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.