A. How the moving water break up rocks.
B. How the heat of sun break up rocks.
C. What forms the earth’s surface layer.
D. What the soil is made up of.
E. Winds break up the rocks.

56___________

At first, the earth’s surface layer(层)was made up of huge blocks of hard, solid rocks. Then slowly, the rocks broke up into smaller and smaller pieces. Finally stones, pebbles(卵石)and grains of sand were formed. Many sorts of plants and animals lived and died on the surface of the earth. Their remains mixed up with the stones, pebbles and formed soil. The outmost layer of the earth is soil.

57___________

There are many ways in which large pieces of rock break into smaller ones. Wind, water, heat and cold help to break up rocks. When winds blow grains of sand against a large rock for a long time, the softer layers of the rock are slowly worn away. These leave holes and cracks(裂缝)in the rock, which become bigger and finally the rock breaks up into smaller pieces.

58___________

The moving water of streams and rivers also helps to break up rocks. As the water moves along, it varies with it small pieces of rock. These rub the larger ones. Thus, the larger rocks are worn down to smaller pieces.

59___________

The heat of the sun is also helpful to breaking up rocks. When the sun shines, the rocks become very hot. If these rocks are suddenly cooled, they may crack. Ice also plays a part in making soil. Water in the cracks can turn into ice in cold weather. As this happens, it becomes bigger and the cracks become wider, and finally break into smaller pieces.

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