25 * 
prevent. It has been one of the most important forces and factors 
in the geological changes which have so modified the surface of the 
earth. The present surface of the largest portion of the United 
States' is made up of this s< sedimentary ” or lt drift ” material which 
has been moved from the place where it was formed through the 
disintegration and decay of the old crystalline rocks, by water, wind, 
or moving ice, and which has accumulated to a depth of hundreds 
or thousands of feet over nearly the entire surface of the country. 
It is estimated that the general surface of the land in the area of the 
crystalline rocks of the Piedmont Plateau has been lowered at least 
2,000 fe-t by this continual washing. This vast amount of material 
has been slowly removed and deposited elsewhere by the very same 
agents which we are contending with to-day in our gullied fields; 
for this denudation, or erosion, is still going on, as it has been for 
ages past. 
As a rule this denudation is exceedingly slow and the general 
level of la»ge tracts of country is not lowered more than an inch or 
two in a hundred years. Where the change is as slow as this it is 
undoubtedly of benefit to the human race, as in the course of time 
it must carry off the soil which have been used over and over again 
for vegetation and expose fresh material to the roots of plants. 
With this slow change the natural forces are amply sufficient for 
the decay of the subsoil and for the conversion of this freshly ex- 
posed material into a good = oil. When the rate of denudation is 
excessive, however, and more rapid than the natural decay of the 
subsoil material which is exposed, it may work serious injury to 
agricultural lands. 
Along the banks of the Ohio River and in very many portions of 
the South hundreds of fields that were once covered with sturdy 
forests of o ik, maple, walnut, and pine, and which bore under cul- 
tivation, being cleared of the natural growth, large crops of wheat, 
maize, tobacco and cotton, may now be seen furrowed with gullies 
as with the wrinkles of age, and abandoned to brush and briers. 
A surface layer of goo.l agricultural soil 6 inches deep resulting 
from the slow and gradual disintegration and decay of rocks and 
accumulation of humus may have required hundreds of years for 
its natural formation, and yet it is liable to be washed away in a 
single storm. 
This excessive erosion, or washing, of lands may be prevented, 
and the already gullied fields may be recovered, and steep slopes 
of loose material may be held and prevented from washing: — 
(1) . By chemical means, in the application of manures and fer- 
tilizers and in the accumulation of organic matter, which change 
the texture of the soil and make it more porous and more absorbent 
of water, so that there is less to run off over the surface. 
(2) . By means of cultivation and under-drainage, which prevent 
erosion by distributing the surface flow over the ground and in- 
crease the amount carried off by under-drainage. 
