The Doming of the Black Hills
Rock Strata being shifted into a dome at the time of the great continental uplift.
The forces of erosion—wind and water—have levelled the dome and opened the seams to view.
In order to get an even clearer picture of how this amazing phenomenon came about through the aeons, let us fold back the ages to the very birth of this planet.
For centuries men have attempted to determine the earth’s exact age, but except for the famed Archbishop James Ussher of Ireland, who gravely calculated that the earth was formed at precisely nine o’clock on the morning of the twelfth of October in the year 4004 B.C., no scientist has been able to come closer than a few million years in the figures. Through a number of trustworthy measurements, however—including, in recent years, the examination of the deterioration of radioactive elements in rocks—geologists have agreed that the oldest known ingredients of the earth’s crust have been in existence at least two billion years, and, according to some very recent calculations, possibly as long as three and a half billion.
In what is known as the Archean period, the most ancient of which we have any geological knowledge, a vast sea covered much of North America, bounded by certain masses of land, the extent of which has never been discovered. From this land mass remnants of mud and sand were broken away by waves and deposited on the floor of the sea. Eventually, under the pressure of its own weight, this material formed shales and sandstones to an undetermined depth—many thousands of feet. Those particular sandstones and shales underlie the entire Black Hills area and extend in nearly every direction for a considerable distance, suggesting perhaps that the area of the Hills was at one time the bottom of this watery bowl.
The Archean period came to an end some five hundred million years ago. By then the seas had withdrawn, and the new land formations which had lain under the early ocean merged with the vestiges of the first land mass. But this metamorphosis, which can be described in such calm fashion, was by no means a gentle affair. It took place largely as the result of a shifting and rising of certain ocean bottom areas, among which was the region where we now find the Black Hills.
At the time of this uplift, and possibly contributing to it, there was a tremendous disturbance in the lower regions of the earth which sent great streams of molten matter up into the several-mile-thick layer of shale, through which it poured toward the surface, breaking through in monolithic forms and hardening into granite. The New Mexico writer, Eugene Manlove Rhodes, describing a similar geologic phenomenon in the valley of the Rio Grande, has called it “like sticking a knife through a tambourine,” and indeed it was. Harney Peak, in Custer State Park, is just such a granite finger pointing up through the original shales toward the sky.
When this disturbance took place the granite juttings did not rise above the surrounding landscape as they now do. In many cases they did not even reach the surface of the shale beds, but ceased their flow and hardened short of the crust of the earth, as it was then to be found. When, however, the region was domed, many millions of years later, the subsequent weathering of the huge blister did not attack these granite formations with anywhere near the vigor with which the softer sandstones and limestones were eroded. What actually occurred, then, was a peeling away of the softer rocks, leaving the granite formations near their original sizes, but at last above the ground level in the form of peaks, needles, and spires.
But we have gotten ahead of our story. Following the Algonkian period, when the molten matter was injected into the layers of shale, there came what is known as the Cambrian period. The Cambrian occupied the first 80,000,000 years of the Paleozoic era, which in itself covered the entire period from 510,000,000 to 180,000,000 years ago.