CHAPTER TWO

The Formation of the Black Hills

One of the most rewarding features of a visit to the Black Hills is the opportunity for the average individual, who has no technical training, to see with his own eyes a museum of the earth’s ages and a living sample of practically every one of the many aeons of the planet’s history.

The Hills, which is to say the rock substances of the region, are older by hundreds of millions of years than the stone out-juttings of the Rocky Mountains. Layer after layer of slates and schists from the very foundations of this globe lie visibly exposed as the end result of a doming of the region, a vast blistering, as it were, which raised the entire structure, layer upon layer, several thousand feet in the air. Following this doming process, a vigorous program of erosion commenced. Stratum by stratum the winds and rains cut across this huge blister in a horizontal plane, eventually laying the core open at the height above sea level at which we find the Black Hills today. From that core, extending in every direction in the general form of a circle, the various strata which once lay so smoothly one upon another have been laid open as one might slice off the top of an orange.

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.

During the Cambrian period the land subsided again, perhaps because of the weight of the uplifted sedimentary formations. During this subsidence the waters once again covered vast portions of North America, and additional muds and slimes were deposited on the bottom. It was at this period that life first appeared on the earth, in the form of simple marine organisms which have left fossil remains. These deposits made in the Cambrian period can be seen in outcroppings all through the region, although they are most notably found in the area about Deadwood. Because of their structure they indicate to the geologist that the shoreline of the ancient Cambrian sea was near at hand, and also that this covering of water was by no means as extensive or as deep as the earlier Archean sea.

The deposits of sand and mud, which were eventually pressed into stone, occasionally reach a depth of as much as five hundred feet, although they were laid down extremely slowly, as eddying mud is laid at the bottom of a pond. In the locality of Deadwood they contained a rich infiltration of gold, and the entire conglomeration was thoroughly intermixed with a vast outcropping of much older rock—this effect undoubtedly having taken place later, during the great continental uplift, when the final doming occurred.

THE AGES OF EARTH

MILLIONS OF YEARS AGO (Pre-Cambrian Existence back to 3½ Billions of years)

PALEOZOIC ERA 510 Cambrian Period—First fossils deposited. Marine life. 430> Ordovician Period—Invertebrates increase greatly. 350> Silurian Period—Coral reefs formed. First evidence of land life. 310> Devonian Period—First forests. First amphibians. 250> Mississippian, Pennsylvanian, and Permian Periods.—Reptiles and insects appear. Continental uplift at end of this period. 180 MESOZOIC ERA Triassic Period—Small Dinosaurs. First mammals. 150> Jurassic Period—Dinosaurs and marine reptiles dominant. 125> Cretaceous Period—Dinosaurs reach zenith of development then disappear. Small mammals. Flowering plants and development of hardwood forests. 60 CENOZOIC ERA & PERIOD Paleocene Epoch—Archaic mammals. 50> Eocene Epoch—Modern mammals appear. 35> Oligocene Epoch—Great apes appear. 25> Miocene Epoch—Grazing types of mammals appear. 10> Pliocene Epoch—Man appears. 0

The next period of the earth’s age—the Ordovician period, which extended from 430,000,000 to 350,000,000 years ago—has left its mark just as visibly upon the Black Hills. It was during this period that the many species of invertebrate marine life reached a zenith of development, and that a bed of sediment was laid down and later compressed to a pinkish limestone. The fact that this Ordovician bed is less than forty feet thick indicates that the land mass from which the muds and sands were drawn was very low, and that the Cambrian sea was relatively shallow, entertaining only minor erosive currents along its shores.

The next two ages, the Silurian and the Devonian, which brought our earth down to a scant 250,000,000 years ago, did not see the deposit of any silting in the Black Hills region. No doubt the waters which covered the locality dried up gradually. The Mississippian period, however, was a time of great depositional activity. A layer of limestone between five and six hundred feet thick was set down over the entire section. In later periods this limestone underwent much decay and water erosion, which formed the amazing caverns for which the Black Hills are known. Wind Cave, now the site of a National Park, Crystal Cave, and Jewel Cave are the best-known tourist attractions among the many, although there are a number of lesser ones, some even today only partially explored.

The chemical activity which accomplished this erosion was caused by the seeping of rain water down through later accumulations of sediment on top of the layer of limestone. As it seeped through rotting vegetation and timber the water collected carbonic acid gases which, when it reached the level of the Mississippian limestone, eroded the structure and ate out huge hollows in it.

The thickness of the Mississippian deposit indicated that at this time the earth had again sunk beneath the waters to a considerable depth. The shallow sea which had not offered sediment to a greater depth than a few feet was replaced by active currents which carried heavier sedimentary materials from great distances, laying them down on the floor of the sea in various strata to a depth of several hundred feet. Finally, after an unknown number of millions of years, but perhaps during the Triassic period, the land again rose above the level of the waters. A red shale suggests a time of great aridity when the region must have been a near desert, and certain discernible patterns in the shales suggest periods of rapid evaporation and a consequent change in chemical activity.

Finally the land subsided again, for the last time to date. At times salt water covered the region, and at other times fresh water left its chemical mark. At some levels in this last layer of sedimentary rocks an abundance of fossils can be found, indicating deep water, and at others ripple-marked rock indicates very shallow water. It remains a period of great mystery. How long this final submersion continued we do not know; but in all probability it lasted nearly a hundred million years, and then was terminated by the vast upending of North America which created the Rocky Mountains. This upheaval did not take place suddenly, as a volcanic eruption or a series of earthquakes, but apparently commenced about sixty million years ago and lasted, as a continuous series of shiftings and slow upheavals, for about twenty million years.

At the beginning of this mighty uplifting the region of the Black Hills was covered by the various layers of sedimentary deposits to a depth of nearly two miles. Slowly this rectangular area was lifted as a dome over the surrounding prairies. We do not know how high above the level lands this dome reached, but we do know that several thousand feet of later deposits overcapped the granite upthrusts which were planted in the fundamental shales. Those granite fingers, which have now been exposed to view, stand from five hundred to four thousand feet above the plains, and thus the original dome may be assumed to have extended from eight to ten thousand feet above our present-day sea level.

Almost as soon as this era of upheaval first started, as soon as the first land of the Black Hills was elevated above the great sea, the forces of erosion set to work. Wind and rain worked their terrifying magic on the slowly rising terrain, carving away the softer rocks and the loose dirt and leaving only the granite outcroppings. Down from the sides of the great dome poured the waters of melting snows, gushing springs, and torrential rainfalls, digging out rivers, canyons, and the deep and narrow cuts which characterize this beautiful region. Slowly the land continued to rise and the oceans to fall until at last an equilibrium was reached, a static state of affairs which remained much the same until our own period, when the base of the dome was at last revealed, with the surrounding lands drifting away in every direction on a gentle incline.

From that day the structure of the Black Hills has changed but little. The high winds off the Dakota plains and the annual spring run-off and seasonal rains cut their minute etchings in the landscape; but Nature’s greatest effort in the Black Hills part of the world has, it seems, been made. It must be remembered, though, that Nature has had other responsibilities. At the time of the doming of the Black Hills the Alps had not yet been formed, nor the Pyrenees, nor the Caucasus. And on the site of the mountains we know today as the sky-piercing Himalayas, the swampy waters still moldered.