9 2 



PHYSICAL GEOLOGY 



When streams on their way to the sea pass from the hard to the 

 soft rocks they flow over rapids or falls, because the less resistant 

 rocks are cut down more easily than the hard. The boundary 

 between the Coastal and Piedmont plains is for this reason called 

 the "Fall Line," and it is here that many cities are located, both 



because the falls fur- 

 nish water power and 

 because they deter- 

 mine the head of 

 navigation. 



(5) When a stream 

 B in cutting its bed en- 

 counters a hard rock 

 mass, the erosion of 



Fig. 68. — A fall formed when resistant rock is en- ^ ne valley is retarded 

 countered by a stream. The rock CD is hard gneiss, 1 • 1 



while that represented by lines is softer schist. The line ^ ' 



AB is the course of the stream. may continue farther 



down the valley. A 

 fall or rapids (Fig. 68) will naturally be formed at such a place, and 

 the hard rock mass will constitute a temporary base level which will 

 prevent the stream from deepening its bed above the fall. As a 

 result of the lateral erosion of the stream and of the action of the 

 weather, the valley above the fall may be greatly widened, forming 

 arable land. A case somewhat similar to the above is that of the 

 falls of the Yellowstone, which are the result of the presence of lava, 

 made more resistant by thermal action (Fig. 69). When rocks are 



Fig. 69. — The falls of the Yellowstone River. The rock is lava, and the falls 

 at A and B are due to the superior hardness of the lava at these points. 



less jointed or fractured, in one portion of a valley than in another, 

 they are less affected by erosion and may produce a fall or rapids. 



(6) Falls also result where rocks have strongly vertical joints, as 

 vertical joints in homogeneous rocks have the effect of vertically 

 inclined beds. 



