188 PHYSICAL GEOLOGY 



5. Compare the downstream slope of the higher and lower 

 terraces of a given valley. 



6. Interpret Figure 198, indicating (1) the successive steps in the 

 development of the features shown, and (2) how the several changes 



FIG. 198. Diagram of stream terraces. 



that are recorded in the work of the river may have been brought 

 about. 



7. Why are the materials brought by great rivers to the sea 

 usually fine (though unequally so) ? 



8. Interpret the fact that limestones containing marine fossils 

 are sometimes found interbedded with delta deposits. 



SUMMARY 



The mission of running water is to wear the land to base 

 level. The material it carries toward and to the sea is pre- 

 pared for transportation largely by the agents of weathering, 

 and in subordinate amount is worn from the rocks by the 

 streams themselves. The irregular reduction of the land 

 produces most of the familiar relief features of the surface, 

 whose characteristics are determined by several factors, 

 especially by the character and structure of the rocks from 

 which they were carved, and the stage of development which 

 they have reached. The waste of the land is often laid aside 

 on its way to the sea by overloaded streams, forming topo- 

 graphic features subject to later destruction by eroding waters 

 or by other agencies. 



The getting of the land into the sea has been the great task 

 of streams throughout all the geological ages since lands and 

 seas existed, and the materials of the sedimentary rocks of 

 existing lands represent for the most part the stream-borne 



