Guest post by Lisa Winters
"Know The Canyon's History, Study Rocks Made By Time, Very Slowly."
The line above is a useful mnemonic; the first letter of each word represents the first letter of each rock layer in the Grand Canyon. The Colorado River has been hard at work carving a gorge more than a mile deep into the Colorado Plateau, and because of it, we have one of the most awe-inspiring landscapes in the world. No need to be a geologist to get excited about these colorful stones.
You’ll notice a couple things immediately when you look across the Grand Canyon. First, it’s a really big ditch. Second, the canyon is made up of different-colored horizontal layers of rock stacked on top of one another. Each layer has a story to tell.
The dark black rock down at river level is a big leap back in time. This Vishnu Schist first appeared almost 2 billion years ago — as lava exposed to the heat and pressure of colliding volcanic islands with the North American landmass.
As you move up toward the rim, the rocks become younger, meaning they were added more recently. Each layer is a page of the story describing life at that time. Sandstones are sand compressed together, typically from old sand dunes or beaches. Shales are solidified mud, deposited in the waters of ancient river deltas. Limestones form at the bottom of warm, shallow seas (which tells us Arizona used to be underwater)! You can also find many fossils of sea creatures preserved in the rock, like trilobites, brachiopods, and crinoids.
ALEX LEONARD
The Grand Canyon was formed by a variety of events. First there was deposition, when sediments and soils accumulated on the landscape and eventually hardened into rocks. Next came uplift, or the raising of the Earth’s surface, likely associated with earthquakes or mountain-building as plate tectonics shifted 70-30 million years ago. The Colorado River then began slowly cutting through this newly-uplifted landscape, a process called downcutting. And finally, continued erosion by rockslides and tributary streams has led to the temple and butte formations that we see today.
Need an acronym to remember the process? D.U.D.E.!
MICHAEL QUINN, NPS
Not old at all. While the rock layers have been around for millions (even billions) of years, the canyon itself is young. The Colorado River started carving into the rocks of the Grand Canyon only 5-6 million years ago. The steep-walled canyon results from our arid climate — the Colorado River cuts down faster than rain water can erode the sides of the canyon. Otherwise, we would have a more typical wide, flat river valley!
What about Glen Canyon Dam? Great question. Big spring floods used to carry lots of rocks and sediment, which acted like sandpaper, wearing down the river channel and side slopes. Since 1963, Glen Canyon Dam has prevented dramatic changes in water level, so the canyon is likely eroding much slower. Nonetheless, these brightly colored rocks are constantly moving and shifting around us, adding more to the story every day.
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