142 THE EARTH AS MAN'S ABODE within a few hours. Less rapid currents of water are slower in their action but no less efTective in the long run. They constantly round off the boulders and rocks as they jostle them about against each other and against the rocky canyon walls and transport the lighter particles thus formed farther down stream. The Mississippi River carries a million tons of silt and sand to the Gulf of Mexico every twenty-four hojjrs. Large deltas are formed by such rivers as the Mississippi, the Nile, and the Yangtze as they dump their debris into the sea. Valleys like the great Sacramento and San Joaquin River valleys in California have been filled to a depth of as much as 1500 feet by the sand and silt washed down from the nearby mountains. Huge gashes have been cut into high plateaus by swiftly flowing rivers such as the Colorado River. The Colorado River Canyon is 214 miles long, from 8 to 12 miles wide, and over a mile deep. At the foot of mountains the debris resulting from the disintegration of the rocky materials is carried out by the water into the valleys, forming large delta-like fans. As time passes, the mountains are gradually worn down, and these fans creep up the canyons until the mountains become level plains. Eventually these deposits become cemented together by chemical action of the water and its contents with the sediments, and new layers of stratified rock come into being. Oceans and Lakes Disintegrate Their Shore Lines. Oceans constantly pound their way into the rocky shores, wearing down the rocks to form the beach sands and to form still smaller particles which are carried farther out to form shallow submarine plains. Later these plains may be elevated several thousand feet, and the rivers commence once more the eroding process. At other times the plains near the sea are lowered so that the water covers them, forming large bays and estuaries. While as a rule the waves in lakes are smaller than those in the oceans and therefore subject their shore lines to a more gentle pounding, on the other hand, lakes in cold climates freeze over, and the expanding ice grinds the shore lines. Underground Waters Contribute Largely to the Disintegration of Rocks. Underground waters seep through porous layers and into cracks, dissolving out soluble substances, and eventually re-emerge as springs to feed the rivers. Billions of tons of dissolved matter, nearly a quarter of the total material transported by rivers, are thus carried in solution to the oceans every year.
THE GRADATION OF THE EARTH 143 Slichter, consulting engineer of the United States Geological Survey, has estimated that the amount of ground water is nearly one-third as great as that in the oceans. Generally, the ground waters settle on an impervious layer of rock and reach a level called the water table, sometimes near the surface and at other times quite deep. When streams cut below these water tables, springs appear along the banks. Water seeps into wells when they are driven below the water table. Sometimes portions of land lie below the water table, and a swamp or lake results. Ground waters may move through the soil or rocks very slowly, sometimes only a few hundred feet per year. These waters frequently Fig. 38. Thousand Springs, Snake River Canyon, Idaho. Outlet for groundwater flowing through porous lava. (From Reeds, Chester A.. The Earth, published by the University Society.) become charged with carbon dioxide, which gives them the solvent action necessary to dissolve limestone and form such huge series caves as the Carlsbad Caverns in New Mexico and the Mammoth Cave in Kentucky, which extend scores of miles underground. Sometimes ground waters collect in fissures between rocks and pour out as veritable rivers. sunk into ground water that is under pressure. Occasionally the pressure of the steam formed in Artesian wells are produced when pipes are the water to shoot upwards suddenly as a geyser. of the lower portions of fissures causes This action of geysers may occur regularly, as in the case of Old Faithful in Yellowstone National Park, or irregularly, as in the case of Eaimangu in New