WIND AS AN AGENT OF EROSION

In Canada we cannot say that wind is an important agent of erosion except locally where it might actually be the most important one. We certainly know that it is active when we travel some beaches in windy weather and see wind spraying sand along the ground, or building dunes which inundate and bury the marginal forests. We can see it again in some of the very dry areas of Western Canada in swirling clouds of dust and fine soil. And in any city street we may be reminded of the carrying power of the wind when we find ourselves with a speck in the eye.

If wind is thought of as moving air then its importance in erosion is very great indeed. Without the winds to blow moisture from ocean areas over the continents there would be no rain and snow and so no erosion by rivers and glaciers. Without the winds of the world to blow to and fro and mix cold air and warm air the climates of the world would be far different from those we know now. Climates affect weathering profoundly and largely control erosion. Without the wind, there would be no waves on the face of the sea to cut into the land and erode the cliffs, for most waves are the result of wind moving over the water.

The waves which gnaw away at the shore of Great Britain may be born of the wintry winds far out in the North Atlantic, off Newfoundland and south of Greenland. Surf on the Pacific Coast of the United States may be offspring of the great air-currents far south of the equator in the distant reaches of the Pacific. Erosion of Australian coasts may be done by rollers from thousands of miles away, generated in the storms of the 'roaring forties' far below the southern continents. These are indirect ways in which wind is an agent of erosion.

When we examine the ways in which wind is directly active in erosion we must recall that all modifiers of scenery have two different aspects—those which tear down something already there, which we can call the destructional effects, and those which build something not there before and which we can call the constructional effects. With wind we can see at once that where sand and dust are picked up by the wind, something must be in the process of being torn down. When the wind subsides the sand and dust it was carrying are deposited. Where this takes place something will be added or constructed. Part of nature's energies are devoted to tearing down and part to the building of something new. What happens at any one place will be the result of the balance between the two.

Where is Wind Active?

Now, you may ask, what would be the conditions under which wind would be an effective agent of erosion? In regions where sufficient rain and snow fall to keep the ground moist, wind is but a minor factor in the erosion of rocks and soils. Water is not a very strong binding material yet it is strong enough to defeat the best efforts of wind, for the air is of very low density. Plants bind soils together with their roots in addition to fending off the wind with their above-ground parts. Plants also serve to hold and maintain moisture in some areas where, without them, there would be times during the year when the soils would dry up and be vulnerable to wind attack. For wind to be effective, there must be some material to be blown about. One would hardly expect the wind to be an active eroding agent in a region where there are no soils or loose materials. Wind will lift loose, dry materials and carry them off somewhere else, so for wind to continue to be effective in a region, one would require that rocks, that will go to pieces to produce more fine-grained materials for the wind, must also be present.

So obvious as to be easy to miss among the requisites for effective wind erosion is wind itself. Strong, prevailing winds would be best. Thus, to find places where wind would be most effective we would look for dry places, with no vegetation, with strong winds, with loose sands and rocks underneath which would give rise to more loose material when the surface bit is gone. One of the places on the earth best fitting all these is the Libyan Desert, and to a lesser degree it is also true of an estimated one-fifth of the land surface of the earth where neither moisture nor vegetation can effectively deter wind erosion. These places are more or less what we can call the desert regions of the world.

There are some other places in which we may find wind effective because of the extra strength of one of the factors we have listed. Along some river banks, for example, large quantities of sand and silt may be left high above spring watermark to dry out during the summer. Along some lakeshores, and commonly along ocean beaches, great quantities of sand may be pushed up by waves. The sand is so porous that it dries rapidly and thus becomes available to the passing winds. Retreating glaciers leave vast areas of loose debris open to wind erosion as soon as it dries. Parts of southern Canada and the northern United States show evidence of having been in this condition at the end of the great continental ice-age. Occasionally seacoasts are cold and barren of vegetation and yet have sand. Many parts of the Labrador Coast show strongly the action of wind because of this peculiar combination of circumstances. The tops of mountains are often subjected to very high winds which, combined with the absence of vegetation, result in cleanly scoured rock surfaces.

Deflation and Abrasion

Wind effects are of two kinds. The simple picking up of a grain of sand in a gust of wind and dropping it somewhere else is called deflation. Vast areas in the Libyan Desert and in parts of Mongolia have been deflated to the point of developing considerable relief directly from this activity of the wind. Deflation was very obvious in the thirties for those who lived in the Dust Bowl country of the southwestern United States or the western Canadian prairies. Giant dust clouds darkened the sky and swept across the country. Great areas of topsoil were completely stripped and blown away. The mere presence of these dust clouds proves deflationary powers of the wind, for where they deposit their millions of tons of dust and sand they show conclusively that 'what comes down must go up', if we may be permitted to twist an old one.

The second way that wind is an effective agent of erosion is by abrasion. Lighthouse-keepers of Sable Island off Nova Scotia find the glass in their windows being pitted and frosted. In some of the desert regions, railways find the natural sand-blast cuts into and thins the steel rails and cuts off wooden telegraph poles. Millions of little particles of hard rock or mineral, pounding the surface of the rocks, wear and tear the surface away, and add to the sand in the wind, in the process.

Sand Dunes

When the wind dies out or the land obstructs the motion of the air, deposition of what it is carrying will take place. Sometimes the sands are piled irregularly into drifts, even as snow is. When the drifts get larger they eventually become dunes. Occasionally where the wind blows from the same direction over long periods the piles of sand may develop perfect crescentic forms, although in most regions of drifting sands such forms are rare. When wind blows in one direction, sand from the front of the dune may be blown up and over the crest to fall at the back. This removal at the front and addition at the back will mean that the dune will appear to move downwind. Such moving dunes are common in deserts and in any coastal regions.

Here, at Cavendish Beach in Prince Edward Island National Park, waves have scalloped the edge of the beach into cusps and winds have blown the dry sand above high tide level into hummocky dunes. The reddish sandstones, which supply the sand when they disintegrate, are just visible in the left foreground. © Canadian Government Travel Bureau, 1963

Migrating sands are often very destructive. In Africa and in central Asia old cities have been overwhelmed in shifting sand. In the southwestern United States, along the shores of some of the Great Lakes, in coastal regions of France, and along the Baltic, creeping dunes have covered roads, forests, and even villages. Sometimes they inundate and cover buildings only to release them again, years later, as the sand mass moves on.

Other Effects

Grains of sand in wind-blown deposits are different from grains of sand developed in other places such as streams and beaches, for the abrasion of particles by other particles leaves their surfaces pitted on a minute scale which makes them look frosted. Rock surfaces in such areas are commonly fluted, pitted, or even faceted. Boulders sometimes have flat faces ground on them by the abrasion of the moving sand and become known as ventifacts. They are characteristic of windy areas and when they are found in regions of heavy vegetation and abundant rainfall they are indicators of formerly drier times.

Dust-size particles of wind-blown material sometimes travel great distances and accumulate in great deposits called loess. Because loess is derived from desert regions where leaching by water is almost non-existent, it usually forms soils of wonderful fertility. Thus it is that the extensive loess deposits of China have been able to support millions of Chinese for many centuries. The Yellow River and the Yellow Sea get their names from the loess suspended in their waters. In Northern Argentina, in New Zealand, along the Rhine and the Mississippi River systems, other deposits of this wind blown material are to be found.

Volcanic Dust

When volcanoes explode into activity and after that belch large amounts of fine dust into the air, the winds of the earth may carry the dust over the whole world. This settles out gradually as a thin film everywhere and contributes something to all deposits. Since it is in the air everywhere it means that we are breathing it all the time. Theoretically we are taking into our lungs, every time we breathe, a tiny amount of dust from the volcanoes of the world, a tiny amount of dust from each or all of the deserts of the world, and a tiny amount of dust from all the other contributors. People used to look askance when geologists talked of this but the atomic bomb blasts have put into the atmosphere tiny amounts of dust which because of their radioactivity, can be traced. Now when the snow or the rain falls on Vancouver or Winnipeg or Halifax we can show certainly that it contains small fragments of coral atolls in the Pacific Ocean, which were blown to bits by atomic explosions and then wafted over the surface of the earth by winds.

Summary

To summarize, then, we have seen that wind is vital to most processes of erosion because it moves water from the oceans over the land and drops it there to begin rivers and glaciers; that wind generates the eroding waves and currents in the oceans; that it blows dust around the earth so that all of us breathe the Sahara, southern Saskatchewan and the volcanic explosions of past times. We have seen that wind moves great masses of shifting sand and that rocks in dry areas may be polished and abraded by natural sand-blast. In deserts, however, we must note that most of the scenery is the result of erosion by the rare rains that fall, rather than by wind.