Ice helps handle boulders in still another way; but before I tell you what it is I want you to imagine you are an Indian, away back in the days before Indian schools, and see if you wouldn't be as superstitious as they were. Just suppose then that you are a red child of the forest, and that along a certain lake you saw near the shore a lot of boulders scattered about in a disorderly way. This, say, was in the fall. But when you came back the following spring you found them all piled up into a wall along the lake, and you positively knew no member of your tribe or of any other had done the piling. Wouldn't it make you feel a little superstitious? HOW MR. WINTER BUILDS BOULDER WALLS It was Mr. Winter that built these walls. With the spring break-up on lake shores big cakes of ice, blown by stiff gales, pry up the boulders along shore, and force them further up the bank. Then another gale and another push, and more stones are crowded up on top of the first course, and so there is built a rude wall. Some of the stones may be crowded together side by side. This makes what is called a "boulder pavement." But even this isn't all of nature's engineering in the handling of boulders. Here is another example. Ice is formed on lakes early in the winter when the air is but little below the freezing point of water. Under these circumstances ice expands. Then, with the first severe cold spell it contracts and so cracks. Water, rising from below, fills these cracks, and is itself, in turn, frozen to ice. Then comes a warm wave, these ice wedges swell, and so the ice sheet expands, pushes up along the shore and, if there are any boulders there moves them about; or sometimes drives them deep into the bank so that the following spring it looks as if somebody had been shooting at the bank, using boulders for bullets. The sun shapes boulders somewhat as the blacksmith shapes iron, but instead of striking with a hammer it strikes with its rays. Rock is a poor conductor of heat, so the heat from the sun only goes into the rock a little way. The result is that the surface expands and so loosens itself from the rock beneath and in course of time falls off. With the cooling of the atmosphere at night just the opposite thing takes place; the surface cools off first and so, contracting, loosens itself from the body of the stone. It seems to be a regular tug of war between the heat of the day and the cool of the night. First of all the corners and sharp edges break away because, being thinner, they are heated and cooled more quickly. The boulders owe their rounded shapes most of all, however, to the fact that they were ground together in the body of the glaciers as those great ice sheets flowed along. GOOD TALKS BY LEARNED BOULDERS