Now the brook leads us into a dripping woodland, and just ahead we can hear the roar of a little waterfall, for at this point the cutting stream flows upon the bed rock with its alternating bands of hard and soft rock through which the busy brook is cutting a miniature gorge. Here is a hard layer which the stream has undermined until it stands out as a shelf, over which the water leaps and falls in one mass with a drop of nearly ten feet. Watch how the water below boils and eddies; think with what force it is hammering its stone-cutting tools upon the rocky floor. Surely here is a place where the brook is cutting fast. Notice that swirling eddy where the water is whirling about with the speed of a spinning top; let us remember this eddy and when the water is lower we will try to see what is happening at its bottom. On the other side of the woods our brook emerges into a broad meadow; let us follow it and see what becomes of its load, whether it is carried onward, or whether the tired brook lays it down occasionally to rest. Out of the woods, the brook dashes down a steep incline until the foaming tide comes to rest in a deep pool. What becomes of the large pebbles which have been swept down? Do they go on or do they stop? If you go to the outlet of the pool you will see that the water is coming 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 important of the brook's lessons, for anything that tends to check the current makes it drop some of the sediment that it carries (Fig. 44). Yonder is an old tree stump with its crooked roots caught fast on the bottom; the mid-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 overflow 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 _flood 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, each spring when the stream is in flood, a fresh coating of soil mixed with fragments of vegetable matter, and thus grow deeper and richer year by year. The flood plains of the Mississippi and of the Nile are notable examples of this important form of stream deposit. [Illustration: _Fig. 45. A delta built by a tiny rill flowing from a steep clay bank._]