131 



84. 



And now let us make one more rainy day observation l^ofore 

 going back to our warm, di-y homes. Just aliead on the other side 

 of that clump of alders and willows lies the pond into which the 

 brook flows and where its current is so checked that it gives up 

 almost all of its burden of sediment. Close to the shore it has 

 dropped its heaviest fragments while the sand and clay have been 

 carried farther out, each to be 

 dropped in its turn, carefully 

 assorted as to size and weight. 

 Here you can see that the 

 stream has partly filled this 

 end of the pond and is now 

 sending its divided current out 

 over the deposit which it has 

 made in a series of branching 

 rivulets. This deposit is called 

 a delta (Fig. 84) and deltas 

 are another important form of 

 stream deposits. In the lakes 

 and ponds deltas may grow outward until the lake is filled when 

 the stream will meander across the level plain without nmch cur- 

 rent and hence without much cutting power (Fig. 85). In the sea 

 great deltas are being formed in some places, like those at the 

 mouths of the Mississippi, the Nile and the Ganges. Large areas of 

 dry land have thus been built. Deltas, like flood plains, atford rich 

 farming lands when they are built high enough to remain above the 

 water. 



Here let us end our study of the brook for to-day, and wait until 

 the rain ceases and the water runs clear again ; then we can see the 

 bottom and can also learn by contrast how much more work the brook 

 has been doing to-day than it does when the volume of water is less. 



On the road home, however, we can notice how the temporary 

 streams have been cutting and depositing, as well as the everflowing 

 brook. See where this tiny rill has run down that steep clay bank 

 until its current was checked at the foot. Notice how it has spread 

 out its sediment in a fan-shaped deposit. This form of deposit is 



533 



A delta built by (i tin i/ rill Jlaiting from 

 a steep cluy bunk. 



