120 INTRODUCTION TO BOTANY 
leaves and branches. Not infrequently it is covered with moss. 
The soil of the forest, abounding in humus, retains water much 
longer than average soil in open fields. All of these factors 
cooperate to hold back the run-off from wooded areas. It is 
therefore of the highest importance that such regions as the 
White Mountains,! the Adirondacks, the central and southern 
Appalachians, and western mountain ranges which are used 
as sources of water for irrigation should be forested. 
112. Forest growth prevents erosion. Along with the value 
of the forest in regulating the flow of streams, account must 
be taken of its importance in preventing the washing away, or 
erosion, of the earth’s surface. Not only mountains and hill- 
sides but cultivated slopes everywhere are subject to great 
losses by washing during thaws after snows and during rain- 
storms. How much earth is thus annually carried to the Gulf 
by the Mississippi alone has already been stated (sect. 2+). 
Figure 239 represents an early stage in the formation of gul- 
lies on a steep slope after clearing. The land is already past 
the stage in which it can be cultivated in the ordinary way. 
Left to itself the tendency is for the washing to continue 
until the hillside becomes a series of miniature ravines, strewn 
with bowlders and separated by bare ridges. Thousands of 
acres in the southern United States and hundreds of thou- 
sands in some of what were once the most fertile parts of 
southern Europe have been ruined in this way. Such de- 
struction may be prevented by retaining hillsides in a wooded 
condition, or at least by leaving belts of trees at intervals, run- 
ning at right angles to the lines of slope. The early stages of 
erosion may be checked by damming the principal gullies with 
logs, stones, and brushwood, and then replanting with suitable 
1 Recent investigations in the White Mountain region by the United States 
Geological Survey show that the run-off on two drainage basins, each of 
about five square miles, on the Pemigewasset River, was very unequal. One 
basin was covered with virgin forest ; the other had been deforested and 
burned. The run-off from the latter basin during seventeen days in April 
(including three extensive storms) was about double that from the forest- 
covered basin. 
