146 THE EARTH AS MAN'S ABODE changing to ice as a result of the heat of the sun and pressure of the accumulating mass. Glaciers produce typical amphitheater-like or U-shaped valleys, called "cirques." The best evidences of glaciation, however, are the scratches that are left on the surfaces of rocks. The last ice age was in full retreat about 25,000 years ago. Evidence of previous ice ages, many millions of years ago, both in the southern and northern hemispheres, have been found in some of our older rocks. During the Pleistocene age, great glaciers spread over Europe and North America, retreating and returning again at least four times. Great and relatively rapid changes must have taken place as plants and animals tried to adjust themselves to these changing conditions. Perhaps this period marked the dawn of mankind. Oceans Unceasingly Wear Away Their Shores. Two types of motion in the oceans are responsible for their erosive action along a million miles of shore. The first of these, the tides, has little effect except where the waters rush into narrow channels, sometimes piling up as high as 50 feet. The constant hammering of the waves is the main means by which the oceans gradually change the shore lines of the continents. An example of marine erosion is seen in the decrease in the size of the Island of Heligoland at the mouth of the Elbe in the North Sea. Its circumference was reduced from 120 miles in 800 a.d. to 45 miles in 1300 A.D. ; by 1900 it was only 3 miles, and the island would probably have soon entirely disappeared if the German Government had not reinforced it with a wall of concrete, so as to preserve it for a naval base. STUDY QUESTIONS 1. What would the surface of the earth be like if it had no atmosphere? 2. Show how the atmosphere is the ultimate cause of the gradation of the earth. 3. What is the most active agent of erosion? 4. Explain the formation of caves such as the Carlsbad Caverns. 5. Explain the formation of such U-shaped valleys as Yosemite Valley. 6. How do glaciers bring about erosion? 7. Explain the existence of swamps. 8. Why is the Dead Sea more saline than the Great Salt Lake? 9. Explain how rivers change the landscape. 10. Explain the formation of lakes. 11. Do you think that the northern part of North America will ever be covered by glaciers again? 12. Give a possible explanation of hot springs and geysers. 13. List the most important weathering agents.
THE GRADATION OF THE EARTH , 147 14. What are the most important types of chemical changes that are involved in weathering? 15. Name the outstanding scenic spots in the United States and state the most important agents of erosion which have been responsible for them. 16. Explain how temperature changes can bring about weathering. 17 List the various agents of erosion under the three topics: disintegration, decomposition, and transportation. 18. Mention four different ways in which water brings about erosion. 19. Why are the desert winds so effective in erosion?