146 



PHYSICAL GEOLOGY 



early maturity they are roughly U-shaped (Fig. 134), instead 

 of V-shaped, as before. In later maturity they have conspicu- 

 ous flats. These changes in cross section signify that down 



FIG. 137. Diagram of an area in a youthful stage of erosion. The area is 

 situated some distance from the sea. The bottom of the diagram repre- 

 sents sea level. 



FIG. 138. Diagram showing mature topography in a region situated some 

 distance from the sea. The bottom of the diagram is sea level. The 

 area shown in Figure 137 will in time closely resemble the present appear- 

 ance of this area. 



FIG. 139. Diagram showing old topography in a region situated some 

 distance from the sea. The bottom of the diagram is sea level. Unless 

 diastrophism interferes with the work of the streams, the areas represented 

 in the two preceding figures will finally closely resemble this area. 



cutting has come to be very slow, and that the processes which 

 widen valleys and reduce their sides to gentler slopes have 

 become much more important, relatively. Valleys are old 



