|Sixtymile - Grand Canyon Notes|
About Grand Canyon Rocks
This is more about hiking than it is about geology, but anyone who goes down very far into the Grand Canyon more than a couple of times is sure to develop some kind of interest in the geology. Geology not only defines the magnitude, form and color of the Grand Canyon. It determines points of access for entry, and where and how you can get from place to place.
You don't need to know the age of the rocks, how they got there, or how the Grand Canyon was formed (still a subject for debate). But you should know the names and appearance of the different formations to make sense of trail and route descriptions. And you need to be familiar with the distinctive character of the more important structures. I think it is George Steck who describes certain rock types as "friendly" and, at least by implication, others must be "hostile." Which are these and why?
Cliffs may not be friendly, but they are the essence of the appeal and physical drama of the landscape.
The major cliff zones are related to (in order of descent) the Kaibab Limestone at the rim, the Coconino Sandstone a short distance below, the Supai Group, and then the prominent Redwall and Muav Limestone. Tapeats Sandstone forms the cliff at the rim of the inner gorge.
The primary barriers are the Kaibab, Coconino and Redwall. An isolated break in one of these formations without a corresponding break in the other two generally means there is not a feasible route. For this reason, there are only 12 places where trails reach the river from the rim. Secondary cliff formations are Supai, Muav and Tapeats. These cliffs are more often broken through someplace with a passable route.
Kaibab and Coconino breaks are the most rare. Quite a few are located on points and ridges where erosion has had the greatest effect.
Supai cliffs can be a major challenge in persistence and route-finding. More often than not, breaks don't line up and lateral scouting is required to connect one with the next, and perhaps without complete assurance that there is a next one within reach. Supai layers in different parts of Grand Canyon vary in thickness and in the formation of cliffs. Surface exposures of Supai occur in small sections (eastern areas) or large expanses (western areas). There are many water catchment basins, or potholes, on the surface in these areas. But these potholes are exposed to the sun and the rainwater quickly evaporates. Many pothole water sources can be found after a rain along the trail to Thunder River.
Redwall breaks are also a challenge to discover and it seems a certainty that all the accesses that exist have been found and recorded. Most breaks are associated with large-scale faulting (Horseshoe Mesa) or slumping (Tanner Canyon). Redwall is friendly for traction; anyplace you put your foot will grab the sole of your boot and be secure. Redwall is unfriendly to skin. A scrape or fall can shred a large area of skin. Even a touch takes away a few cells with each contact and can leave fingertips supersensitive or even bleeding by the end of a trip.
Muav breaks are not helpful unless there is also a Redwall break in the same place. However, there may be a bench between the Muav and Redwall and sometimes this is a way to get above a fall in a side-canyon. Muav tends to erode in thin layers and form climbable steps. Another friendly characteristic of Muav is that it is impermeable to water. Springs in the Muav range from obscure seeps (Page Springs) to powerful gushers (Thunder River), and most northern drainages seem to have either large or small springs in this layer.
Tapeats breaks are not unusual, but there are many places where an access would be useful and none is there. Tapeats pouroffs often occur as obstacles to travel down sidecanyons. Tapeats is another impermeable rock layer and springs often surface at the upper or middle layers. Water catchments, some of considerable size, also can be found where a drainage cuts the Tapeats. Such catchments may not be visible from above and may not be very accessible. The shelter of an overhanging cliff hides the water from view and keeps it shaded. A cord with a dipper or a weighted bottle can be used to collect water. Suprisingly, some of the better Tapeats breaks are found along the sides of small drainages or directly above the river. These breaks can be a minor crack broken into a convenient series of ledges, or a broad slope with a series of low, traversable cliffs.
Other cliffs found in isolated sections are Shinumo Quartzite, Bass Limestone, and Cardenas Lava.
The dominant slope formation throughout Grand Canyon is the Tonto Platform associated with the Bright Angel Shale. Others are Toroweap Formation and Hermit Shale. Local features include the Hakatai Shale, Dox Sandstone, and Galeros Formation.
Toroweap is the light tan soil between the Kaibab and Coconino cliffs. Because it is sandwiched between these cliffs this slope is rarely useful for travel. Toroweap slopes below the north rim can be very brushy.
Hermit is a bed of fine red shale above the Supai layers where the upper Supai, or Esplanade, forms a terrace.
Supai layers may form a series of narrow terraces and cliffs. A slope of sandstone and red earth rubble typically forms at the base of these cliffs along the Redwall rim. Supai cliff terraces are not easy travel but can often be followed for considerable distance, making it possible to connect breaks in the cliffs above and below.
Bright Angel is many layers of shale that can be seen in a range of colors and textures when traveling along the Tonto Platform: yellow, purple, or green, some rare outcrops of red, visible sedimentary surfaces and fossils. This slope can be followed along a continuous trail on the south side of the river from Hance Rapid to Garnet Canyon. Although there is no corresponding trail on the north side, a similar degree of access is good for off-trail travel except where split by sidecanyons, which are much larger on the north than the south.
Grand Canyon Supergroup
The Supergroup layers are found in isolated sections below the Tapeats layer and are often exposed in the northern sidecanyons. These layers lie on a tilt to other Grand Canyon rocks and create (relatively) convenient ramps for access from the Tonto Platform into many of the sidecanyons.
Shinumo Quartzite is possibly the toughest of Grand Canyon rocks and is an obstacle to travel wherever it appears. This material forms the impressive narrows of Seventyfive Mile Canyon and the cliffs around Utah Flats.
Hakatai appears in sections on the north side. The trail into Clear Creek descends to the bed on a Hakatai slope. This material is finely crumbled and intensely orange-red in color and can affect water quality where much of the flow passes through it, such as in Hakatai Canyon.
Dox slopes predominate in the area between Tanner Canyon and Unkar Rapid (also see Furnace Flats) and this makes one of the few places where it is possible to travel very far along the river corridor. However, Dox is not a favorite rock formation for hikers because the slopes are often steep, crumbly, blazing hot and dry, with unpredictable "ball bearing" surfaces and inconvenient small cliffs.
Galeros is a crumbly, gray material that erodes easily and does not support vegetation. Where Galeros has filled streambeds or ravines, access and travel are often relatively easy even without a trail. Galeros is found only in the several drainages to the east of Walhalla Plateau, where hikers go less often.
The inner gorge is the oldest, most complex and least predictable structurally. There is no consistency as to the presence of layers or their position and angle. Softer and harder materials are randomly distributed with the result that side canyons may offer sections of easy travel segmented by major pouroffs and obstacles. Bypasses to these obstacles often exist but can require technical climbing skills or extensive excursions above the streambed. Inner gorge and sidecanyon slopes, where foot travel is possible (and frequently it is not), are typically hot, crumbly, unstable and difficult to navigate. However, a bench where travel is easier is often found at the top of these slopes just under the Tapeats cliff.
Other Information Sources
To gain a good understanding of the relationship between Grand Canyon hiking and geology, careful study of the color-coded geologic composite map from the Museum of Northern Arizona is recommended (see Essential Maps). Suggested reading is Grand Canyon Geology, S. Beus and M. Morales editors, a collection of academic papers "...not intended for the casual reader" with each section by a specialist in the formation, and which has nothing at all to do with hiking. Hiking Arizona's Geology, Ivo Lucchitta, has several Grand Canyon sections. For a quick lesson by web, see Bob Keller's Grand Canyon Geology Overview.