The Bright Angel Shale, together with the formations above and below it, form the perfect recipe to create a plateau. You have a group of three nearly horizontal rock formations. On the bottom you have a hard, cliff forming formation, the Tapeats Sandstone, on top another hard, cliff forming formation, the Muav Limestone, and sandwiched in between you have a thick but soft layer that erodes easily, the Bright Angel Shale.
Since the Bright Angel erodes much more quickly than the Tapeats, a flat area forms behind the Tapeats cliffs. And as the Bright Angel retreats, it erodes out from underneath and undermines the Muav, causing landslides that retreat the Muav as well, creating the platform between the Redwall/Muav cliffs and the Tapeats cliffs that we see today. The Tapeats, Bright Angel, and Muav are all over 500 million years old, and were deposited one after another over a period of 30 million years. Geologists bunch them together and call them the Tonto Group, and the plateau they create is called the Tonto Platform.
The main portion of the Tonto Platform stretches for some 50 miles ‘as the crow flies’ from the eastern canyon until it finally peters out near Elves Chasm, or for over 100 miles ‘as the Tonto turns’ (as I like to say) if you hike the entire length of the Tonto Trail, which twists and turns following its southern rim wherever it goes.Here we are looking down at the Tonto Platform from above along the South Kaibab Trail. To me it looks like a roiling, gray sea from up here, mostly flat but with plenty of bumps and dips to tire your legs as you hike it. The gray color is blackbrush, the dominant plant on the Tonto, gnarly little bushes a couple of feet high, popping up every 5 or 6 feet, dissolving into a gray smear of pixels from up this high. As you get down closer to the platform you begin to make out the individual bushes as little dots covering the entire surface. When you are down hiking the Tonto Trail you pass by one blackbrush after another right by the trail, each ready, if you are wearing shorts, to tear off another bit of skin from your legs with their stiff branches.
But look again at this scene from above. You see another inner canyon, gouged out from the Tonto, splitting the Tonto into two pieces north and south. The Tapeats Sandstone forms the rim of the inner canyon, and below that rim you see the ancient basement rocks of the canyon, the Vishnu complex of granite and metamorphic schists, continuing to fall steeply for another 1000 to 1500 feet down to the Colorado River, at the very bottom of the entire Grand Canyon. You can’t see the river from here, and with the inner canyon as steep and deep as it is, you can’t see much of it unless you go right to the rim of the Tonto Platform and peer over (as I have done for you in scores of panoramas taken along the Tonto Trail).Here we are down on the Tonto Platform on a spot along the Tonto Trail, in its usual position somewhere near the rim of the Tonto Platform, and near the boundary of the Bright Angel Shale and the Tapeats Sandstone. We are looking from the east side of Grapevine Canyon across to its west side only about a quarter of a mile away. The Tonto Trail takes you directly from here to there, but if you are a strong, fast hiker, you can get there in a two hour forced march, or if you are a pokey hiker like me, ambling along enjoying the scenery, stopping every once and a while to shoot a panorama, more like half a day.
The Tonto Trail does its best to minimize going up and down. But in the many large, complex side canyons like Grapevine Canyon, following that steady contour line is like tracing the edges of an oak leaf, from one edge of its stem, then in and out of one branching side canyon after another finally to the tip of the leaf, then in and out of all the side and side side canyons finally out to the other side of the stem, just a short distance from where you began.Well then, why not just cut across between the leaf tips, the heads of the side canyons? Here we are along the Tonto Trail on the point separating the drainages of Monument Creek and Hermit Creek, looking back at Cope Butte. Most of the major drainages, including here, are separated by high ridges that go up at least as high as the top of the Redwall Limestone cliffs, so you certainly are not going to go up and over anything like that. But look closely at the upper portions of the Bright Angel Shale. You can see at least four distinct cliff bands in the Bright Angel below the Muav, and this is typical. So even if you try to shorten the sweep around a point on the Tonto Platform by cutting across the inside curve of the Bright Angel Shale, you will have to deal with not just several hundred feet of climb and drop, but will have to scramble through some cliff bands as well. The long, slow, relatively flat amble following the twists and turns of the Tonto Trail is the way to go. Water is scarce on the Tonto. Here we are looking at the small but steady flow of water coming from Grapevine Springs. There is no reliable water west of here along the Tonto until you get to Burro Spring for 16 miles, though if you know where to look you are likely to find water in a few other places in the cooler winter months.
Very, very few people hike the entire length of the Tonto Trail in a single hike. Most hikers like me break it up into segments: hike down one of the major trails from the South Rim, then follow the Tonto until you reach another major trail, and follow that trail back up to the rim. But any hike on the Tonto is a serious endeavor. You must know what you are doing and you must know where and when you will get water. And do all you can, like calling the park service’s back country office, to know what current water conditions are.Nearly all of the Tonto Platform south of the Colorado River is accessible by trail, but not so north of the river. The Clear Creek Trail heads east from the North Kaibab Trail and crosses nine miles of the Tonto north of the river, and here you see the two mile section of the Tonto you cross on the North Bass Trail, but that’s about it. Beyond that, to visit the Tonto Platform north of the river you will be heading off-trail into deep wilderness areas. I have not ventured that far afield in the Grand Canyon, at least not yet, but you can read about them in Grand Canyon Loop Hikes and other books written by George Steck. Here, on the Beamer Trail, you see the last bit of the Tonto Platform in the eastern part of the park, before it disappears underwater and underground for good at the mouth of the Little Colorado River. Already by this point the Tonto Platform is lower and much closer to the river, is much narrower, and is cut into pieces by deep ravines that make the Beamer Trail much more difficult to hike than the Tonto Trail. You can see one last bit of Bright Angel Shale here, further north along the Nankoweap Trail. It is a long arduous descent down the Nankoweap Trail, and I remember hoping for relief near the bottom from the Bright Angel Shale and a stretch of fairly flat hiking. The Nankoweap does traverse through the Bright Angel Shale for about a mile, but it most definitely is not flat.
The Nankoweap Trail descends through the Redwall, Muav, Bright Angel, and Tapeats in an area that has been tilted dramatically by a major geological feature, the East Kaibab Monocline. When the trail reaches the Bright Angel Shale, it tracks right along a cliffband that would have been nice and level on the Tonto Platform, but here bobs up and down, up and down due to the tilt of the formation. You do not get any relief descending the Nankoweap Trail until you are past the Tapeats Sandstone and nearly all the way down to Nankoweap Creek.
I know that this description of the Tonto makes it sound ominous and forbidding, and you do need to be a level-headed, well-experienced Grand Canyon hiker before you take it on. But it is worth it. It is incredibly beautiful. I will focus on that beauty in the next geology article on the Tapeats Sandstone.
To read more detailed geological information about the Bright Angel Shale, visit the United States Geological Service page for the Bright Angel Shale.