Our Window on Nature

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Drifting Dunes

Filed under: Geology — Lowell Christie -- February 15, 2006 @ 2:44 pm

Rock on DunesBased upon what you see in the movies, you would expect deserts and drifting sand to be synonymous, but actually a sand dune is a rather unique occurrence. So unusual are sand dunes, in fact, that several of the finest (and most colorful) dune fields have been preserved as state parks or national monuments.

You can scramble up black sand dunes near Moses Lake in Washington or surf down shimmering slopes at White Sands National Monument in New Mexico. Coral Pink is both the name and the color of a series of dunes in Utah. But the most massive fields occur in none of these places. The real skyscrapers appear in, believe it or not, a mountain valley in southern Colorado.

Now it’s true that sand crops up nearly everywhere – in the mountains, on the beaches, in your back yard – but it piles up into dunes only where grains of the correct size confront winds of sufficient velocity. The raw material, most often quartz because it resists chemical breakdown, must be heavy enough that it doesn’t simply blow away, yet light enough to bounce or roll along the surface of the ground ahead of the wind.

It takes only a seven-mile-per-hour breeze to start grains of sand as small as .08 inches tumbling across the ground.
But like a flowing river carrying pebbles and boulders down a ravine, the amount of weight that the wind can carry increases faster than its speed. Doubling the wind velocity more than triples the size of sand grains involved. Even then it takes 17-mile-per-hour gusts to start any type of mass sand movement.

All things being equal, the sand would spread out evenly over the ground, but all things aren’t equal.

Moving sand travels easily over hard surfaces, since it bounces three times as often as it takes to the air. But a soft spot on the earth has an almost magnetic attraction for bouncing sand, and as more sand accumulates it makes a larger trap for grains blown in during the next windstorm. Eventually, a dune is born.

Or perhaps sand builds up behind a plant or a rock. The end effect is the same. Instead of an open plain where the wind has its way, the small mound grows as the wind continues to bring in more material.

You can discover the direction of the prevailing winds by studying the shape of a dune. Where the wind blows primarily from one direction, the windward side of the dune has a gradual slope, while the sheltered side is steeper.

This simplest type of dune actually “travels” in the direction of the wind. Gusts not only sweep new sand up and over the dune, but they also gather up sand on the windward slope and scatter it down the other side. Thus the entire dune structure turns over on itself constantly, much like a wheel, and the grain of sand that falls over the crest today will be buried, only to reappear as the dune moves on. And so it revolves again and again.

Because of the constant rolling motion, this type of dune cannot reach great height – not so with the more stationary dune that forms when conflicting winds take turns chasing sand grains back and forth across the countryside.

At Great Sand Dunes National Monument in Colorado, two different prevailing winds create what’s called a reversing dune. Since the winds alternate directions throughout the year, the dunes cannot continue moving in one direction. Although most of the wind comes from the southwest, periodically a violent storm blows in from the northeast, and in several hours’ time cancels many weeks of work by the prevailing southwesterly wind. Because of this, these Colorado dunes grow upward, now stretching as high as 700 feet above the valley floor.

But a reversing dune is an unusual phenomenon. Most sand dunes are vagabonds, continually on the move. They typically progress from several to perhaps a dozen feet a year, but there have been important exceptions. In California one dune was measured as having traveled 82 feet in one year. But that’s nothing compared to the dune at White Sands National Monument that moved nearly the length of a football field. This dune is said to have shifted 264 feet in a single year.

As you can imagine, the slower movement of more typical dunes still creates a problem for the few plants that try to colonize this everchanging terrain. Species survive only if they can grow faster than the dune that’s building around them. As the plant keeps pace with the rising level of sand, its roots may still be gathering moisture and nutrients from soil 20 or 30 feet below.

When the dune tumbles on, it sometimes leaves a monstrosity behind. In one case, yuccas with trunks 45 feet tall were found abandoned by a passing dune. And at White Sands you see natural towers of gypsum sand knitted together by roots – the only evidence that there a dune once stood.

There are dune fields worth visiting in many states, each with its own story to tell. The slopes near Yuma, Arizona, will look familiar to you. They’re the site of most “Sahara” movies filmed in Hollywood. And besides providing a pleasant spot to work on your suntan, the sandy shores of Lake Michigan provide the raw materials necessary to mold automobile engines. It requires 400 pounds of sand per engine.

But the best reason for visiting sand dunes is to enjoy the view. They appear stark, uncluttered, sometimes bleak, but with a texture that changes hourly as the sun shifts across the sky. And the quiet – there’s something about walking on sand that makes you feel alone and attuned to nature. Try it and see if you don’t agree.


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