Earth Study 823 



water is to drop all the load which it is carrying and it drops the heaviest 

 part first. We find the pebbles at the bottom of the jar, the sand and 

 gravel next, and the fine mud on top. The water may become perfectly 

 clear in the jar and yet, when stirred a little, it will become roily again 

 because of the movement. Every child who wades in a brook, knows that 

 the edges and the still pools are more comfortable for the feet than is the 

 center of the stream under the swift current. This is because, where the 

 water is less swift at the sides, it deposits its mud and makes a soft bottom; 

 while under the swifter part of the current, mud is washed away leaving 

 the larger stones bare. For the same reason, the bottom of a stream cross- 

 ing a level field is soft, because the silt, washed down from the hills by the 

 swift current, is dropped when the waters come to a more quiet place. If, 

 across a stony brook, the pupils can build a dam that will hold for two or 

 three months in the fall or spring when the brook is flooded, they will be 

 able to note that the stones will soon be more or less covered with soft mud ; 

 for the dam, stopping the current, causes the water to drop its load of silt. 

 It would have to be a very recently made pool in a stream, which would not 

 have a soft mud bottom. The water at times of flood is forced to the side 

 of the streams in eddies, and its current is thus checked, and its load of mud 

 dropped. 



It should be noted that at points where the brook is narrowest the cur- 

 rent is swiftest, and where the current is swiftest the bottom is more stony. 

 Also, where there is a bend in the stream the brook digs deeper into the 

 bank where it strikes the curve, and much of the soil thus washed out is 

 removed to the other side of the stream where the current is very slow, and 

 there is dropped. (See Introduction to Physical Geography, Gilbert and 

 Brigham, pp. 51 and 52.) If possible, note that where a muddy stream 

 empties into a pond or lake, the waters of the latter are made roily for some 

 distance out, but beyond this the water remains clear. The pupils should 

 be made to see that the swift current of the brook is checked when its 

 waters empty into a pond or lake, and because of this they drop their load. 

 This happens year after year, and a point extending out into the lake or 

 pond is thus built up. In this manner the great river deltas are formed. 



References The Brook Book, Mary Rogers Miller; Brooks and Brook 

 Basins, Frye; Up and Down the Brooks, Bamford; Physical Geography, 

 Tarr; Introduction to Physical Geography, Gilbert and Brigham. 



LESSON CCX 

 How A BROOK DROPS ITS LOAD 



Leading thought The brook carries its load only when it is flowing 

 rapidly. As soon as the current is checked, it drops the larger stones and 

 gravel first and then the finer sediment. It is thus that deltas are built up 

 where streams empty into lakes and ponds. 



Method Study the rills made in freshly graded soil directly after a 

 heavy rain. Ask the pupils individually to make observations on the 

 flooded brook. 



Experiment Take a glass fruit jar nearly full of water from the brook, 

 add gravel and small stones from the bed of the brook, sand from its borders 

 and mud from its quiet pools. Have it brought into the schoolroom, and 

 shake it thoroughly. Then place in a window and ask the pupils to observe 

 the following things: 



