The gradual thickening and shrinking of the earth's crust as it cools have made the wrinkles we call mountain systems. Through millions of years the globe has been giving off heat to the cold sky spaces through which it swings in its orbit around the sun. The cooling caused the contraction of the outer layer to fit the shrinking of the mass. When a plump peach dries on its pit, the skin wrinkles down to fit the dried flesh. The fruit shrinks by loss of water, just as the face of an old person shrinks by loss of fat. The skin becomes wrinkled in both cases.

The weakest places in the earth's crust were the places to crumple, because they could not resist the lateral pressure that was exerted by the shrinking process. Along the shores of the ancient seas the rivers piled great burdens of sediment. This caused the thin crust to sink and to become a basin alongside of a ridge. The wearing away of the land in certain places lightened and weakened the crust at these places, so that it bent upward in a ridge.

Perhaps the first wrinkles were not very high and deep. The gradual cooling must have exerted continued pressure, and the wrinkles have become larger. It is not likely that new wrinkles would be formed as long as the old ones would crumple and draw up into narrower, steeper slopes, in response to the lateral crushing.

We can imagine those first mountains rising as folds under the sea. Gradually their bases were narrowed, and their crests lifted out of the water. They rose as long, narrow islands, and grew in size as time went on.

Why is the trend of the great mountain systems almost always north and south? Study the map of the continents and see how few cross ranges are shown, and how short they are, compared with the others. The molten globe bulged at its equator, as it rotated on its axis. The moon added its strong pulling force to make it bulge still more. As the crust thickened, it became less responsive to the two forces that caused it to bulge. The shrinkage was greatest where the globe had been most pulled out of shape. The rate of the earth's rotation is believed to have diminished. Every change tended to let the earth draw in its (imaginary) belt, a notch at a time. The forces of contraction acted along the line of the equator, and formed folds running toward the poles. In this early time the great mountain systems were born, and they grew in size gradually, from small beginnings.

These mountains of upheaval, made by the bending of the earth's crust, and the formation of alternating ridges and depressed valleys, are many. The earth is old and much wrinkled. Other mountains have been formed by forces quite different. Volcanic mountains have been far more numerous in ages gone than they are now.

Mt. Hood and Mt. Rainier are peaks built up by the materials thrown out of the craters of volcanoes dead these thousands of years. Vesuvius is at present showing us how volcanic mountains are made. Each eruption builds larger the cone—that is, the chimney through which the molten rocks, the ashes, and the steam are ejected. Side craters may open, the main cone be broken and its form changed, but the mass of lava and stones and ashes grows with each eruption. The mountain grows by the additions it receives. Ætna is a mountain built of lava.

A third mountain system grew, not by addition, but by subtraction. The Catskills illustrate this type. This group of mountains is the remnant of a table-land made of level layers of red sandstone. The rest of the high plain has been cut down and carried away, leaving these picturesque hills, the survival of which is as much a mystery as the disappearance of the balance of the plateau of which they were once a part.

The fold that forms a typical mountain ridge has a cone of granite, the original rock foundation of the earth, and on this are layers of stratified rock, ancient deposits of sediment carried to the sea by streams. When exposed to wind and rain, the ridge is gradually worn down. In some places the water cuts away the soft rock and forms a stream-bed, that cuts deeper and deeper, using the rock fragments as its tools. Often the layers of aqueous rocks are cut through, and the granite exposed.

Sometimes the hardest stratified rock-beds resist the water and the wind and are left as a series of ridges along the sides of the main range. The crumpling forces may crack the ridge open for its whole length, and one side of the chasm may slip down and the other go up. The result is a sheer wall of exposed rock strata, layers of which correspond with those that lie far below the top of the portion that slid down in the great upheaval and subsidence that parted them. These slips are known as faults.