And now let us make one more rainy-day observation before going back to our warm, dry homes. Just ahead 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 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 it 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. 45), 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 much current and hence without much cutting power (Fig. 46). 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, afford rich farming lands when they are built high enough to remain above the water. [Illustration: _Fig. 46. A brook flowing across a pond which has been filled._] 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, as well as the everflowing brook, have been cutting and depositing. 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 sometimes made by larger streams, especially in a mountainous country with plains at the foot of the slopes. They are called _alluvial fans_ or _cone deltas_ (Fig. 47), but they are not as important as flood plains and deltas. [Illustration: _Fig. 47. A brook building a delta into a lake. Formerly the brook flowed straight ahead, but its own delta has caused it to change its direction._] The first dry, sunny morning that comes we 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 we look closely, we shall see glittering particles of sand moving along the bottom. The clear water, however, allows us to study the bottom which before was hidden by the load of mud.