132 



sometimes made by lai'ger streams, especially in a mountainous coun- 

 try with plains at the foot of the slopes. They are culled alluvial 

 fans or coiie deltas (Fig. 86), but they are not as important as flood 

 plahis and deltas. 



The next dry, suiniy morning that comes let us visit the brook 

 again : it no longer roars, but its clear waters now sing a pleasant 

 melody as they ripple along the stony bed. We can see at a glance 

 that comparatively little work is going on to day, and yet, if \yq 

 look closely, we shall see glittering particles of sand moving along 

 the bottom. The clear water, however, allows us to study the bot- 

 tom which before was hidden by the load of mud. 



85. — A brook floioing across a pond wTiich has been filled. 



First we see the rounded boulders and pebbles of all sizes which 

 must liave been rolled about for a long time to make them so smooth. 

 Some of them are so very hard that we cannot even scratch them 

 with our kives ; others are soft and easily broken. Wliat would be 

 the effect of rolling together stones of such varying hardness? 

 We must think of these stones as the brook's tools with which it 

 cuts and grinds, for water without sediment can do little more than 

 slifflitly to dissolve the rock. 



Let us go at once to the little waterfall, for we shall be curious to 

 see what lies at the bottom of the whirling eddy that drew our 



524 



