178 ELEMENTARY GEOLOGY 
be entirely destroyed. Sometimes they are built so 
high and so large that the stream is divided into two 
channels, one of which is ultimately abandoned. 
Floodplains. — Rivers which are not enclosed between 
rock walls, are often bounded by plains which they them- 
selves have constructed. When they rise in floods and 
Fig. 90. 
A small alluvial fan forming part of a talus at the base of a shale precipice. 
overflow their banks (Fig. 92), the muddy torrents 
spread out on either side of the channel. In these 
places, being shallower, and more retarded by friction 
against the bottom, the water loses some of its ve- 
locity and is unable to carry as much sediment as it 
can in the deeper and more swiftly flowing channel 
