Forest Influences 309 



spongy blanket to absorb the water as it falls and there 

 is an immediate surface flow toward the lower valleys. 

 This flow gathers volume and velocity as it goes and 

 finally joins the torrential streams in the valley in a very 

 short time after the rain has fallen. This is the beginning 

 of the flood which probably does great damage in the 

 valley on the lower stretches of the river, where millions 

 have been invested in bridges, quays and other improve- 

 ments. 



The exposed mineral soil, heretofore protected and mixed 

 with the humus, is, upon the disappearance of the humus, 

 a loose mass of mineral particles. A drop of water rolling 

 over this surface carries the lightest of the mineral par- 

 ticles with it in suspension. As these individual drops 

 join and the stream gains in volume and velocity, larger 

 particles are carried along with it, until the great mountain 

 torrent rolls huge bowlders down its worn bed. The first 

 little trickle of water makes an almost imperceptible 

 track in the soft earth; the water from the next rain 

 naturally follows the track of the first, and the track 

 grows deeper. The deeper the cut, the larger the stream 

 of water collected in it ; the larger the stream of water, 

 the greater its carrying capacity; and so erosion pro- 

 ceeds apace. 



First the humus and with it the fertility of the soil 

 is carried away ; then the surface soil follows, leaving the 

 subsoil exposed in great gullies ; finally nothing but rock 

 and hard packed gravel remain. This process occurs in 

 every hillside forest that is cut over and abandoned. 

 There are millions of acres of cut-over land in the hilly 

 parts of the United States now in various stages of erosion. 



