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.