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.
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 minutes)
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.
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.)