This is a scene in August in Glacier National Park. It illustrates how boulders of the Ice Age travelled by water, when icebergs containing them broke from the glaciers and floated away on rivers and lakes.
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
Of course, the boulders, like other people, differ in their tastes—as you can tell by their talk. The granite boulders have the most to say about travel because they are so hard that they can take longer journeys than weaker rocks, and so have more to tell. But there is another branch of the family that is still more "bookish" as you may say. These are the "pudding stone" boulders—conglomerates. In that most interesting biography, "The Story of a Boulder," Professor Geikie describes a stone that was not only made up of a variety of pebbles, but in which there was a section of sandstone. The sandstone and the conglomerate had been neighbors in some rock ledge just as the pebble section and the smooth sand section are always neighbors where the shores descend into the sea. So when the rock mass, which was finally rounded into a boulder, broke away it included portions of both sandstone and conglomerate.
WHERE THE SEA HELPS SHAPE THE BOULDERS
The upper part of this boulder—the sandstone—had in it stems and leaflets of plants of the Coal Age, changed to coal. The pebbles below were fragments of more ancient rocks made at a time when frogs as big as the oxen of to-day lived in the marshes.