IOO 



PHYSICAL GEOLOGY 



the fracturing of the surface during earthquakes, but they are so 

 few as to be unimportant. Some great valleys, nevertheless, were 

 ready made for the rivers which flow through them. The Great 

 Valley of California, through which the San Joaquin and Sacra- 

 mento rivers flow, was formed, not by stream erosion, but by 



BLACK FOREST 



W^.7 



Fig. 8i. — Section across the Vosges and Black Forest, Germany, showing the 

 graben in which the Rhine flows. (Penck.) 



the sinking of the land along a valley-like depression, or by the 

 uplift of parallel mountain folds, and is called a structural valley. 

 Into such a depression streams may flow from the high lands 

 on the sides and unite (unless the region is arid) to form a 

 river system. The Great Basin region of Utah is also a structural 

 valley, but because of the aridity of the climate no streams flow 

 through it. The River Jordan and the Dead Sea are in a valley 



Fig. 82. — Diagram A illustrates the development of parallel consequent streams 

 on a sloping surface. Diagram B is the same region after the streams have become ad- 

 justed to the structure of the underlying rocks. The streams entering the main at 

 right angles are subsequent streams. The main stream flows through its water gap in 

 the hard ridge. The gaps on either side were eroded by former streams but no longer 

 have streams in them and are called wind gaps. 



formed by the sinking (faulting, p. 261) of a long and comparatively 

 narrow block of the earth's crust. Such a valley is called a rift 

 valley. Owen's valley in California and a portion of the Rhine 

 valley in Germany (Fig. 81) are other examples of valleys due to 

 Faulting. Glaciers excavate valleys in the solid rock, which may 

 afterwards become occupied by streams, but these are usually merely 



