Sculpting the Earth

Article excerpt

Featuring complex shapes, precipitous drops, And multiple geologic traits, canyons manifest Some of nature's most spectacular scenery.

Mother Nature's recipe for a good canyon: In a large basin, place a layer of soft rock a couple hundred feet thick, run a stream over it, and occasionally sprinkle on a pinch of rain. (For an extra special canyon, uplift a generous portion of the rock.) Bake the mixture for about a million years, but not too much longer.

Mother Nature has been making canyons for a long, long time. And she's gotten pretty good at it. The best way to enjoy one of these tasty treats is to go on a canyon hike. A walk through a steep-walled canyon can be one of the most stimulating outdoor experiences you will ever have. The combination of flowing water, water-polished rock, and sharp cliffs makes the occasion truly memorable. And each canyon is unique. One can be huge--Arizona's Grand Canyon covers an area larger than New York City-while another can be a slot no wider than your arm span.

As encapsulated by the above recipe, canyon building involves a few simple but essential components. A canyon starts to form when a river or stream passes over soft rock, frequently sandstone or limestone. The soft rock permits downward erosion by the running water. The area needs only sparse rainfall, to ensure that erosion occurs mainly at the bottom of the canyon, with little effect on the sharp edges of the canyon walls. If these conditions persist over time, the river can continue to cut into the underlying bedrock and steep-sided cliffs will be formed.

Canyons are therefore found in arid regions that typically receive most of the annual precipitation in one season. When rain comes to a desert, it is usually sudden, intense, and dramatic. Large amounts of rainfall-- sometimes the entire year's precipitation-can occur in just a few hours.

On the other hand, if the area is wet with regular rainfall, the river banks will be continually eroded and wide valleys instead of canyons will be formed. Moreover, in northern latitudes, ancient glaciers appear to have scoured out certain areas, generating wide rivers today. Canyons will occur only where limited rain causes little weathering over time.

Streams and rivers continue to cut down into bedrock until they reach what is called their base level, when they can no longer move sediment away. When a stream is young, it is elevated above base level, allowing its flow to be rapid enough to continue eroding the bed. But an old stream has a slower flow that is unable to carve out much more material, and the bed has been eroded down to base level.

When an area is uplifted by geologic forces, the base level may undergo sudden readjustment, so that old streams can begin to cut downward again. For instance, the Colorado Plateau experienced rapid uplift, so that old rivers such as the Colorado were suddenly able to erode more material. This process has generated dramatic features with complex scenery, including the unusual combination of steep canyon walls with wide meanders in the river's course.

The combination of water running over soft rock in an arid region can form a beautiful canyon in about a million years or so. But after several million years, canyons are eroded away and the river valley becomes wide.

How is it possible?

If you've been to a canyon, you may have wondered how a narrow stream could have the energy to carve out such huge amounts of rock. It seems just impossible! But tourists seldom glimpse the stream's true power, observable only when it's flooding.

If the area's topography is such that it funnels nearly all the rainwater into a single streambed, the volume of water is focused and magnified. In a narrow canyon, the result is a huge flash flood--a wall of water that unleashes tremendous power, changing the stream from a petty trickle to a raging torrent 10 feet deep, within minutes. …