RELIEF FEATURES 15 



in the same way. The wind is, therefore, one of the agents which 

 is changing the face of the land. 



2. Rain or snow falls almost everywhere on the land. A part of 

 the rain which falls on the land runs off over the surface. When the 

 snow of the land melts, much of the water follows the same course. 

 The water which runs off over the land in streams carries much mud, 

 sand, and gravel from the land to the sea. Running water is the 

 most important agent modifying the surface of the land, and its 

 general effect is to make the land lower. The rain and snow water 

 which sink beneath the surface also help to lower the land by dis- 

 solving mineral matter from the rocks. 



3. Great bodies of ice, called glaciers, move slowly over the sur- 

 face of the land in some places, especially on high mountains and 

 on the cold lands near the poles. Glaciers make great changes in 

 the valleys through which they move. 



4. The waves of the sea and of the many lakes are constantly 

 modifying the position and the outlines of their shores. These 

 changes are slight in short periods of time, but they have been very 

 great in the course of the long ages of the earth's history. 



The winds, rivers, glaciers, and waves are agents of gradation. 

 They degrade (wear down) the surface at some points, and aggrade 

 it (build it up) at others. They degrade the land far more than 

 they aggrade it. 



5. Still another series of changes in the surface is brought about 

 through the agency of life. Man, for example, grades down eleva- 

 tions and grades up depressions, as along railroads. He makes 

 dams across rivers, converting portions of them into ponds; he raises 

 and changes the banks of streams, shifting their natural courses and 

 their natural work; he drains marshes and lakes, and more import- 

 ant than all else, he clears (removes the forests) and tills the land, 

 thus preparing the way for the more effective action of wind and 

 running water. Burrowing animals loosen soil, so that it is more 

 easily blown or washed away. Plants, on the other hand, protect 

 the surface from erosion. Little dust is blown from a surface well 

 covered with vegetation, and the water which runs down a hillside 

 carries much less mud and sand from a slope covered with plants 

 than from one freshly plowed. 



