152 THE EARTH AS MAN'S ABODE majority of lakes are relatively shallow, although there are notably deep lakes such as Crater Lake (2000 feet deep) and Lake Baikal in Siberia (5618 feet deep). Lakes usually have outlets, so that the soluble salts do not accumulate. The insoluble suspended matter settles out from the quiet waters of lakes, gradually filling them, until they eventually turn into swamps. Many lakes which have been formed in arid districts have no outlets, and salts accumulate in them just as they do in the oceans, except that the more rapid evaporation in these lakes produces more highly concentrated salt solutions. Ocean Shore Lines Change As the Land Sinks or Rises. It is unlikely that ocean beds have ever risen to form continents, and there is no evidence that the continents have ever been completely covered by the deep ocean. The sedimentary rock formations were probably produced for the most part in great arms of the sea (like Hudson Bay and the Yellow Sea), lakes, and other shallow waterways rather than in great ocean depths. Constantly the ocean wears down the shore at one place and produces beaches and bars at other places, and about the time that the ocean seems to be making real headway into the land areas, they are elevated by internal disturbances (to be discussed later), causing the formation of new shore lines. The coast in the far eastern part of the United States has been gradually sinking, producing channels and islands where rivers and hills once prevailed. On the Pacific coast of the United States, on the other hand, the wave-cut terraces of the Southern California coast show that there has been a recent gradual uplift amounting to as much as 1500 feet. The continents are surrounded by shelves varying in width from 5 to 300 miles and averaging about 75 miles. These are smooth, gently sloping plains, whose final depth is about 600 feet. At the outer edges the underwater platforms give way rapidly to the deep ocean basins. These continental ledges catch most of the sediment carried from the land into the ocean. A mountainous ridge almost 10,000 feet above the adjoining basins runs down the middle of the Atlantic Ocean, extending 8000 miles south from Iceland. If the oceans were to be lowered only 600 feet by the formation of huge sheets of ice, Ireland would be joined to England and England would become a part of the mainland of Europe. North America would be connected with Asia by a strip 1500 miles in width. New Guinea would become a part of Australia.
THE HISTORY OF THE LANDSCAPE 153 The discovery of submarine canyons, which closely resemble those cut by rivers on land, suggests that these canyons were actually cut in the present ocean bottom at some previous time when there was so much less water in the oceans than at present that these portions of the ocean bottom were dry land. According to one theory, the ocean may have been lowered as much as 3000 feet during the ice ages, in which the glaciers may have been as thick as 10 miles. The average height of all of the land on the earth's surface is 2300 feet above sea level, while the average depth of the oceans is about bUj. 42. View ul lubeiiuLe V cille> Iruiii liispiraLioii Puiiii. Aote Bridalveil Falls. This is a typical glacial valley. (Courtesy of the U. S. Dept. of the Interior.) 13,000 feet below sea level. The summit of -the highest mountain, Mount Everest (29,141), is only about 12 miles higher than the lowest spot in the oceans, Swire Deep, east of the Philippines (35,410 feet). Given enough time, erosion alone would carry all the land areas into the ocean, and the whole earth would be covered with water. all the land were reduced to a common level, it would be covered by the ocean to a depth of two miles. As it is, the oceans cover 70.8 per cent of the earth's surface to an average depth of 13,000 feet, as mentioned above. In spite of this, it is probable that the land and water areas have not greatly changed during the past few millions of years. If