  {uncaptioned} Stone Mountain, sixteen miles east of Atlanta, is the world’s largest exposed granite monolith. It is as great a wonder to geologists today as it was to Indian medicine men of ancient times. While geologists know how it was formed and what it is made of, they still are amazed at its tremendous size, its wonderful symmetry and its location, high and alone on a gently rolling plateau over thirty miles from its nearest mountain neighbor. This mountain is a perfect example of the unbelievably powerful forces and the eternal patience of nature, for it was a million years in the making and lay a hundred million years incubating before it arose like a great egg on a vast plain in another hundred million years. Stone Mountain is 1,683 feet above sea level, and 825 feet above the surrounding land which is itself a dividing ridge. Rain water running off the eastern slope goes into the lake and out by the Yellow River. That on the west finds its way to South River. The streams join 50 miles away at Lake Jackson and flow on by the Ocmulgee and Altamaha to the Atlantic Ocean. Three or four miles to the north, headwaters of Peachtree Creek start their long trip to the Gulf of Mexico by way of the Chattahoochee and Apalachicola. The exposed granite of Stone Mountain covers 25 million square feet, or 583 acres. A surveyor figured the mass at 7,532,750,950 cubic feet. Since that time several million cubic feet have been quarried and shipped away, but all of man’s endeavors show as insignificant peelings taken from the western and eastern slopes. Granite weighs 167.9 pounds per cubic foot, if you are interested in computing the weight of Stone Mountain. Granite is the universal stone, containing practically all the natural elements from uranium and aluminum to iron and silica and the rarer minerals. It decomposes into fertile soil, as is readily seen by the growth that springs up where a little dirt and moisture collect on the gentler slopes of the mountain. Stone Mountain is near the foot of the Appalachians, an extremely ancient mountain chain originally composed of granite gneiss. The peaks, in their youth, rose much higher than the brash young Rockies, or even taller than the Himalayas. Three hundred million years ago, when Stone Mountain was born, the land in the area stood perhaps 10,000 feet higher than it does now. During a period that may have lasted a million years or more, molten stone under tremendous pressure was pushed upward from deep in the earth. If the force behind it had been sufficient to drive it out at the surface, the rock would have cooled rapidly and would have assumed a different form. The weight of two miles or so of rocks and earth overhead was sufficient to contain the tremendous pressure of the molten flow, so the upper crust literally floated on a hot liquid base. Something had to give as liquid rock thrust into an area where there was no space. Forty miles or so to the northwest is a chain of mountains, of which Kennesaw is the tallest, formed by pressure from the side which buckled underlying rocks up like a steep roof, or folded layers over each other. Some of that pressure may have been applied by Stone Mountain. This admittedly is theory—upper layers which held much of the factual story have long since washed away. {uncaptioned} Since the intruding material was contained in its original prison cell and held under constant pressure, it cooled gradually, a process which took perhaps a hundred million years. By cooling slowly, the molecules formed compact, uniform crystals. Meanwhile the older, softer granite overhead was weathering and turning to soil and eroding away. Some went to extend the coastal area of southeast Georgia and some to help build up the rich black belt of South Alabama. In the two hundred million years since the intrusion, the two-mile-thick overlay has eroded down to its present level, leaving the hard core of Stone Mountain standing up like a great gray egg. The surface of the mountain wears very slowly—scuffing feet of millions of visitors have left barely discernible marks along the western trail. Meanwhile the original crust is still wearing away at a rapid rate, so Stone Mountain is continuing to grow taller in reference to its base. Around the base have been noted fingers of Stone Mountain granite extending outward into the old rock, or sometimes soil, where the molten material was forced into crevices during the lateral movement of underground strata. The mountain is a natural target for lightning. Thunderclouds bombard it with their heaviest artillery. A bolt of lightning behaves very much like the thermo-jet torch. Its extreme heat converts moisture in underlying molecules to steam and literally blasts off the surface crystals, making a slight saucer-shaped depression four to six inches across. Heat fuses the bottom of the depression, leaving a slick, glassy surface. Every lightning bolt for many years has left its mark. It is noticeable that they are thickest not on the highest points, but in depressions. Meteorologists say that is where the first drops from a shower soak into the granite and therefore make the best ground to attract the lightning. From the time Gutzon Borglum began carving in 1923, stone rubble piled up at the base of the mountain below the monument. Hardly a man alive could remember what lay under it. After nearly fifty years, when the rubble was removed, there was revealed a low hill of the original granite gneiss peeping out from under the mountain, or more accurately, pushing into its side. The old rock clearly shows how it was twisted, turned and tortured by the great pressures of two hundred million years ago. Unable to shove aside this lot of rock, the molten mass tried to engulf and digest it. We see only the tip top of Stone Mountain. The shape around the base that will be revealed when more of the surrounding rock erodes away in the next million years is anybody’s guess. The depth surely is infinite, for it is still connected down through the channel by which it spewed upward. Stone Mountain is unique. Chemical makeup of the granite, and its physical characteristics, are different enough from all other stone in the Southeast to indicate that this is the only portion of that ancient flow of molten rock that has yet reached the surface. |
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