THE GEOLOGIC STORY
Approximately 300 million years ago, the entire Black Hills region was at the bottom of a sea. In the sea lived many marine animals somewhat similar to the clams and snails of today's seas. The sea bottom became littered with the calcium carbonate shells of dead marine animals, accumulating to depths of hundreds of feet. These calcium carbonate deposits were gradually formed into a thick bed of limestone. Today this bed of rock is called the Pahasapa Limestone after the Sioux Indian name for Black Hills. As time passed, the sea retreated and advanced; sometimes exposing the area to erosion, then piling more sediments on, until thousands of feet of sedimentary rocks eventually lay on top of the Pahasapa limestone. Within the bed itself, solutions of silicon dioxide replaced the limestone in some places, forming a type of rock called chert. The chert formed in a few thin layers and lenses. About 70 million years ago began a series of uplifts associated with the forming of the Rocky Mountains and, at the same time, the Black Hills. The Black Hills were warped upward like a giant bubble in the earth's crustan eliptical, flattopped dome approximately 120 miles long by 60 miles wide. As this dome was being uplifted, its top was gradually being stripped away by stream erosion. Today, of course, the seas are gone from the interior of the continent; and the Black Hills dome stands in the midst of a sea of plains formed by the redistributed sediments which once covered the central portion of the hills. The Pahasapa Limestone is exposed today as a ring of limestone plateau surrounding the central hills. It is in this bed of rock that Jewel Cave and the other Black Hills caves occur.
As a result of the flexing movements in the earth's crust, these sedimentary rocks were forced to bend. Limestone, however, does not bend very well; and the stretching caused the rock to fracture in essentially three planes, each at nearly right angles with the others. These fractures or "joints" vary from a few inches to several feet apart. At Jewel Cave there is one set of vertical joint planes running more or less north-south and another almost east-west. These combined with horizontal bedding planes form a pattern like a giant layer cake which has been cut into squares. It was this pattern which controlled the development of the cave.
Initially, the Black Hills may not have been very far above the surrounding terrain. Ground water moved slowly in the fissures in the limestone. Rainwater falling to earth in this warm climate absorbed carbon dioxide and other gasses from the air and soils. In this way, the ground water became a weak solution of carbonic acid. This solution with its capability of dissolving calcium carbonate percolated downward until it reached the water table somewhere above the Pahasapa Limestone. (In this booklet, the terms "water" and "solutions" are used more or less interchangeably, since nearly all cave waters would include these and other impurities in solution.) Since water cannot percolate through limestone between tiny spaces as it does in sandstone, it followed the joint planes, slowly dissolving limestone as it moved along. Gradually as the limestone was dissolved and carried away in solution, the fractures were enlarged into passages. Water movement was preferentially westward, the direction of tilt of the strata, so that the east-west passages became higher, wider, and longer than the north-south passages. The passages also developed at several different levels, but not all fractures at any one level developed equally. This was probably due mainly to the varying solubility of the limestone. The zone of chert, being much harder and less soluble than limestone, helped to separate levels. The result is an irregular, three-dimensional, grid-like network of interconnecting passages. It took several million years for the initial Black Hills uplift to take place, and it took another several million years for the passages to be developed by solution. During this latter period the uplifted area apparently remained quite stable, judging from the evidence that water moved through the limestone quite slowly.
Even today we hear about change in the world climatea gradual warming trend, advance and retreat of glaciers, etc. Evidences of many climatic changes and events exist in Jewel Cave. For instance, it appears that the cave was drained after solution of the passages. Then, waters again flooded the cave; but instead of dissolving more limestone, this time, they deposited the calcium carbonate in the form of calcite crystals. Calcite is the mineral form of calcium carbonate and can take many different crystal forms. In Jewel Cave, most of the large crystals of calcite are sharply pointed crystals called "dogtooth spar," or a blunter variety called nailhead "spar." These crystals were undoubtedly formed underwater from very slowly moving solutions. Why the solutions began to deposit calcium carbonate instead of dissolving it is something of a mystery, but many geologists feel that it was due to a change in the temperature of the water. This probably resulted from warming of the climate. At any rate, the solution-filled passages were lined with a layer of calcite crystals up to seven inches thick.
Following the formation of these crystals, there appears to have been a series of alternate drainings and floodings caused by fluctuation in the water table. Many of the calcite crystal deposits show layering with clay sediments which were carried into the cave. The glass-like transparency of the calcite crystals is in most cases obscured by a clay "cap." This must have been deposited during one of the last of these floodings when silt-and clay-laden solutions inundated the cave. By contrast, small crystallined cavities called "vugs," which formed in joints off the main galleries, contain dogtooth spar that is "crystal clear." These vugs were more or less sealed off from the main passage and were less influenced by the sediment-filled waters.
During one of these later floodings, the waters again were able to dissolve calcium carbonate, and the outer ends of some crystals were dissolved away, leaving a flat surface of hexagonal crystal cross sections.
Today the later table is at least 160 feet below the lowest point in the cave. As it lowered to this level long ago, it drained the cave a final time. Ceilings, which were partly supported by the water, collapsed leaving piles of limestone blocks or "breakdown" as it is called, on the floor. Where major passages intersect, larger chambers were formedthe ceilings collapsing to form a single tension arch ceiling. At these points occur the largest rooms in the cave with characteristic high-domed ceilings and a high pile of break down in the center of the room. Evidence for the dating of this event lies in the fact that the exposed ceilings and upper walls have no crystal coating, but crystal layers do show on the bottoms of the breakdown blocks.
Resuming uplift of the Black Hills may have caused this final lowering of the water table. Certainly uplift did occur, for streams were accelerated, eroding more of the sedimentary rock cover of the Hills and excavating deep canyons. One such canyon is known today as Hell Canyon. As Hell Canyon was deepened by stream erosion, it cut across one of the passages forming a natural entrance to the cave. The stream occasionally dumped mud and clay into portions of the cave, completely filling one passage on the north wall just inside the natural entrance. When the canyon deepened to its present level, the natural entrance was left high and dry in the canyon wall, 100 feet above.
Last Updated: 02-Feb-2007