at different places. In the Province of Shansi, China, are deposits several hundred feet thick. Near Kansas City, Missouri, there is a deposit at least thirty feet thick. Sand is also carried by the wind, but since it is heavier than dust, a much stronger wind is necessary for carrying it. Sand never rises to any great height, but is carried near the earth. There are many obstacles on the earth to stop it. Sand blown by a strong wind has considerable cutting force. For this reason sand blasts are sometimes used for etching glass. In the semi-arid regions of western United States, projecting rock cliffs are often carved into strange and fantastic forms. The softer parts of the rock are cut more rapidly so that the harder parts are left projecting. Even the hard rocks are not able to withstand. the long continued action of blown sand. Photograph by Detroit Publishing Co. Sand dunes. Mounds and ridges of wind-blown sand are called dunes. In the dry parts of western United States there are thousands of square miles where dunes are abundant. On the east and south shores of Copyright by Underwood & Underwood, N. Y. FIG. 62. A forest on Cape Cod planted as a windbreak is being buried by drifting sand. Lake Michigan are other sand dune regions. Dunes vary much in size. Very large ones occur in central Wyoming. The largest ones are found in the Sahara Desert where they reach a height of several hundred feet. The growth of a dune begins when the sand is stopped by some object on the earth's surface. Once started, the dune increases by the addition of more sand. They are not stationary, but migrate in the direction toward which the prevailing wind blows. The migration of the dunes may do great damage by covering up farm land, forests, railroads, and some FIG. 63. The pines and grasses shown at the left hold the sand and prevent the migration of the sand dune. times buildings. Trees, shrubs, and grasses that will grow in sand are sometimes planted on dunes to prevent their migration. The roots of the plants hold the sand. Effect of change of temperature on rocks. Changes in the temperature of the atmosphere also have a mechanical effect on the earth's surface. You have already learned that heat expands and cold contracts objects. Exposed rock surfaces are heated in the day and cooled at night. On mountain tops this daily change of temperature is very great. This continual change of temperature causes rocks to scale off. In mountainous regions great piles of this broken off debris collect at the base of mountains and cliffs. Such deposits are called talus. The total effect of this shell FIG. 64. Permission of U. S. Geological Survey. Great piles of debris, called talus, have collected at the base of these mountain peaks in Colorado. ing off of exposed rock surfaces through the ages has been very great. If you examine the surface of a boulder or rock, you can notice this shelling off effect. Chemical. The oxygen of the air is its most active chemical element. If a piece of iron is exposed to the air in the presence of moisture, it rusts. The oxygen of the air unites with the iron and forms a red rust; in time the whole of the iron is converted into the oxide. Oxygen attacks many rocks in a similar manner. As the oxidizing process is continually going on, the total effect is very considerable. Other constituents of the air may also attack rocks chemi cally, but oxygen is the most powerful. Work of rain in causing relief. All have observed that streams are muddy after a heavy rainfall. This FIG. 65. the streams. Permission of United States Forest Service. Gullies in a hillside formed by running water. muddiness is caused by the rain carrying soil into With every rain more or less earth is discharged into streams; the total effect of rain erosion is enormous. Perhaps rain has had more to do with producing relief features than any other one agent. Its effect as an erosive agent is more notice |