Page:Popular Science Monthly Volume 89.djvu/427

 Balanced Rock, in the Garden of the Gods, Is So Perfectly Poised That Formerly Two People Could Sway It. Now It Is Cemented Firmly in Place

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��Rocking Stones and Their Romantic Story

��IN some of the accompanying photo- graphs arc to be seen three different kinds of rocks, perclied by Nature one on top of another. How could they have been placed in such positions? They weigh many tons.

Millions of years before the coming of the first man on the earth, the two top detached boulders were gently placed in their present resting places by the liand of a veritable giant — the North American glacier. During the (ircat Ice Age the ■whole of the northern jiortion of the United States was covered hundreds of feet deep with glacial ice. A glacier is snow, which, by melting and intense packing, is formed into solid ice banks. But the glacier is a constanth' mo\ing ice mass. It trav'els slowK- but wiili enormous grinding and carrying power, down the slopes and valleys.

A mass of ice se\'eral hundred feet thick, constantly replenished at its source, and sliding down a mountain slope with a weight of many tons to the square foot must have been well-nigh irresistible. That such was the case is illustrated by the many enormous boulders which were picked up from their original moorings by the huge glaciers of the Ice Age and transported many miles before they were deposited by the melting of the ice.

Without this now well established

��explanation of glacial transportation it would be impossible to account for the queer i)ositions in which boulders are often found as well as for the inter- mingling of entirely different kinds of rocks in the same place.

In one of the photographs the upper boulder is a rock about fi\e b>' eight by eight feet, of coarse Massachusetts granite. It is securely perched on a different kind of rock of nearly the same size — a rock known as a gneiss. Both are resting on a granite ledge, but of a different texture from that of the upper granite rock. Even assuming that the two granite rocks were alike, without knowledge of glacial action, it would be difticult to account for the presence of the middle boulder weighing at least ten tons.

Nowhere in the I'nited States are the evidences of the tremendous force of the great glaciers of the Ice Age more striking than in the Sierra Nevada of California. Rugged \'-shaped moun- tain gorges have been scoured and smoothed out into broad I'-shaped \alle\s by the great descending ice masses. The dirt and rocks have been spread about on the plains below or at the mouths of the canyons, while the glacial scratches and furrows can be plainly seen on the remaining rocks in

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