THE CHANGING FACE OF NATURE 



out as bold headlands, only in time to also yield to the continu- 

 ous charges (Frontispiece) . When it is realized that this battle- 

 line of sea and land is 175,000 miles long and that in a storm 

 the waves strike with the force of several tons per square foot 

 and bombard the shore with rock fragments that weigh up to a ton, 

 it is evident that the oceans and the great lakes have been and 

 still are mighty agents of 

 erosion, in time destroying 

 even the most resistant land 

 masses. 



Running water in rivu- 

 lets, creeks, and rivers is 

 another powerful agent of 

 land erosion. Follow any 

 stream and you will not have 

 traveled far before you will 

 find some place where it is 

 cutting its banks (Fig. 3). 

 Thorn Creek, near Glenwood, 

 Salt Creek, one-half mile up- 

 stream from Brookfield, the 

 Desplaines at Lockport, and 

 Fraction Run, above Delwood 

 Park, give excellent demon- 

 strations of the cutting power of a stream. In newly upheaved 

 sections, like mountain areas, the streams are torrents (Fig. 4) 

 that wear down their narrow valleys, cutting deeply and 

 rapidly, both by virtue of their terrific force and by the 

 angular rock fragments they carry that act as graving tools. 

 When the stream has cut down to somewhere near the level of the 

 body of water into which it flows, it runs more slowly, meanders 

 back and forth across its valley, attacking and undermining the 

 bounding hills, thus gradually widening its influence. Then its 

 valley changes from the deep a- 1 '.. narrow canyon of a young 

 stream to the broader basin of an old river (Fig. 448) . A river 



FIG. 2. Ranks of billows 



