THE WORK OF STREAMS 



III 



Fig. 97. — Diagram showing the profile 

 of young, mature, and old valleys. 



inhabitants are isolated. There are many such regions in the United 

 States; for example, large portions of West Virginia, southeastern 

 Ohio, eastern Kentucky, and Tennessee are in maturity. As a rule 

 a master stream reaches maturity earlier than its tributaries, and in 

 its lower course earlier than in its upper course. A region in maturity 

 may be traversed by a stream which flows through a broad, old val- 

 ley, and a youthful region may be traversed by a mature stream. 



Old Age. — Continued erosion will gradually cut down the valley 

 sides (Fig. 97) to gentle slopes, lower the divides, and thus tend to 

 reduce the surface to an undulat- 

 ing plain. The sluggish streams 

 will meander (p. 121) in wide 

 valleys. The region is then in 

 old age (Figs. 98, 91). An abso- 

 lute plain may, perhaps, never 

 be reached, since elevations will 

 be left here and there because of some favoring condition, such as 

 (1) hardness of rock or (2) a favoring position with reference to 

 the drainage of the plain. Such hills or mountains rising above the 

 general level of the surface are called monadnocks, from a mountain 

 of that type in New Hampshire. Portions of Kansas have passed 



through youth and 



^H ISBSy maturity and are now 



St in the stage of old 



J&fr- '* J age. 



^ ^" " ^^^ ^1_ - The time required 



" tfe? *^^m^jf for the production of 



. .__ a base-leveled condi- 



tion or for pene- 

 planation " is called 

 the cycle of erosion. 

 It will take, perhaps, one hundred thousand times as long to pass 

 from maturity to old age as from youth to maturity. It will be 

 seen from the above that the age of a region is not recorded in years 

 but in the work accomplished or to be accomplished. 



Effect of Elevation and Depression on Streams. — If a region 

 is elevated after it has been reduced to base level (peneplain), the 

 streams will be quickened and will again be enabled to deepen their 

 valleys. If the streams meandered (p. 121) on the peneplains, they 

 may intrench themselves in their old courses until they flow through 



CLELAND GEOL. — 8 



Fig. 98. — Block diagram showing a region in old age. 

 Sea level is represented by the bottom of the block. 



