ORIGIN OF TOPOGRAPHIC FORMS. 121 



rapids, and falls. As the age of the region increases these inequalities are 

 cut away. The lakes are drained, the falls and rapids disappear. The 

 mountains and hills are worn down, and finally the entire surface is reduced 

 -to a low rolling expanse. The region approaches base level. It is in its 

 old age. Plains represent old age among topographic features. 



The life of a topographic area is not to be measured in years, but in 

 its cycle of changes, which have little reference to time. The time required 

 to run through its life differs with the conditions under which and the ma- 

 terials upon which erosion acts. It varies with the intensity of erosive 

 action and with the amount of work to be done. 



Sometimes a region after being reduced nearly to base level has been 

 again elevated. Such elevation brings again into action the erosive agen- 

 cies to carve and plane the terrain a second time. A region thus restored 

 to youth by elevation is the mountain region of North Carolina. The 

 bench level of the country is an old base level, which has been raised. In 

 this the streams are now cutting and regulating their courses, while the 

 bench level, in its gentle undulations, shows the old base-level surface, 

 little affected as yet by recent erosion. 



DEPOSITION FROM WATER. 



When the swift current of a stream is checked, as by a reduction of 

 slope or by a widening of its bed, it deposits a part of its load. It is thus 

 that river banks, river and lake terraces, and bars at the mouth of streams 

 are made. ( >f the building of river banks, tine examples are seen in south- 

 ern Louisiana. Before the stream was lined with levees the Mississippi 

 river overflowed its banks at every considerable rise. Loaded with detritus, 

 it suddenly spread over its banks to the dimensions of an inland sea; its 

 velocity was thereby checked and much of its load was quickly deposited, 

 the greater part, including the coarsest material, falling on its immediate 

 banks, which were thereby built up higher than the adjoining country. The 

 river and bayous of this region flow on the tops of ridges of their own con- 

 struction, the only laud above the swamps. The highest ground every- 

 where is that on the immediate river bank, whence the slope is away from 

 the stream on either hand to the swamp, as shown in PI. xv. 



