176 



PHYSICAL GEOLOGY 



(Fig. 182). (What would be the character of a curve drawn 

 on the surface of a fan along a line all points in which were 

 equidistant from the apex of the fan?) 



FIG. 182. Profile of a large alluvial fan near Cucamonga, Cal. Length of 

 section, 6V 2 miles. 



The growing fans of neighboring streams in arid regions 

 unite in many cases to form extensive alluvial slopes or 

 plains (Fig. 183). 



Certain rivers have been ponded back by the fans of 

 tributaries, forming broad, lakelike expansions of the river. 



FIG. 183. A piedmont alluvial plain, Silver Peak Range, Nevada. Waste 

 from the mountain valleys unites to form a compound fan. (Sketch from 

 photograph by Spurr, U.S. Geol. Surv.) 



Lake Pepin in the upper Mississippi, and Lake Peoria in the 

 Illinois River (Plate II), are of this origin. 



Flood plains. The portion of a valley bottom subject 

 to inundation is called the flood plain (Plate IX). Flood 

 plains, or flats, are usually formed primarily by the side 

 cutting of relatively sluggish streams (p. 140), and subordi- 

 nately by deposits made during overflow. In exceptional 

 cases rivers occupying narrow-bottomed valleys are forced 

 to aggrade their channels, and flood plains result that are 

 due entirely to deposition (Fig. 184). The alluvial deposit 

 may cover the underlying rocks thinly or thickly. 



