150 



PHYSICAL GEOLOGY 



plains (almost plains). Above their otherwise flattish sur- 

 faces occasional unreduced elevations rise abruptly. These 



FIG. 140. A peneplain near Camp Douglas, Wisconsin, with several monad- 

 nocks in the distance. (Sankowsky.) 



elevations are called monadnocks (Figs. 140 and 141), after 

 a mountain of this type, in New Hampshire, and owe their 

 preservation either (1) to the superior resistance of their 



rocks, or (2) to a favorable posi- 

 tion among the streams. 



Cycles of erosion. A cycle of 



7 /?wJ\/^^^.' erosion is the time required for 



the production of a base-level plain. 

 From the preceding paragraph it is 

 evident that erosion cycles have 

 been rarely completed. Usually 

 they are interrupted and a new 

 cycle inaugurated. 



The erosion history of a region 



FIG. 141. A monadnock on i s often recorded by the character 



the flood plain of a river. . . , m, 



of the topography. Thus, mean- 

 dering courses are developed by rivers only on valley flats, but 

 many a meandering river occupies a valley scarcely wider 

 than the stream itself, and much narrower than the belt 

 within which the river winds (Plate VI). This means that 

 after the development of the meandering course, the region 

 was elevated in such a manner as to increase the gradient, 



