204 



ROCKS AND SOILS 



mountains and hills, transporting dissolved materials 

 and sediments to the sea. Much of the material that 

 a stream carries is supplied by the weathering of its 

 valley slopes. Some of its load is secured by eroding 

 away its beds and banks. Erosion by streams is car- 

 ried on chiefly by means of the sand, stones, pebbles, 

 and fine mud they sweep along. These materials 

 moved along by the stream grind, scrape, and scour 



Courtesy Popular Science Monthly 



FIG. 328. GLACIAL MAP 



\\fhite shading shows extent to which ice cov- 

 ered North America in The Ice Age. 



the river bed and banks, acting much the same as a 

 file or a piece of sandpaper. 



Work of the wind. On windy days, especially in dry 

 weather, bits of dust and sand are raised from the 

 earth and carried along by the strong air currents. The 

 presence of these solid particles in the air increases 

 its erosive power. In arid regions of the world where 

 vegetation is scarce, wind plays an important part in 

 soil making. In regions with moist climates the earth 

 is generally protected by a covering of leaves, stems, 

 and roots of plants. 



Work of glaciers. When more snow falls in a region 

 than melts, the snow slowly accumulates. As the mass 

 of snow becomes thicker and thicker it is changed into 

 ice, forming a glacier. A glacier moves slowly down 

 its slope, seldom exceeding a few feet a day. Glaciers 

 erode, transport, and deposit materials, but the man- 

 ner in which the work is done and the results achieved 

 are different from the action of rivers. As the glacier 

 moves along, earth and stones are frozen fast to its 

 bottom and sides. These scour and grind away the 

 bed rock and the sides of the glacier valley. 



At the present time glaciers are confined to the polar 



regions of the earth and high mountains such as the 

 Alps in Europe and the Rockies in North America. It 

 is believed, however, that thousands of years ago, 

 at least four different times, great ice sheets moved 

 down from the north and covered Canada and a large 

 part of the United States, as far south as the Ohio and 

 Missouri rivers (see Fig. 328). As these glaciers re- 

 ceded, or melted back, they deposited their materials. 



FIG. 329. ROCK SHOWING GLACIAL SCRATCHES 



Glacial deposits are easily distinguished from sedi- 

 ments deposited by rivers. They are unassorted ; great 

 boulders, stones, pebbles, and clay are all mixed to- 

 gether, whereas sedimentary deposits by rivers are 

 stratified. Stones and pebbles in glacial deposits are 

 not generally as round and smooth as pebbles carried 

 by streams. Glacial erosion often leaves bed rock 

 marked with scratches and grooves made by pebbles 

 and sand held firmly in the frozen ice (see Fig. 329). 



If you are living in a region over which a glacier 

 passed, make a field trip and observe evidences of 

 glacial erosion. Report your findings to your class. 



The work of tides, waves, and currents. The sea is 

 ever restless. Storm waves strike the ocean shore with 

 tremendous force. Tides and shore currents are con- 

 stantly beating on the coast. Sea water helps to dis- 

 integrate some kinds of rock by its solvent action on 

 them. Other mechanical agencies break off pieces of 

 rock. These fall into the sea and are then driven vio- 

 lently by the impact of the waves against the rocky 

 shore. The combined action of these forces slowly 

 grinds back the shore, depositing the weathered and 

 eroded substances in the sea. 



What are the dangers of too much erosion? Con- 



