If you could observe 2 billion years pass in a single hour, you'd see the land from which the Grand Canyon is carved wander across the globe, traveling as far south as the equator -- perhaps even farther. You'd see it dip below sea level, rise as mountains, dry into dunes, and get smothered under swamps. You'd watch as water deposited different sediments -- such as silt, mud, and sand -- atop it. Out of sight, compacted from above and cemented together by minerals, these sediments would eventually form sedimentary rocks such as sandstone, siltstone, limestone, and shale. Some of these rocks would resurface later, only to be eroded by wind and water. Others would remain buried. Because the canyon itself may be as little as 6 million years old, you probably wouldn't recognize it until the last 11 seconds of the hour, when two or more rivers began to cut down through the Colorado Plateau's rocks. As a frame of reference, keep in mind that all of human history would take up only a quarter of a second at the end of the hour.
Although you can't personally experience the canyon's 2-billion-year history, the layers of rock in the Grand Canyon record much of what happened. Because the rocks are both well preserved and exposed down to very deep layers, the canyon is one of the world's best places for geologists to learn about the Paleozoic era, and eras even earlier.
The record starts with the Vishnu Schist and other basement rocks, which consists of schist, gneiss, and granite. The canyon's oldest and deepest layer, it's the black rock draped like a wizard's robe directly above the Colorado River. Originally laid down as sedimentary rock, the layer was driven deep into the earth, and underneath a mountain range, more than 1.7 billion years ago. There, it got to temperatures so extreme (1,100°F/593°C) and was under pressure so great that its chemical composition altered, changing it to metamorphic rock, which is much harder and glossier than the others.
The Grand Canyon Supergroup (layer 11), a group of sedimentary and igneous (volcanic) rocks laid down between 1.2 billion and 800 million years ago, appears directly above the schist in numerous canyon locations. These pastel-colored layers stand out because they're tilted at about 20 degrees. Desert View is one good place to see them. Once part of a series of small mountain ranges, erosion shaved off the Supergroup, so that it disappeared from many parts of the canyon.
Where the Supergroup has disappeared, the Tapeats Sandstone layer (layer 10) sits right on top of the Vishnu Schist, although more than a billion years separate the two layers. Erosion created this huge gap, commonly referred to as the Great Unconformity. Because of it, the layers have little in common. While the Vishnu Formation predates atmospheric oxygen, Tapeats Sandstone contains fossils of sponges and trilobites from the Cambrian era's explosion of life. It also tells of the beginnings of the Tapeats Sea's incursion 525 million years ago. At that time, the water was so shallow and so turbulent that only the heaviest particles -- sand -- could sink. That sand eventually formed the sandstone.
The Bright Angel Shale (layer 9) forms the gently sloping blue-gray layer (known as the Tonto Platform) above the Tapeats Sandstone. It tells of a Tapeats Sea that had become deeper and considerably calmer in this area. Some 515 million years ago, the water was calm enough to let fine-grained sediment settle to the bottom. The sediment formed a muck that eventually became the shale. Above it is the Muav Limestone (layer 8), which dates back 505 million years. The Muav layer recalls a Tapeats Sea that was deeper still in this area -- so deep that feathery bits of shell from tiny marine creatures sank. These bits of shell, together with other calcium carbonate that precipitated naturally out of the water, created the limestone. Where not stained by the layers above, the Muav appears as a yellowish cliff underneath an obvious layer known as the Redwall Limestone. The Temple Butte Formation (layer 7), averaging 385 million years old, is made of purplish-colored dolostone in the east deposited from former tidal channels that aren't coastal and were laid down in the intertidal environment (marine or brackish). The Temple Butte is easier to distinguish in the western regions, where the cliffs extend hundreds of feet and marine fossils are prevalent.
About halfway between rim and river, the Redwall Limestone (layer 6) forms some of the canyon's steepest cliffs -- 800 feet high in places. This imposing rock layer reveals a Mississippian-age sea that deposited calcium carbonate layers across all of what is now North America about 340 million years ago. Silvery-gray under the surface, the Redwall is stained red by iron oxide from the rocks above. To see the Redwall's true color, look for places where pieces have recently broken off.
Just above the Redwall is the Supai Group (layer 5). Formed about 300 million years ago, these layers of sandstone, shale, and siltstone were deposited in tidal flats along shorelines. They usually form a series of red ledges just above the Redwall cliffs. Right above them, and even deeper red, is the Hermit Formation (layer 4), deposited in the flood plain of one or more great rivers around 280 million years ago. This soft shale usually forms a gentle slope or platform directly below the Coconino Sandstone (layer 3), which is the easiest layer in the canyon to identify. The third layer from the top, it's the color of desert sand and forms cliffs that are nearly as sheer as those of the Redwall. The Coconino was laid down as dunes in a Sahara-like desert that covered this land about 275 million years ago. Everywhere in this layer, you'll see slanted lines caused by cross-bedding -- where new dunes blew in atop old ones. While the other layers display fossils that become increasingly complex through time (the Supai contains fossils of insects and ferns, and marine invertebrates are common in the Redwall), the Coconino's only imprints are lizard and arthropod tracks that always go uphill. (This seems odd until you watch a lizard on sand. It digs in while going up, making firm marks in the process, then smears its tracks coming down.) Some of these fossils are visible along the South Kaibab Trail.
On the top are the canyon's youngest rocks -- the yellow-gray Toroweap Formation (layer 2) and the cream-colored Kaibab Formation/Limestone (layer 1), which forms the rim rock. Both were deposited by the same warm, shallow sea at the end of the Paleozoic era (270-273 million years ago), when this land was roughly 350 feet below sea level. Younger layers once lay atop the Kaibab Formation, but they have eroded away in most areas of the canyon. To see examples of this, look east from Desert View to nearby Cedar Mountain or northeast to the Vermilion and Echo cliffs.
Today, the ancient rocks are part of the Colorado Plateau. Between roughly 65 and 38 million years ago, this land was lifted by a process known as subduction (some observers believe that uplifting is still occurring). When a continental plate butts up against an oceanic plate, the heavier, denser oceanic plate is forced underneath it. Like an arm reaching under a mattress, this slipping -- or subduction -- can elevate land on the upper plate that's far inland from continental margins.
This happened in the Four Corners area (a region on the Colorado Plateau) during an event known as the Laramide Orogeny, which involved the Pacific plate subducting under the North American plate, pushing 130,000 square miles of land that was in the Four Corners area up to elevations ranging from 5,000 to 13,000 feet. This area, which consists of many smaller landforms, has six individual plateaus -- the Coconino and Hualapai on the South Rim; and the Kaibab, Kanab, Uinkaret, and Shivwits on the North Rim -- that are all part of the larger Colorado Plateau. The Laramide Orogeny also marked the beginning of the uplift of the Rocky Mountains.
Because the earth's crust is very thick under the Grand Canyon and its surrounding area, the layers of rock here rose without doing much collapsing or shearing. Where significant faulting did take place, the rocks sometimes folded instead of breaking. Monoclines are places where rocks bend in a single fold. As you drive up the 4,800-foot climb from Lees Ferry to Jacob Lake on the North Rim, you'll ascend the East Kaibab monocline. Driving east from Grandview Point, you'll descend the Grandview monocline. In both cases, you'll remain on the same rock layer, the Kaibab Formation, the whole time.
The Colorado Plateau is an ideal place for canyon formation for three reasons. First, it sits at a minimum of 5,000 feet above sea level, so water has a strong pull to saw through the land. This makes the rivers here more active than, say, the Mississippi, which descends just 1,670 feet over 2,350 miles. With an average drop of 8 feet per mile, the Colorado River in the Grand Canyon is 11 times steeper than the Mississippi. Second, the Colorado Plateau's desert terrain has little vegetation to hold it in place, so rain quickly erodes it. Third, rain often comes in monsoons that fall hard and fast, cutting deep grooves instead of eroding the land evenly, as softer, more frequent rains would.
The different layers and rock types make the resulting canyons more spectacular, perhaps, than any in the world. In addition to being different colors, the rocks vary in hardness and erode at different rates. Known as differential erosion, this phenomenon is responsible for the stair-step effect at the Grand Canyon.
Here's how it works: The softer rocks -- usually shales -- erode fastest, undercutting the cliffs above them, which are made of harder rock. During winter's melt-freeze cycles, water seeps into cracks in these now-vulnerable cliffs, freezes, and expands, chiseling off boulders that collapse onto the layers below. These collapsed rocks tumble down into boulder fields such as those at the bases of the canyon's temples. The biggest rock slides sometimes pile up in ramps that make foot descents possible through cliff areas. Where soft rock has eroded off of hard rock underneath it, platforms form. One such platform, known as the Esplanade, is obvious in the western canyon. The end result is a series of platforms and cliffs.
Runoff drives the process, and more of it comes from the North Rim. This happens for two reasons. First, the land through which the canyon is cut slopes gently from north to south. So runoff from the North Rim drains into the canyon while runoff from the South Rim drains away from it. And more precipitation falls at the higher elevations on the North Rim -- 25 inches, as opposed to 16 inches for the South Rim. As this water makes its way -- often along fault lines -- to the Colorado River, it cuts side canyons that drain into the main one.
These side canyons tend to become longer and more gradual through time. Since the runoff can't cut any lower than the Colorado River, it eats away the land near the top of each side canyon. As this happens, each canyon's head slowly moves closer to its water source -- a process known as headward erosion.
Standing at Grand Canyon Village looking down the Bright Angel fault, you may notice that the gorge formed along it is longer on the Colorado River's north side. This is typical of the Grand Canyon's side canyons. Because more water comes off the North Rim, more erosion has taken place on that side of the river, and longer canyons have been formed.
Then Came the Floods
Eroded material has to go somewhere. The rocks that fall into the side canyons are swept into the Colorado River, usually by flash floods during the August monsoon season. While it may be hard to imagine a current this strong in what is usually a nearly dry side canyon, look again at how thousands of tiny drainages converge like capillaries into a single significant creek bed. In most cases, the water over several square miles of hard land drains into one relatively narrow rock chute. A downpour, then, can generate floods that are immensely powerful and very dangerous.
Below each significant side canyon are boulders, which are swept into the Colorado River by these floods. These boulders form dams in the larger river, creating rapids where the water spills over them. The water above each set of rapids usually looks as smooth as a reservoir. Below the first rocks, however, it cascades downstream, crashing backward in standing waves against large boulders. Before Glen Canyon dam began blocking the river flow in 1963, the Colorado River broke up many of the biggest rocks during its enormous spring floods. These floods, which commonly reached levels five times higher than an average flow today, swept along small rocks, which would in turn chip away and break apart boulders, eventually moving them downstream. For the canyon to have reached its present size, the river had to have swept away more than 1,000 cubic miles of debris. Now, with the sizable spring floods a thing of the past, less debris moves, and the rapids have become steeper and rockier.
While it's fairly easy to explain how the side canyons cut down to the Colorado River's level, it's much harder to say how the Colorado cut through the plateaus that form the sides of the Grand Canyon. Unless the river was already in place when these adjoining plateaus started rising roughly 60 million years ago, it would have had to first climb 3,000 feet uphill before it could begin cutting down. The explorer John Wesley Powell, who mapped the Colorado River in 1869, assumed that the river had cut down through the land as the land rose. The river in the eastern canyon may indeed be old enough to have accomplished this. The western canyon, however, is much younger. In fact, there's no evidence of a through-flowing Colorado River in the western Grand Canyon before about 5 million years ago.
Geologists have proposed a number of theories about how the river assumed its present course, none of which is supported by a strong body of evidence. The difficulty is that only scattered pieces of evidence can be used to date the canyon precisely. Most theories center on the idea of an ancestral Colorado River that flowed through the eastern canyon, exiting the canyon via a channel different from its current one. This ancestral river would have been diverted onto its present course by another, smaller river that probably reached it via headward erosion. One theory holds that this "pirate" river cut headward all the way from the Gulf of California, while another maintains that it may have originated on the Kaibab Plateau during a period when the climate was wetter than it is today. No one is sure what happened, and the debate is still open.
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