Fill in each numbered blank with a suitable word. Throughout the ages, the (1) surface of the earth has been built up in some places and worn down in other places. The wearing down of the land is called erosion. Wind, water, air, ice and heat all help to (2) cause erosion. As the wind (3) blows over the land, it often picks up small grains of sand. When these grains of sand strike (4) against solid rocks, the rocks are slowly worn away. Later, the wind may pick up these new rock particles, and with them wear away other rocks. In this way even very (5) hard rocks are worn away by the wind. When particles of rock or soil become loosened in any way, running (6) water carries them down the hillsides. Some rock and soil particles are carried into streams. The streams may then carried them into rivers, and the rivers may carried them into the (7) ocean. Land that is (8) covered with trees, grass and other plants wears away very slowly, and so (9) loses very little of its soil. The (10) roots of plants help to hold the rocks and soil in place. When rains fall in a forest, the leaves of the trees and the soft soil beneath them are able to hold a great deal of (11) water. Water that falls on grasslands runs away more slowly than water that fall on (12) bare does. Water that flows slowly carries (13) fewer soil particles than water that flows rapidly. Thus, forests and grasslands help to (14) slow down erosion. Even (15) where the land is thickly covered with plants, some erosion goes on. Sometimes there is a long period of rainy (16) weather. In the spring the (17) melting snow turns to large quantities of water. At these times the soil cannot (18) hold all the water. It then runs down hills in streams. As the streams carry away some of the soil, the stream (19) beds get deeper and deeper. After thousands of years of such (20) erosion, wide valleys are often formed.