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." [Illustration: _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. [Illustration: _Fig. 44. A pile of brook debris deposited by the checking of the current._]