"What makes the corn such a beautiful green?" asked Susie. "That is quite a question," said Uncle Robert. "We will try and find out some day. But I want to know what makes the bottom land richer than the land up on the prairie?" "Well," said Frank slowly, "I suppose that the dirt brought down by the river and spread out over it makes it richer." "Where does that dirt come from?" "Way up the river." "If I should call the bottom land a flood-plain," said Uncle Robert, "would you know why?" "Oh, I know," said Donald. "Because the water covers it when there is a flood." "Now what made that flood-plain?" "Wasn't it always there?" "No," said Uncle Robert. "The river made it." "How could the river make the flood-plain?" asked Susie. "Why, you told me a moment ago that the river brought down great quantities of dirt and left it all along the shores," said Uncle Robert. "But it wouldn't bring down enough to make all that field, would it?" asked Donald. "The river is a great worker," said Uncle Robert. "It is at work now, and has been working for many, many long years. It has not only made this flood-plain, but many others. Sometimes the river carries this dirt clear out into the sea, and sometimes it piles it up at its mouth so that a delta is formed." "Oh, yes," said Donald, "we studied about that in geography when we had school, but I didn't know a delta was made that way." "Are there any deltas in this part of the river?" asked Susie. "There may be," replied Uncle Robert, "wherever one stream flows into another." [Illustration: The mill and dam.] "Is there one at the mouth of our creek?" asked Frank. "We will look when we go back," replied Uncle Robert. "Shall we take a walk now?" When they reached the upper end of the island they sat down on some large boulders that formed part of the tiny beach. Just above them was the flood of water pouring over the dam. The bright sunshine made the foam look white and glistening, lighted here and there with colors of the rainbow. The water rumbled and roared as it rushed out of the mill pond. To the left were the flour mill and the village. They could hear the mill wheel turning. They could see a little white church half hidden among the trees. A kingfisher swept by them with a voice like a watchman's rattle. "He knows how to catch fish better than we do," said Donald. Susie picked up some pebbles and put them in her apron. She tried to get a number of colors. Some were nearly red, some were blue, and some were white. "Can you find one that is exactly round?" asked Uncle Robert. "Here's a white one that's almost round," and Susie held up a quartz pebble. "Where do you suppose this little white pebble came from?" asked Uncle Robert. "Did it come from away up the river--a long way?" said Donald. "I think so. One day this pebble was a part of some rock or quarry. How it was broken off, how it came down, how it was made round, is well worth studying."