172 EARTH FEATURES AND THEIR MEANING 



cause of some favoring circumstance rise to greater heights above 

 the general level of the peneplain, are known as monadnocks after 

 the type example of Mount Monadnock in New Hampshire (Fig. 



179). 



The river cross sections of successive stages. To the suc- 

 cessive stages of a river's life it has been common to carry over 

 the names from the well-marked periods of a human life. If 

 neglecting for the moment the general aspect of the upland, we 



fix our attention up- 

 on the characteristic 

 cross sections of the 

 river valley, we find 

 that here also there 

 are clearly marked 

 characters to distin- 

 guish each stage of the 

 river's life (Fig. 180). 

 In infancy the steep, 

 nar row, and sharp- 



Age 



Comparison 



FIG. 180. - Comparison of the cross sections of river ansled cafion ig ft char . 

 valleys for the different stages of the erosion cycle. 3 . . 



acteristic ; with youth 



the wider V-form has already developed ; in adolescence the angles 

 of the canon are transformed into well-rounded shoulders, and the 

 valley broadens so as in the lower reaches to lay down a flood 

 plain ; in maturity the divides and the double curves of the line 

 of beauty appear ; while in the decline of old age the valleys are 

 extremely broad and flat and are floored by an extended flood 

 plain. 



The entrenchment of meanders with renewed uplift. - Upon 

 the reduced grades which are characteristic of the declining stage 

 of a river's life, the current has little power to modify the surface 

 configuration. On the old land of this stage a renewed uplift 

 starts the streams again into action. This infusion of driving 

 power into moving water, regarded as a machine capable of ac- 

 complishing certain work, is like winding up a clock that has 

 run down. Once more the streams acquire a velocity sufficient 

 to enable them to cut their valleys into the land surface, and 

 so a new erosional cycle may be inaugurated upon the old land 

 surface the peneplain. After such an uplift has been accom- 



