagrams from Ancient Landscapes of the Colorado Plateau by Ron Blakey (Colorado Plateau Geosystems, Inc.) and Wayne Ranney.
P.S. This book is a must read!

Probably named by the early geologist Clarence Dutton in the 1880’s, the Grand Staircase’s alternating configuration of southward-facing cliffs, terraces, and slopes is due to the varied erosion rates of the different rock types. Harder rocks, such as sandstone and limestone, erode slowly and make up the cliffs and terraces. Softer rocks, such as shale and siltstone, erode faster and make up the slopes. This high, rugged, and remote region, where bold plateaus and multi-hued cliffs run for distances that defy human perspective, was the last place in the continental U.S. to be mapped.

Navajo Rug Making: A Looming Creation

A Navajo Poem (First Stanza)

Today I will walk out, today everything evil will leave me,
I will be as I was before, I will have a cool breeze over my body.
I will have a light body, I will be happy forever,
nothing will hinder me.
I walk with beauty before me. I walk with beauty behind me.
I walk with beauty below me. I walk with beauty above me.
I walk with beauty around me. My words will be beautiful.

She sits on the floor of her octagonal, wooden hogan for hours at a time, weaving a rug in the traditional Navajo method. It will take 2 to 3 months to weave an average 4’ x 6’ rug, maybe 5 to 6 months for an exceptionally complex weave. The pattern seen here has taken 6 weeks to create. There are no short cuts in the weaving process. They are all hand woven on a traditional upright loom. As she works, the completed portion is rolled to the bottom of the loom.

In Navajo weaving, the warp is one continuous length of yarn, not extending beyond the weaving as fringe. A traditional Navajo geometric pattern is determined by the weaver before the rug has begun, and every nuance of the design is recalled from memory. One way to tell the quality of the rug is to count and compare the geometrics at the beginning and the end to see if they are the same.

Navajo textiles were originally utilitarian blankets for use as cloaks, dresses, saddle blankets and similar purposes. Toward the end of the 19th century, weavers began to make rugs for tourism and export. The Navajo did not produce rugs until export markets expanded at the end of the 19th century, and their textiles served no specific religious or ceremonial function. Today, more than 300,000 Navajo people live in the 16 million acre Navajo Nation, about the size of West Virginia. Many traditional weavers raise the sheep and shear, wash, card and spin the wool themselves. They dye it in methods which have been passed down through generations with native plants such as wild walnut, lichen and rabbitbrush. The Navajo blankets and rugs are truly borne from the land.

A Navajo rug illustrating the tradition of bold geometrics,
precise symmetricality and rich colors

The Navajos believe that part of their spirit or soul gets trapped in the rug as it is woven on the loom. They purposely leave a small piece of yarn, called the “spirit string,” sticking out slightly from the surface of the rug. This will allow their spirit or soul a way to get out of the rug. They also believe that only God is perfect; therefore, they will make sure their woven creation has a small, hard-to-find imperfection. On a Navajo rug, it’s the loose piece of yarn.

Handmade Navajo rugs are truly works of art. Each rug is a one-of-a-kind item taking many months to create, and true in every sense of the word “handmade.”

Stately Ship Rock and the Navajo Volcanic Field

I took this photo of Ship Rock from Buffalo Pass 65 miles to the west, high in the Chuska Mountains

along the Arizona-New Mexico border. A fortuitous break in the clouds allowed the sun to illuminate the two black monoliths, reflecting back as an unearthly, metallic-white gleaming in the high desert. Typically, radial dikes can be seen to emanate outward from the base of Ship Rock on the desert floor. Dikes are magma-filled fractures in the Earth’s crust that serve as conduits for molten rock. There is also a smaller diatreme juxtaposed in the foreground called The Thumb.




Differential erosion has left Ship Rock and the Thumb towering above the surrounding plains. Interestingly, the Chuska Mountains are "held up" (and hence created) by a lava flow of the same geological ilk that created other volcanics within the region, but here, a trachybasalt which is the extrusive equivalent of minette (please read on). The resistant igneous rock prevented the Chuska's from eroding, while the neighboring regions of the plateau were unroofed. Seen in the photo, the pass was covered by the flow's underlying Chuska Sandstone. 

SHIP ROCK
Looking like a scene from a sci-fi movie, Ship Rock stands 1,583 feet above the high-desert plain in the northwest corner of New Mexico. It is the 30 million year old or so erosional remnant of the neck (or plug) of a volcano called a diatreme. Diatremes most likely form when rising magma in basic to ultrabasic volcanic fields creates a sudden gaseous explosion deep underground, when the magma comes in contact with subterranean water. The heated groundwater under pressure causes a hydro-volcanic (or phreatomagmatic) eruption, which results in the formation of a diatreme volcano. A series of explosive eruption excavates a shallow crater (or maar) at the surface, flanked by bedded pyroclastic ejecta. Over time, the diatremes have become exposed or exhumed by uplift of the Colorado Plateau. They typically survive erosion, since their composition is more resistant than the surrounding rock, which has worn away. This accounts for the Navajo (Dine') American Indian's reference to them as "black rocks protruding up" in their language.

Called Tsé Bitʼaʼí meaning "rock with wings", Ship Rock stands within the Navajo Tribal reservation and is of great religious and mythological significance to the tribe, being mentioned in many of their legends. According to one legend, after being transported from another place, the Navajos lived on the monolith, "coming down only to plant their fields and get water." One day, the peak was struck by lightning, obliterating the trail and leaving only a sheer cliff, and stranding the women and children on top to starve. The presence of people on the peak is forbidden "for fear they might stir up the chį́įdii (ghosts), or rob their corpses." Rock climbing on the monolith is strictly forbidden by the Navajos.

THE NAVAJO VOLCANIC FIELD
Ship Rock is one of over 80 late Oligocene to early Miocene-age (ca. 28-19 Ma) volcanoes and intrusive structures standing within the Navajo Volcanic Field. The Field covers roughly 20,000 square kilometers and is situated within a greater physiographic province called the Four Corners Platform, itself a member of the even greater Colorado Plateau. The platform includes the four corners states of Arizona, New Mexico, Colorado and Utah. The intrusive structures within the field include tuff pipes, dikes, intrusions and diatremes. Many of the diatremes are roughly clustered along Laramide-age monoclines such as the Comb Ridge, Defiance and Hogback monoclines. No faults are present at the surface, but magma ascent was probably facilitated by NE-SW trending Laramide fractures at depth.




Schematic diagram of the diatremes of the Navajo Volcanic Field clustered around the Four Corners region.

The letters identify diatremes: ME, Mules Ear; MR, Moses Rock; CV, Cane Valley; GR, Garnet Ridge;
RM, Red Mesa; GN, Green Knobs; BP, Buell Park; AG, Agathla; CR, Church Rock; SR, Ship Rock; MR, Mitten Rock;

RB, Roof Butte; WP, Washington Pass; FR, Fluted Rock; TB, Twin Buttes. Map from Roden and Smith (1979).

Map of the central Navajo Volcanic Field. Notice clustering of volcanic structures along Laramide-age monoclines.
 Dark circles indi­cate minettes; open triangles represent SUMs. Monoclines are indicated by heavy lines.
Abbreviations: AP, Agathla Peak; AZ, Arizona; BB, Boundary Butte; BP, Buell Park; CO, Colorado;
CRM, Comb Ridge Monocline; CV, Cane Valley; EDM, East Defiance Monocline; GN, Green Knobs;
GR, Garnet Ridge; ME, Mule Ear; MHM, Mesaverde Hogback Monocline, MR, Moses Rock;
NM, New Mexico; RM, Red Mesa; SR, Ship Rock; UT, Utah. Major communities are also shown. 
After Smith and Levy (1976) and McGetchin et al. (1977).

PETROLOGY OF THE ROCKS
Typical of the magma of many diatremes within the Navajo Volcanic Field, Ship Rock is composed of a tuff-breccia or technically, a serpentized ultramafic microbreccia (or SUM). In fact two principal rock types are found within the Navajo Volcanic Field, the other being a "minette" which is a greenish-gray, ultramafic, highly potassic, orthoclase biotite lamprophyre. After the intermediate, basic and ultrabasic scoria cones, monogenetic maar-diatreme volcanoes are actually the second most common volcano type on continents and islands. The majority of the maar-diatreme volcanoes represent the phreatomagmatic (referring to the interaction of water and magma) equivalent of the magmatic scoria cones and their associated lava flows.

ANATOMY OF A DIATREME
A diatreme volcano generally consists of a maar-crater at the surface with a tephra-ring surrounding the crater. The more-or-less cone-shaped diatreme underlies the maar, and the irregular-shaped root zone beneath that. At the very bottom enters the feeder dike, the source of the magma. In the photo of Ship Rock, the maar-crater has been removed during the process of exhumation that uplifted the surrounding plateau, resulting in the exposure of a portion of the previously-bedded diatreme.

From "Maar-Diatreme Volcanoes..." by Lorenz

The common chronological-thread amongst the many diatremes is end-Laramide emplacement in association with faults at depth, exhumation in association with post-eruptive uplift and dissection of the Colorado Plateau (the regional bedrock is eroded away in addition to the maar-crater which is composed of unconsolidated material) , and finally, erosive sculpting of the exposed necks. 

Diagram of a diatreme with its maar-crater illustrating its exhumed position (land surface today) 
as a volcanic neck following its erosive exposure.
Source unknown.

TECTONIC ORIGINS
The origin, unique chemistry and emplacement of the minettes is likely related to the underlying Farallon Plate, which during the Oligocene, increased its angle of subduction beneath the North American Plate at the end of Laramide-time. This possibly created a pulse of volcanic activity as the plate sank into hotter regions of the mantle and subsequently melted.The region of the Four Corners began to experience extensional forces. Volcanic activity also accompanied the opening of the Rio Grand rift as magmas penetrated the thinning, subsiding crust. Mafic minette-magma may have been derived from fractional crystallization within the upper mantle.

Baars (Colorado Plateau, 1972) theorizes that the most abundant volcanoes are found on the margins of the Colorado Plateau prior to its uplift. The plateau at that time, still a basin, received Rocky Mountain sediments causing it to futher depress. Baars goes on to explain the origin of the diatremes to have occurred largely at the folded-edges of the various basins as they sank.  

OTHER DIATREMES WITHIN THE NAVAJO VOLCANIC FIELD



Somewhat north of Monument Valley, is this small, possibly unnamed diatreme. Notice the remnants of its dike running off to the right projecting vertically through the strata that it dissected.

Agathla Peak, also known as El Capitan in Spanish (named by Kit Carson), stands over 1,500 feet south of Monument Valley. Situated in Arizona, it is about 85 miles west of Ship Rock, in New Mexico. It too is a diatreme within the Navajo Volcanic Field. Notice the Navajo dwellings in the foreground for scale.

Looking south-southeast from Goosenecks State Park near Mexican Hat, New Mexico

, this exhumed diatreme is called Alhambra Rock. Typical of other Navajo Volcanic Field diatremes, erosion has left the resistant neck standing as a lone sentinel. In the foreground, erosion has exposed the Pennsylvanian rocks of the Hermosa Group's Honaker Trail Formation within the Goosenecks of the San Juan River. Alhambra is framed against Comb Ridge in the distance, the eroded, upturned eastern limb of the Monument Upwarp.

From the crest of Comb Ridge looking south into Comb Wash and the San Juan River (11 miles to the east of Gooseneck State Park), the Mule Ear, a resistance flank of Comb Ridge, points skyward with the eroded remnants of the Mule Ear diatreme situated to the right in the strike valley. The diatreme is positioned within the eroded eastern flank of the Laramide-age Monument Upwarp. Interestingly, the Mule Ear's prominence above Comb Ridge, of identical sedimentary composition, may be related to it having been subjected to low grade metamorphism from the neighboring diatreme which conferred a resistance to erosion.

This is a closer view of the Mule Ear (left) and the Mule Ear diatreme (center and right) taken from the San Juan River as it heads south just before turning to the west and heading into Lime Ridge and the Raplee Anticline.

The effects of Farallon Plate subduction and its consumption beneath North America has been manifested with transformational faulting in California, extension through the Basin and Range Province in Nevada and Arizona, with volcanic activity within the fields of the Colorado Plateau, and extension further east into New Mexico with the graben of the Rio Grande rift and horst of the Sandia Mountains. The result of crustal thinning in the Four Corners has allowed the ascension of magma through the crust and the emplacement of the many diatremes within the Navajo Volcanic Field. The uplift and unroofing of the Colorado Plateau has exhumed the diatremes and exposed them to the effects of erosion.