212 THE LURE OF THE LAND 



higher levels of the earth's surface is a process which 

 no human precaution can wholly prevent. It has been 

 one of the most important forces and factors in the geo- 

 logical changes which have so greatly modified the sur- 

 face of the earth. The present surface of the largest 

 portion of the United States is made up of this ' sedi- 

 mentary ' or ' drift ? material which has been moved 

 from the place where it was formed through the disin- 

 tegration and decay of the old crystalline rocks, by wa- 

 ter, 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 crys- 

 talline rocks of the Piedmont Plateau has been low- 

 ered at least 2,000 feet 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 large tracts of country is not low- 

 ered 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 has been used over and 

 over again for vegetation and expose fresh material 

 to the roots of plants. With this slow change the nat- 

 ural forces are amply sufficient for the decay of the 

 subsoil and for the conversion of this freshly exposed 

 material into a good soil. When the rate of denuda- 

 tion 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. 



