If we should dip up a glassful of this muddy water, we should find that when it had settled there remained on the bottom of the glass a thin deposit of sediment. The amount of this sediment is small, no doubt, for a single glassful, but when we think of the great quantity of water constantly flowing by, we can see that considerable sediment is going along with it. But this sediment in suspension is not all the load that the brook is moving. If you will roll up your sleeve, plunge your hand to the bottom of the brook and hold it there quietly, you will feel the coarser gravel and small stones rolling along the bottom.
All this load of sand and gravel comes, as we have seen, from the valley sides, the banks of the brook, and from its bed. It is moving downward away from its original resting place; and what is the result? For thousands upon thousands of years, our brook may have been carrying off its yearly load of sediment; and though each day's labor is small, yet the added toil of centuries has been great. The result of this labor we can see in the great trough or valley through which the brook flows. Tennyson speaks of the ceaseless toil of the brook in the following words:
"I chatter, chatter, as I flow To join the brimming river, For men may come and men may go, But I go on forever."
Fig. 43. A brook cutting under its bank and causing a landslide.
We have seen how the rills and torrents bring into the brook their loads of sand, clay, and gravel; now let us walk along the bank and see what the brook is doing to increase this load. Just here there is a sudden turn in the channel and so sharp is the curve that the rushing stream is not able to keep in mid-channel, but throws itself furiously against the outer bank of the curve, eating into the clay of which it is composed, until the bank is undermined, allowing a mass of clay to slide down into the stream bed, where it is eaten up and carried away by the rushing water ([Fig. 43]). Farther on, the brook dashes down a steep, rocky incline, and if we listen and watch we may hear the thud of boulders hurled along, or even see a pebble bound out of the muddy foaming water. These moving pebbles strike against each other and grind along the bottom, wearing out themselves as well as the large unmovable boulders of the rocky bed of the brook. Thus the larger stones are ground down, rounded at first but in time reduced to sand, adding in this way to the moving burden of the brook. By this slow process of cutting and grinding, the deep rock gorges of New York state, like those at Watkins, Ithaca, Au Sable Chasm, and even the mighty gorge of Niagara, have been made. The Grand Canyon of the Colorado, over a mile in depth, is one of the greatest examples of stream cutting to be found in the world.
Fig. 44. A pile of brook debris deposited by the checking of the current.
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.