The story of rock formation tells how fire and water, and the two combined, have made, and made over, again and again, the substance of the earth's crust. Chemical and physical changes constantly tear down some portions of the earth to build up others. The constant, combined effort of wind and water is to level the earth and fill up the ocean bed. Rocks are constantly being formed; the changes that have been going on since the world began are still in progress. We can see them all about us on any and every day of our lives.
GETTING ACQUAINTED WITH A RIVER
I have two friends whose childhood was spent in a home on the banks of a noble eastern river. Their father taught the boy and the girl to row a boat, and later each learned the more difficult art of managing a canoe. On holidays they enjoyed no pleasure so much as a picnic on the river-bank at some point that could be reached by rowing. As they grew older, longer trips were planned, and the river was explored as far as it was navigable by boat or canoe. Last summer when vacation came, these two carried out a long-cherished plan to find the beginning of the river—to follow it to its source. So they left home, and canoed up-stream, until the stream became a brook, so shallow they could go no farther. Then they followed it on foot—wading, climbing, making little détours, but never losing the little river. At last they came to the beginning of it—a tiny rivulet trickled out of the side of a hill, filling a wooden keg that formed a basin, where thirsty passers-by could stoop and drink. They decided to mark the spring, so that people who found it later, and were refreshed by its clear water, might know that here was born the greatest river of a great state. But they were not the original discoverers. Above the spring, a board was nailed to a tree, saying that this is the headwater of the river with the beautiful Indian name, Susquehanna.
It was a dry summer, and the overflow of the basin was almost all drunk up by the thirsty ground. They could scarcely follow it, except by the groove cut by the rivulet in seasons when the flow was greater. They followed the runaway brook, through the grass roots, that almost hid it. As the ground grew steeper, it hurried faster. Soon it gathered the water of other springs, which hurried toward it in small rivulets, because its level was lower. Water always seeks the lowest level it can find. Sometimes marshy spots were reached where water stood in the holes made by the feet of cattle that came there to drink. The water was muddy, and seemed to stand still. But it was settling steadily, and at one side the little river was found, flowing away with the water it drew from the swampy, springy ground. All the mud was gone, now; the water was clear. It flowed in a bed with a stony floor, and there were rough steps where the water fell down in little sheets, forming a waterfall, the first of many that make this river beautiful in the upper half of its course. To get from the high level of that hillside spring to the low level of the sea, the water has to make a fall of twenty-three hundred feet, but it makes the descent gradually. It could not climb over anything, but always found a way to get around the rocks and hills that stood in its way. When the flat marsh land interfered, the water poured in and overflowed the basin at the lowest margin.
In the rocky ground the two explorers found that the stream had widened its channel by entering a narrow crevice and wearing away its walls. The continual washing of the water wears away stone. Rocks are softened by being wet. Streaks of iron in the hardest granite will rust out and let the water in. Then the lime in rocks is easily dissolved. Every dead leaf the river carried along added an acid to the water, and this made easier the process of dissolving the limestone.
Every crumbling rock gives the river tools that it uses like hammer and chisel and sandpaper to smooth all the uneven surfaces in its bed, to move stumbling blocks, and to dig the bed deeper and wider. The steeper the slope is, the faster the stream flows, and the larger the rocks it can carry. Rocks loosened from the stream bed are rolled along by the current. Then bang! against the rocks that are not loose, and often they are able to break them loose. The fine sand is swept along, and its sharp points strike like steel needles, and do a great work in polishing roughness and loosening small particles from the stream bed. The bigger pebbles of the stream have banged against the rock walls, with the same effect, smoothing away unevenness and pounding fragments loose, rolling against one another, and getting their own rough corners worn away.
The makers of stone marbles learned their business from a brook. They cut the stone into cubical blocks, and throw them into troughs, into which is poured a stream of running water. The blocks are kept in motion, and the grinding makes each block help the rest to grind off the eight corners and the twelve ridges of each one. The water becomes muddy with the fine particles, just as the drip from a grindstone becomes unclean when an axe is ground. Pretty soon all the blocks in the trough are changed into globes—the marbles that children buy at the shops when marble season comes around.
I suppose if the troughs are not watched and emptied in time, the marbles would gradually be ground down to the size of peas, then to the size of small bird shot, and finally they would escape as muddy water and fine sand grains.
Sure it is that the sandy shores that line most rivers are the remnants of hard rocks that have been torn out and ground up by the action of the current.