154 THE EARTH AS MAN'S ABODE Glaciers Leave Typical Topographical Features. When debris accumulates along the sides of glaciers, lateral moraines, or ridges, are formed. Gentle rolling plains produced by glaciers drop- The material is composed ping their loads are called ground moraines. of some boulders and considerable clay. Glaciers not only carry debris along on their surfaces but also have rocks frozen into them that add to their grinding power. As glaciers slowly move, a few feet per year, perhaps, and a little more rapidly at the top and in the center tha'n at the bottom and along the edges, the bedrock surfaces are polished, scratched, and grooved, while angular boulders arc smoothed. When glaciers move down young valleys, they widen and deepen them, polishing and grooving the valley walls, and change their shape from a V to a U. Yosemite Valley is a glacial valley. When the huge glaciers of the five ice ages covered parts of North America, they rounded off the mountains and hills to form drumloids. An elliptical hill of glacial debris formed beneath an ice sheet is called a drumlin. 1. Compare a glacier with a river. STUDY QUESTIONS 2. How can one recognize glaciated topography? 3. Give the chief characteristics of young rivers and account for them. 4. What are the characteristics of the aging of rivers? 5. How can one tell the age of a given mountain range? 6. What types of scenery are caused by the unequal hardness of rocks? 7. Where are badlands found? 8. Account for the typical badland topography at Zabriskie's Point in Death Valley. 9. How are rivers rejuvenated? 10. What is an antecedent river? Give an example. 11. What happens to shore lines as land areas sink and lift?
UNIT III SECTION 5 DIASTROPHISM AND VULCANISM TEND TO REVERSE THE RESULTS OF DEGRADATION That part of the surface of any heavy body will become more distant from the center of its gravity which becomes of greater lightness. The earth therefore, the element by which the rivers carry away the slopes of mountains and bear them to the sea, is the place from which such gravity is removed; it will make itself lighter and in consequence will make itself more' remote from the center of gravity of the earth, that is from the center of gravity of the universe which is always concentric with the center of gravity of the earth. . . . The summits of the mountains in course of time rise continually. — Leonardo Da Vinci. Introduction. It is reasonable to conclude that the agents of gradation are capable of destroying all of the earth's land areas if given time. Furthermore, there is every reason to believe that more than enough time has elapsed to have brought about complete gradation of the lithosphere. The fact that land areas exist today indicates that there are forces which tend to balance the forces of gradation. Diastrophism and vulcanism are the forces which lift up land areas, and they are produced by the same changes that bring about gradation. There is no reason to think that these balancing forces will not continue as long as the earth has an atmosphere, and therefore as long as erosion continues, because it is erosion itself that disturbs a condition of equilibrium that exists and thus brings these compensating forces into action. It is fascinating to study how erosion thus seems to defeat its own ends. Diastrophism refers to all movements of the outer shell of the earth. Vulcanism refers to all phenomena that are connected with molten rock ma,terial and its movements; the term is derived from Vulcanus, the Roman god of fire, who was supposed to dwell in a volcano. Earthquakes Are the Result of Tremendous Changes in the Earth's Surface. During the past fifty years,- the world has known many disastrous earthquakes. Japan has experienced three such earthquakes — in 155