THE ACTION OF WIND
Only during the last quarter of a century have geologists come to
properly appreciate the really important geological work of the wind.
One reason for this is the fact that people live mostly in humid
regions where the soils are largely effectually protected against
wind action by the vegetation. But even in such regions, wind action
is by no means negligible. One has but to observe the great clouds of
dust raised by strong wind from freshly cultivated fields during a
little dry weather in the late spring. Much of this dust is carried
considerable distances, often miles, and in some cases young crops
are injured by removal of soil from around the roots, while in other
cases young plants are buried by deposition of the wind-blown material
over them. In humid regions, the action of the wind is perhaps most
strikingly exhibited along and near shores of sea and lakes, where
loose dry sands are picked up and transported in great quantities,
often depositing them as sand dunes, which may form groups of hills
covering considerable areas. Very conspicuous examples are the sand
dunes of Dune Park in northern Indiana, and the dunes along the coast
of New Jersey.
But the action of wind is most strikingly effective in desert and
semiarid regions. The importance of the work of wind is made more
impressive when we realize that about one-fifth of the land of the
earth is desert.
In deserts some of the ordinary agents of weathering and erosion
are either absent or notably reduced in effectiveness. Thus, stream
action is, in general, reduced to a minimum; weathering effects due
to moisture in the air are notably reduced, and either frost action,
or wedge work of ice, is relatively unimportant due to lack of water.
Change of temperature between night and day is, however, unusually
important as a process whereby rocks are broken up due to relatively
rapid expansion and contraction in deserts because such temperature
changes are exceptionally great, and rocks and soils are almost
everywhere directly exposed, being free from vegetation.
The finer grained materials, especially sand grains, in deserts
are picked up by the wind and driven, often with great velocity,
against barren rock ledges and large and small rock fragments. By
this process (corrasion) the rocks are worn and often polished by the
materials blown against them. The principle is that of the artificial
sand-blast, used in etching glass, or cleaning and polishing building
and decorative stones. Under favorable conditions wind-driven sand
accomplishes noticeable erosion in a surprisingly short time. Thus, in
a hard wind storm, a plate glass window in a lighthouse on Cape Cod was
worn to opaqueness, while in a few weeks or months the directly exposed
window glass may there be worn through.
The great erosive effect of wind-driven sand is relatively close to
the ground because the larger and heavier fragments are not lifted to
very considerable heights. For this reason ordinary telegraph poles
are difficult to maintain in desert regions because, unless they are
specially protected, they are soon cut down by sand swept against
their bases. In the desert regions of our Southwestern States cliffs
rising above the general level of the country are often undercut by
wind erosion, sometimes with the development of large caverns. (See
Plate 1.) Even the high portions of great ledges are there more or less
fantastically sculptured by wind erosion, the softer portions being
more deeply cut into than the harder. The famous sphinx of Egypt has
been notably roughened by action of this kind.
The enormous power of high winds to transport rock material in desert
regions is strikingly illustrated by the great sand storms of the
Sahara Desert, where sand and dust, forming clouds with cubic miles of
volume, sweep for many miles across the country. Some one has estimated
that every cubic mile of air in such a storm contains more than
100,000 tons of rock material. It is said that an army of 50,000 men
under Cambyses was buried under the sands of a storm in the desert of
northern Africa.
Dust from some of these storms is known to be driven hundreds of miles
out over the Atlantic Ocean, there to settle in the sea. In mountainous
desert regions, like the Great Basin of our Western States, the general
tendency is for the rock materials wind-eroded from the mountains to
be carried into the intermontane basins or valleys. Some basins of
this sort are believed to contain depths of 1,000 to 2,000 feet of
wind-blown material.
A special kind of wind-blown material called "loess," is a sort
of fine-grained yellow, or brown loam which, though relatively
unconsolidated, has a remarkable property of standing out as high
steep cliffs or bluffs along the banks of streams. Many thousands of
square miles of northern China are covered with loess. Among many other
regions, thousands of square miles of parts of the States of Iowa,
Nebraska, and Kansas are covered with loess, which, in this case, is
believed to be fine material gathered by winds from the region just
after the retreat of one of the ice sheets of the great Ice Age, when
there was very little vegetation to hold down the loose soils of
glacial origin.
Much as snowdrifts are formed, so, in many places, the wind-driven
sands are built up into sand hills or so-called "dunes." Dunes are very
common in many places, as for example, along our middle Atlantic coast;
in Dune Park of northern Indiana; and in the great arid and semiarid
regions of the Western States. Where there is a distinctly prevailing
direction of wind, the sand is blown to the leeward side from the
windward side, and the dunes are caused to migrate in the direction of
the wind. The burial and destruction of forests, and the uncovering of
the dead trees is not uncommonly caused by migration of sand dunes, all
stages of this phenomenon being well exhibited in Dune Park, Indiana.
The rate of dune migration is very variable, but study in a number of
places has shown a rate of from a few feet to more than 100 feet per
year. Arable lands, buildings, and even towns have been encroached upon
and buried under drifting sand. An interesting example is a church in
the village of Kunzen, on the Baltic seashore which, in a period of
sixty years, became completely buried under a dune and then completely
uncovered by migration of the dune. Much destruction has been wrought
by shifting sands on the Bay of Biscay, where farms and even villages
have been overwhelmed. The ruins of the ancient cities of Babylon and
Nineveh are buried mostly under wind-blown sand and dust. There is
good reason to believe that the climate of central and western Asia is
now notably drier than it was a few thousand years ago, and this may
help to explain the burial of many old cities and villages there under
wind-blown deposits.