Title: Adventures with animals and plants
Identifier: adventureswithan00kroe (find matches)
Year: 1948 (1940s)
Authors: Kroeber, Elsbeth, 1882-; Wolff, Walter Harold, 1901-
Subjects: Biology
Publisher: Boston : D. C. Heath
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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5'- The Earth and Its Inhabitants Change unit x
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Fig. 467 Rivers of ice, knowii as glaciers, wear away the land, breakiiig tip bard rock. Note the broken rock below the glacier, (geological SURVEY OF CANADA) Fig. 468 As the temperature changes the ex- pansion and contraction of the surface layer causes the rock to scale. This is weathering. (gilbert — U. S. GEOLOGICAL SURVEY) Stream, the small rocks wear away even the hardest rock bv their rubbing and pounding-. Wearing away of soil and rock is called erosion. You have seen it on a small scale after every heavy rain. Even strong winds may erode the soil; and animals, burrowing in the ground, by loosening the particles of the soil, help erosion by wind and water. Rocks are broken up in other ways too. Changes in temperature cause them to crack and crumble. This breaking up of rocks in air is called weathcrh?g. Sometimes a seed falling into a tiny crack grows and splits up the rock. Or acids produced by the roots of plants and by organisms of decay dissolve some rocks, such as limestone. In connection with this try Exercises i and 2. In general, erosion is most rapid on steep mountain slopes. Thus mountains tend to flatten out gradually, throughout the ages. It has been calculated that ero- sion reduces the average height of the United States one foot in every seven thousand to nine thousand years. New land is formed. AVhat becomes of the rock and soil that crumbles and is washed or blown awav? Some of it is piled up elsewhere on the land, and as the mountains flatten, the lowlands tend to be built up. But most of it is carried by streams to the oceans or lakes into M'hich they empty. It has been estimated that

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