Sunday, January 30, 2011

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."

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.    


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.

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.

1 comment:

  1. thank you -- very well written, lovely and helpful photographs, and very clear depictions of the geology. I was just through there and was looking for something just like this!