GEOSC 10
Geology of the National Parks

Hiking through History

 

Hiking Through History

The Penn State CAUSE class did what roughly 1% of the visitors to the Grand Canyon do, and hiked to the bottom and then back out. The trek down is rugged, often dusty, often hot, and safe only for well-prepared hikers. Many of the people who do make the hike report that it is the experience of a lifetime.

The rocks at the Grand Canyon are in order, with the oldest ones on the bottom, so in hiking back up from the river to the rim, we were hiking upward through history. The next section, The Longest Story, is a travelogue of the sites we saw on the way up. A lot of detail is provided, NOT to make you memorize it all, but to give you a small sample of the amazing things that geologists have learned, and how rich and varied the history of our planet really is.

So, lace on your boots, and let’s start the mile-high climb from the Colorado River to the rim of the Grand Canyon, watching the geology all the way.

The Longest Story

At the bottom, the river has cut the narrow, steep inner canyon through the Precambrian Vishnu and Brahma Schists. The older Vishnu has the appearance and chemical composition of metamorphosed sediments. The lava flows of the Brahma preserve the pillow structure of submarine eruptions, but the interbedded volcanic airfall material shows that at times the region was exposed as dry land. The total thickness of three miles of lava flows and interbedded layers, now standing almost on end although they initially were deposited almost horizontally, speaks of an important, long-lasting interval of deposition.

These oldest lava flows and sediments of the Grand Canyon have been "cooked," and are now of metamorphic types that form only in the hearts of mountain ranges at very high pressures and temperatures. During and after the metamorphism, melted rock (magma) squirted into cracks in these rocks, and then froze to form the pretty pink Zoroaster Granite. Yet this whole package of rocks was then brought back to the surface as the rocks of the mountains above them were eroded, with the erosion producing a very smooth, nearly horizontal plain on top of them, and weathering/soil formation causing changes that extend deep beneath that plain into these rocks.

The sea next advanced across this plain, first picking up and carrying and rolling pieces of the rocks and soils on the erosion surface to form a conglomerate, then giving way to sandstones, shales, and limestones that piled up to a thickness of two miles or so. (Such a great thickness does not mean that the sea was two miles deep; rather, in this case, the water stayed relatively shallow, but the warping of the crust by the drifting plates and other processes caused the sea floor to sink as the muds and other deposits piled up; recall that the Mississippi Delta is much more than 2 miles thick.) These rocks include mud cracks, ripple marks, casts of salt crystals that formed when the sea water evaporated in nearshore environments, and stromatolites, which are algal-mat deposits in which the algae trap mud, grow up through it, and trap more mud. All of these are similar to modern features, and indicate gradual accumulation (a layer, then drying for mud cracks, then more mud, then ripples from water flow, then drying for salt casts, and on and on and on).

Death-Valley-type pull-apart faulting then dropped and rotated these layers, so that they now slant (see the figure below). Long-term weathering and erosion then occurred, leading to a low, almost flat landscape broken by a few higher points where especially resistant rocks did not erode as rapidly. Again, deep weathering speaks of long exposure. In some places, the sediments were entirely removed down to the metamorphic rocks beneath, but in other places the sediments are preserved where they were dropped by faulting.

Rock layers with Tapeats, Unconformity, Supergroup, and Sedimentary rocks labeled.View of the east end of the Grand Canyon, showing a little of the two miles of Precambrian sedimentary rocks, labeled “Supergroup”, which were dropped and tilted along Death Valley-type faults that are not visible here.  Erosion for a long time produced the unconformity above, and then the sea returned to deposit the Tapeats Sandstone and rocks on top of it.
Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

The sea then returned, again reworking materials on the erosion surface to make a basal conglomerate, followed by beach sandstone, then offshore shale, and limestone from farther offshore. As the sea deepened and the beach moved towards the land, shale was deposited on sandstone, and then limestone was deposited on shale. These three layers, the Tapeats Sandstone with its thin basal conglomerate, the overlying, Bright Angel Shale, and then the Muav Limestone, form the slope that is known as the Tonto Plateau, and is so evident on the south side of the canyon. The rocks of the Tonto Plateau include fossils of marine animals such as trilobites, and even trilobite tracks. Again, all evidence is of deposition by processes just like those operating today, over long periods of time. A layer with a trilobite track must have been exposed long enough for a trilobite to crawl across it. The thousands and thousands of different layers in the rocks, with ripples and tracks and fossils, indicate long times.

Time then passed of which we have no record in the Grand Canyon, except that stream channels were carved on top of the Muav Limestone, indicating that the region was raised out of the sea and erosion was occurring. Fossils from two of the periods of the Paleozoic are missing, indicating that much time passed. When deposition resumed, the first rocks put down were limestones in the stream valleys, but another time gap sits on top of those in-the-channel rocks. The limestone in the channels, called the Temple Butte, includes coral and shellfish (brachiopod) fossils, and plates from armored fish.

Video: Grand Canyon Strata (8:22)

Credit: R. B. Alley © Penn State. "Grand Canyon Strata." YouTube. May 30, 2024.
Click here for a transcript of the Grand Canyon Strata video.


If you hike out of the Grand Canyon, these are the rocks you walk on. We don't want you to try to memorize everything here, but enjoy the hike and appreciate the history. It's truly amazing. We're going to start down here with the Vishnu and Brahma Shist, the Zoroaster granite. Here's a picture, two pictures of them down by the river. They started out as sediments and lava flows and ash that fell out of the air from volcanic eruptions. Then they were squeezed and heated in the heart of a mountain range, metamorphosed, and then squirted pink granite, the Zoroaster granite, into them and folded to make these beautiful things. But then they got all the way back to the surface so that they could be eroded to make the unconformity, the time gap there, before deposition of the sediments of the Grand Canyon Supergroup on top.

This is a fascinating bit of history here. The same erosion between these old shifts and granites and the Nices and the Grand Canyon Supergroup was cut by a fault, which dragged up the layers of the Grand Canyon Supergroup. But the fault quit moving before the next erosion surface and the deposition of the sediments on top of that, because that surface is not cut by the fault.

Now, we're going to go a little bit east in the canyon. We don't see the oldest rocks here, the Vishnu, is not here. The Vishnu is not here, it's below the river. But we can see the Grand Canyon Supergroup very well, and it is a big pile of sediments. This is that erosion surface on top of it. And how much sediment in the Supergroup? Measure down, walk along a layer, measure down, walk along a layer, measure down. Keep doing that. It's two miles of sediment piled on top of each other, deposited in shallow water next to a sea as the land was sinking and so more sediments could build up, drop down in death Valley-type faults. And they're just ordinary sediments. This, for example, is a little tiny piece of that. This is a stromatolyte. Algae would grow on a rock, mud washes in, the algae grow up through it, mud washes in. Each of those little layers you see in this picture is a day's worth of algae, and growing up and growing up and growing up.

Let's go back now. We've worked up through the Vishnu and Brahma and Zoroaster and the Grand Canyon Supergroup. Now we're going to go up and look at these layers, the Tonto Plateau, the Tapeats, and Bright Angel, and Muav, You can find a trilobite fossil in the Bright Angel. In the modern world, you know that it very often when water flows, it makes ripples. Well, these are old ripples in the Bright Angel Shale. Here's a picture I took of ripples viewed edge on in that tapete sandstone. Here's another picture of looking down on a layer in the tapete. A worm craw through this and left the track.

We'll go back to the canyon, and we're going to move up and see this wonderful story happening right there. It also tells us something about a story that happened out in the west edge of the canyon and gave us what's called the Surprise Canyon. In this picture, we're looking at a closeup of these are all limestone. They were deposited underwater. First, the Muav was deposited, but then it was raised above the ocean, and a river caught into the rocks and gave us that erosion surface. Then the sea came up the river and deposited the limestone of the Temple Butte. But then this was eroded on top and came up out of the water. And then it got back underwater and the Red Wall was deposited. Over here, these are radioactive dates. You can't just look at this and know them, but it was 504 million years ago that the Muav was deposited. Then the erosion, the Temple Butte, the erosion, the Red Wall is only 340 million years ago. And something very similar happened on top of the Red Wall, and that gave rise to the surprise canyon that sits in the Temple Butte position farther to the west.

So we come back, and above the Red Wall, is the Supai. The bottom of the Supai, you may find some limestone with neat fossils like this Trilobite tail or this beautiful snail, this gastropod. Higher in the Supai, you find sediments that were deposited on land, like this fossil trackway. Above the Supai is the Hermit. A lot of them are floodplain deposits. In floodplain, you often find fern fossils like these beauties that were found along the Kyabab Trail in these USGS samples. Then we're going to go above the hermit and look at the Coconino where it lies on the hermit. This one you can see along the trail. Down below there are the river flood deposits, something off the edge of a river that we saw. And above are the fossil sand dunes of the Coconino. Right there is where sand fell down a big mud crack. Think of the Nile River depositing muds and then dunes of the Sahara blowing in and dropping down in this huge mud crack.

Up in the fossil sand dunes of the Coconino, you can find lots and lots of different layers that have little tracks of various creatures, as you see here. These are fairly common. The blue there shows millipede tracks, and there had to be time for the millipede to walk along there. Then we come up here and we're going to look right at the top. It's back to limestone. In the limestone, you find a lot of very interesting fossils. Then we go back, and if you were to go up on top of the Kyabab on the north rim and look north, this is the view when you get the north side of the Kyabab plateau. And if you make a diagram of that, the rocks of the Grand Canyon slant down to the right, and there are younger rocks at Zion on top of that, which slant down, and there are younger yet rocks at Bryce. And on top of those younger rocks of Bryce, we have these old trees, the archeological sites, and all of human history. It's a truly wonderful history. The oldest rocks you can find at the canyon are dated by radioactivity at about 1.84 billion years. So the canyon gives us about 40% of the history of the Earth of 4.6 billion years. So the whole thing is certainly not here at the Canyon. But I hope you see how much fun a geologist has reading this history and how rich and wonderful the history is.

 

The figure below is a static image of what you saw in the "Grand Canyon Strata" video above. Take a look at it and see if you could explain it to a friend.

Diagram showing the rock units exposed in the walls of the Grand Canyon, and the erosional surfaces (called unconformities) that separate some of the rock units.
Diagram showing the rock units exposed in the walls of the Grand Canyon, and the erosional surfaces (called unconformities) that separate some of the rock units.
Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

The marine Redwall Limestone was deposited next, so-named because it makes a red wall. The limestone is actually gray, with the red from rust and clay dripping down from red rocks above. The Redwall Limestone contains fossils of corals, sea lilies (crinoids) and shellfish (brachiopods), but with notable differences from the fossils of those general types found in limestones below, and both sets of fossils differ from those in limestones above. The Redwall Limestone contains caves and sinkholes, which in turn contain sediments associated with the rocks above. Caves generally form on land or possibly very close to land under shallow water, not beneath the open ocean, so the rocks were lifted near or above sea level and eroded after the Redwall was deposited.

Then, the sea flooded in, at least in the region that would become the western part of the Canyon, and deposited the Surprise Canyon limestone in erosional stream channels in the top of the Redwall. These rocks were not even described until the 1980s, and are only reachable by helicopter or arduous climbing. These Surprise Canyon rocks are not indicated in the diagram, above, which is what you would see on the Bright Angel Trail in the central Grand Canyon, but you could reach the Surprise Canyon by following the yellow arrow out of the picture to the left. Erosion cut the top of the Surprise Canyon before the deposition of more layers.

Next are sandstones, siltstones and shales, called the Supai Group and then the Hermit Shale, with plant fossils, lizard and other footprints, etc., at various levels through the rocks, indicating deposition on land in floodplain conditions. Insect fossils appear on the upward trip through the rocks, and then great dunes of the Coconino Formation with sand-blasted, wind-frosted grains and occasional lizard footprints. You might imagine the sand dunes of the Sahara spreading across the flood plain of the Nile River for these rocks. Marine conditions then returned with the Toroweap and Kaibab rocks, providing mostly limestones with sponge fossils and shark’s teeth as well as corals, crinoids and brachiopods, finally reaching the top of the Canyon.

If you're on the North Rim of the Canyon, gaze farther north. The rocks you're standing on slant downward to the north, and you are looking at rows of cliffs with younger rocks, up through the cliffs of Zion from the age of the dinosaurs, up through the lakes of Bryce from early in the age of mammals, up and up and up until finally you reach the trees and Native American sites older than the historical chronologies of Archbishop Ussher.

(By the way, if you’re interested in the carving of the Grand Canyon, have a look in Module 10 Enrichment.)