130 



go on or do they stop ? If you go to the outlet of the pool you will 

 see that the water is coining out with nothing in its grasp but the 

 fine clay and sand, the gravel and pebbles having been dropped by 

 the less rapid current of the pool. This is one of the most impor- 

 tant of the brook's lessons, for 

 anything that tends to check 

 the current makes it drop some 

 of the sediment that it carries 

 (Fig. 83). Yonder is an old 

 tree stump with its crooked 

 roots caught fast on the bot- 

 tom ; the raid-stream current 

 rushes against it only to be 

 thrown back in a boiling eddy 

 and the waters split in twain 

 and flow by on either side 

 with their current somewhat 

 checked. In the rear of the 

 stump is a region of quiet 

 water where the brook is 

 building up a pile of gravel. 

 Farther on, the banks of the 

 brook are low and here the 

 waters no longer remain in the 

 channel, but ovei*flow the low 

 land spreading out on either 

 side in a broad sheet. The increased friction of this larger area 

 reduces the current and again we see the brook laying down some of 

 its load. The sand and gravel deposited here is spread out in a flat 

 plain called a jiood plain because it is built up when the stream is 

 in flood. It is on the large flood plains of rivers that many of our 

 richest farm lands occur. These receive a fresh coating of soil 

 mixed with fragments of vegetable matter each spring when the 

 stream is in flood, and thus grow deeper and richer year by year. 

 The flood plains of the Mississippi and the Nile are notable exam- 

 ples of this important form of stream deposit. 



522 



83. — A jyile of brook debris deposited by 

 the checking of the current. 



