22 



AQUEOUS AGENCIES. 



rate of these two processes. They would continue, however, to cut deeper 

 and deeper, until they finally reached their base-level. Then they would 

 cut no deeper, but sweep from side to side, widening their channels. 

 Meanwhile, rain- wash would continue to cut down their divides. Thus 

 wide channels and low divides, or rounded and sweeping curves, are very 

 characteristic of old topography, while deep and narrow gorges and 

 canons are a sign of recent elevation, and therefore comparatively new 

 topography. Moreover, successive movements are each faithfully re- 

 corded. Thus Fig. 13, which is a section across the Grand Canon of 



the Colorado, shows the 

 following events: 1. The 

 plateau region was raised 

 and the river cut down 

 3,000 feet, and reached its 

 base-level. 2. The river 

 sweeping from side to side, 

 and the crumbling of the cliffs, gradually widened the cation to its 

 width in the upper part, b b. 3. Another rise occurred, and the river 

 again cut 3,000 feet more, and made the inner gorge, a a. This second 

 rise is so recent that the river has not yet reached its base-level. 



On the other hand, suppose a continent, by sinking, carries its 

 river-beds below their base-level : then the decreasing slope will check 

 the velocity of the current, and the rivers will immediately begin to 

 build up again by sedimentation until they again reach their base-level. 

 For example, in the Mississippi Kiver the following events are recorded : 

 1. A higher condition of land, during which it reached its base-level, 

 and formed the broad trough r" r" r". 2. A subsidence of land and a 



Fig. 



13. — Ideal Section across Grand Cailon (after Dutton): 

 a a, inner gorge; b b, outer caflon walls. 



Fig. 14. — Generalized Section across the Mississippi River: r" r" r" '. old bed; r 1 r', second bed; r, 

 present bed; d' d', old deposits; d d, present deposits. 



building up by sedimentation d' to the level 11. 3. A partial re-eleva- 

 tion and a cutting down 200 feet, to the level r' r'. 4. A resinking 

 and building by alluvial deposit cl of about 50 feet. Thus, while on coast- 

 lines, old sea-margins are indicators of crust movements, in the interior 

 of continents river-channels may be used for the same purpose. 



3. — Stratification. 



We have seen that heterogeneous material thrown into still water is 

 completely sorted. This is not stratification, since the various degrees 



