Wind, water, air, ice and heat all work to cause erosion. As the wind blows over the land, it often (26) _______ small grains of sand. When these grains of sand strike against solid rocks, the rocks are slowly worn away. In this way, even very hard rocks are worn away by the wind.

When particles of rocks or soil became loosened in any way, running water carries them down the hillsides. Some rocks and soil particles are carried into streams and then into the sea.

Land that is covered with trees, grass and other plants wears away very slowly, and so loses very (27) _______ of its soil. The roots of plants help to (28) _______ the rocks and soil in place. Water that falls on grasslands runs away more slowly than water that falls on bare ground. Thus, forests and grasslands help to slow down erosion.

Even where the land is (29) _______ covered with plants, some erosion goes on. In the spring, the (30) _______ snow turns into a large quantity of water that then runs downhill in streams. As a stream carries away some of the soil, the stream bed gets deeper and deeper. After thousands of years of such erosion, wide valleys are often formed.