176 THE EARTH AS MAN'S ABODE the western and southwestern portions of the continent were submerged. Mount Rainier in Washington and Mount Shasta in California were active volcanoes, and many lava flows occurred, some 5000 feet in depth. Toward the close of the Tertiary period the Cascade and Coast Ranges in western North America were uplifted, the whole Rocky Mountain area was elevated, and the Colorado River began cutting the Grand Canyon. Even an Epoch Is a Long, Long Time. The Pleistocene epoch of the Quarternary period of the Cenozoic era includes a very brief time, only one million years, in the history of the world ; and yet it was during this period that horses and elephants approached their modern form and the Cro-Magnon man developed. During this epoch there were five great ice ages, in which large sheets of ice, possibly thousands of feet thick, moved down into the part of North America now occupied by the United States as far as Illinois and Ohio and receded again. Great ice floes seem to be peculiar to this period and rank high in importance as terrestrial events. Each of these ice ages lasted for tens of thousands of years, and between each there were warm periods which were at least fifty thousand years long. About twenty thousand years ago the last great glacier retreated, leaving behind it the Great Lakes and thousands of smaller lakes in northern United States and in Canada. Niagara Falls is estimated to have taken 20,000 years to cut its way to its present position from the place where it started as the last glacier receded — hence the conclusion that the last glacier was receding 20,000 years ago. This glacier still covers a large portion of Greenland and regions around the North Pole to an extent of 6,000,000 square miles. If it melts, it is estimated that the level of the sea will be raised about 150 feet. We are still living in a comparatively cool period of the earth's history. A few millions of years ago the earth was warm to the poles, as proven by the rich coal deposits in Alaska. STUDY QUESTIONS 1. What is meant by an unconformity, and of what value is it to the geologists? 2. How is the geologist able to study successive layers of stratified rock? 3. How would you account for the formation of salt beds? 4. When were the coal beds formed? What climatic conditions probably existed when the coal beds were laid down? 5. Which of the mountain ranges in the United States is (a) the oldest, (b) the youngest?
THE HISTORY OF THE EARTH 177 6. How would you account for the rather sudden demise of the dinosaurs? 7. If each minute should represent 1,000,000 years, how long has the earth been in existence? How long has man been in existence? 8. In the place where your home town is located, what was the probable condition of the earth's surface during (a) Ordovician, (b) Devonian, (c) Permian, (d) Jurassic time? 9. What evidence can you give to prove that the earth's surface has undergone many changes? 10. Upon what basis are divisions in geological time made? 11. At what periods was a large portion of North America under water? 12. Of what value is the study of fossils to the scientist? 13. How does the paleontologist identify rock strata?