Narrative Source #1

Part 1
 
Sources for Performance Task:
 
Source #1
This article from Appleseeds magazine is about the formation of the Grand Canyon.
 


The Three Rs of Folding Time
Grand Canyon Style
by Leigh Anderson

 
There is a place—like no other in the world—where time seems to fold in on itself. Where the past meets the here-and-now, mountains meet oceans, beauty meets danger, and discovery meets mystery. This place is the Grand Canyon.
 
The Grand Canyon is 277 river miles long. At certain points, it’s more than a mile deep and as much as 18 miles wide. Going 50 miles per hour, it would take over five hours to drive its entire length! At such speed, you’d hardly see any of what makes the canyon truly grand: dazzling, glittering colors; fossils and wildlife; a great river snaking through . . . rock; and many-layered canyon walls giving glimpses of Earth’s history.
 
Geologists1 have many, different . . . [ideas] about how and when the Grand Canyon was formed. The story of the canyon’s beginnings is like a jigsaw puzzle with many pieces missing. There are a few things geologists do agree on. We’ll call them the three ‪“Rs: Rocks, River, and Rosion (actually, Erosion,2 but we’re going to cheat a bit!).
 
Rocks
The Grand Canyon’s walls are made up mainly of three types of rock: limestone, sandstone, and shale. Over . . . [thousands of] years, the rock built up layer by layer. Each new layer of rock pressed down on the layers beneath it. Then the Colorado River began to cut through these layers like a knife, exposing them for us to see. At the Grand Canyon today, 18 or more layers of Earth’s history are laid out for us to see. We can see backward in time! The rocks near the top of the canyon are . . . [very] old, but those toward the bottom of the canyon are . . . [thought to be over six times older]. What an amazing place for scientists to study the history of Earth.
 
Limestone, sandstone, and shale: Each of these types of rock was formed in a different way. Limestone is made from the fossilized skeletons of tiny organisms that lived in ancient seas. (Fossils are the super hard remains of plants or animals . . .) Sandstone is actually sand, pressed so hard over . . . [thousands of] years that it stuck together into rock. And shale is basically mud, left over from the bottoms of ancient lakes and marshes. Some rock is softer, some is harder, and they erode at different rates. When a layer of hard rock is on top of a layer of softer rock, amazing cliffs are created. . . .
 
River
Without the Colorado River, there would be no Grand Canyon. The river flows southwest from the Rocky Mountains to the Gulf of California, crossing through an area called the Colorado Plateau. As it flows, the river crosses Colorado, Utah, Arizona, and Nevada before flowing into Mexico and the Gulf. But the ancient Colorado was not the same river we know today. In fact, long ago it was probably more than one river. When those ancient rivers joined, the newly formed Colorado began flowing southwest. It bucked over dangerous rapids and frothed like chocolate milk in a blender as it carried mountains of dirt downstream. Like sandpaper repeatedly rubbing the same piece of wood, the fast-moving, sand-filled water slowly carved a groove in the rock beneath it.
 
But the Colorado River didn’t carve the canyon by itself. As ancient glaciers melted, the river and its tributaries3 flooded again and again. The floods cleared away the sand, gravel, and other sediment at the bottom of the river. Then, rocks and boulders, which had tumbled into the river, were able to grind and scrape the bedrock at the river’s bottom, further deepening the canyon.
 
As water moves through the canyon, it flows downhill, dropping in elevation. This makes the water flow faster, with more power to carve out the rock. Also, long ago, the land around the Colorado River began to rise bit by bit, bubbling upward like a giant blister. Known as uplift, this process continued over . . . [many] years. Uplift helped form the canyon we know today.
 
What Do You Think? The Colorado River Today
Today, the Glen Canyon and Hoover Dams tightly control the Colorado River. The river now runs more slowly. Floods no longer sweep away the sediment at the bottom of the river. The river-bottom sediment is 75 feet deep in some places! Because of the slower water and the thick blanket of sediment, the carving of the canyon has slowed down. . . .
 
‘Rosion
When rain falls on rocks, water seeps into the cracks in the rock. When the weather gets cold and that water freezes, it expands, or gets bigger. Over and over, water freezes and expands in the cracks. And slowly, the rock splits apart. Pieces of broken rock (from tiny to huge) fall into the canyon below. As they fall, they might hit another rock and send it tumbling too. When they finally reach bottom, some rocks are carried away by the Colorado. Others remain where they landed.
 
Heavy rains send great slabs of sediment, mud, and rock crashing down cliff faces, widening the canyon and carving new shapes into the giant red walls. The Colorado’s tributaries are busy, too, carving smaller side canyons. Sometimes these side canyons erode into each other, further widening the canyon. This is all part of the process of rocks, river, and ‘rosion that makes this canyon so GRAND!
 
1geologists: people who study rocks, minerals, and soils of the earth or a particular area
2erosion: a process by which rock, soil, or sand is gradually worn away by water, wind, or ice
3tributaries: smaller rivers and streams that flow into a larger river
 
The Three Rs of Folding Time Grand Canyon Style by Leigh Anderson. Copyright © 2008 by Carus Publishing Company. Reprinted by permission of Carus Publishing Company.

Comments