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.