FLOOD PLAINS 1 37 



on the down-stream side, the bar or spit advancing by having sand 

 or gravel pushed up the gentle slope by the current and dropped 

 over the steep face, where it forms inclined layers. Flattened and 

 elongated pebbles arrange themselves so as to offer the least 

 resistance to the current, in a slanting position, with their tops 

 down stream. When the stream is subsiding, the material tends 

 to assume a more horizontal direction, giving an irregular and 

 confused stratification to these deposits. 



Alluvial Cones or Fans. — Where a swift torrent, descending a 

 steep slope, debouches on a plain or wide valley, its velocity is 

 greatly diminished, and a large part of the material which it carries 

 is thrown down and spread in a fan shape from the opening of the 

 ravine in which the torrent flows. The thickness of the cone is 

 greatest at the mouth of the ravine, while its breadth increases out- 

 ward from that point. Where several such torrents open on the 

 plain near together, their fans may coalesce and form a continuous 

 fringe along the base of the mountain. The slope of the cone's 

 surface diminishes with the size of the stream ; in small streams it 

 may be as steep as io°. These cones are formed on much the 

 same principle as deltas, and might, with propriety, be called ter- 

 restrial deltas. Very large alluvial cones are found in the Rocky 

 Mountain and Great Basin regions. (See Fig. 52.) 



Flood Plains. — Rivers, as is well known, are subject to floods, 

 when the volume of water is enormously increased and can no 

 longer be contained in the ordinary channel, but spreads out over 

 the level ground on each side. By this spreading, which may be 

 for many miles in both directions, the velocity of the water is much 

 diminished, and over the flooded area (flood plain) large quan- 

 tities of material are thrown down, while the unchecked velocity 

 in the channel will cause a scouring and deepening there. The 

 nature of the material deposited over the flood plain will depend 

 on the character and swiftness of the flooded stream, and varies 

 from the coarsest gravel to the finest silt. The latter is more 

 usual, for the flood plain is widest along the lower course of the 

 river. A line of maximum deposition occurs where the swift cur- 

 rent of the channel is retarded by contact with the sluggish waters 



