326 INTRODUCTION TO BOTANY 
Rapidly running surface water often carries away part or 
all of the fertile soil.1 In grasslands, meadows, and forested 
areas surface water is retarded in its rate of flow, and conse- 
quently does not carry away much soil. In regions that were 
once forested and from which the timber has now been largely 
Fic. 239, Erosion of the soil following removal of the forest: 
This land was covered with a heavy pine forest and had a rich soil, which was 
held upon the forest floor. When the timber was removed, erosion soon cut 
ditches through the pasture land, and part of the rich soil was washed away 
removed, the surface water soon erodes ditches (fig. 239), 
which, with rapidly deepening channels and developing tribu- 
taries, will in a few years carry away much of the fertile soil 
of the forest floor. After forest fires, which themselves destroy 
much of the humus of the forest soils, the surface water, 
which is no longer retarded and absorbed by humus, flows 
1 Soil Erosion,” Bulletin 71, Bureau of Soils, U.S. Dept. Agr., 1911. 
