Note: The "stages" referred to below are only for convenience in this outline and include only the major events. They are not formal divisions. - Art and Peg Palmer (1999)

Stage 1: Deposition of the Pahasapa Limestone (Madison Limestone) on a shallow sea floor 340-320 million years ago (Mississippian Period). Low areas of the continent were covered by shallow sea water. The major cave-forming limestones of North America were deposited at this time. Several major layers (from bottom to top) = massive dolomite (route to Lakes), bedded dolomite and limestone (major boxwork zones), chert (ceiling of Ice Palace), massive limestone (Fairgrounds, Garden of Eden). Gypsum (hydrated calcium sulfate) and anhydrite (calcium sulfate) were also included within some of the lower and middle layers.

Stage 2: Gypsum and anhydrite are very unstable, both physically and chemically. Soon after the rocks were deposited (about 320 million years ago) they were uplifted slightly above sea level, allowing several things to happen:

1. the pressure of the overlying rocks forced the gypsum and anhydrite to migrate into major cracks in the surrounding rock;
2. anhydrite hydrated to gypsum, causing expansion that formed many small cracks in the surrounding rocks, especially the dolomite beds midway through the Pahasapa-these are the cracks that the boxwork follows;
3. reduction of gypsum and anhydrite in the deeper layers produced hydrogen sulfide, which migrated upward to oxygen-rich areas, where it oxidized to sulfuric acid; this acid formed the earliest cave openings-generally small pockets and fissures-and the surrounding limestone and dolomite were altered to a weak, crumbly, bleached zone. The basic outline of the cave passages were formed at this time;
4. some of the hydrogen sulfide combined with dissolved iron to produce iron sulfide (pyrite, etc.).

Stage 3: The climate became wetter, and considerable fresh water entered from the surface. Gypsum and anhydrite were replaced by calcite. Oxidation of the iron sulfide around the old hydrogen sulfide zones produced red and yellow zones of iron oxide in and around the cave, and the calcite deposited at this time is orange-brown as a result. This includes the veins that now protrude as boxwork fins. In the upper strata, gypsum was simply dissolved away, leaving a fractured jumbled texture in the limestone (as in the Garden of Eden).

Stage 4: Eventually the climate became so wet that sinkholes and solutional fissures formed at the surface, and some caves were formed or enlarged below. Some fissures extended below the chert level. Much of the cave enlargement simply followed earlier openings and fractures from Stage 2. Some of the upper-level openings formed at this time, but most of them are filled with sediment from Stage 5.

Stage 5: About 300 million years ago, a rise in sea level caused the Minnelusa Formation (mainly sandstone) to be deposited, filling in the sinkholes, fissures, and most early caves. The red sand and clay deposits in the Beauty Parlor and Garden of Eden represent the lowest layers of this formation.

Stage 6: Continued deposition of sediments buried the Pahasapa Limestone to a depth of at least a mile (300-70 million years ago-Pennsylvanian through Cretaceous Periods). A layer of white calcite (dogtooth spar) was deposited on the walls of earlier cave openings.

Stage 7: Forces within the earth's crust caused the Black Hills and Rocky Mountains to rise beginning about 70 million years ago. Mobilization of deep fluids early in this stage caused hydrothermal minerals to be deposited in some of the early caves and pockets, including quartz crystals (Crown Jewels, etc.).

Stage 8: As the Black Hills continued to uplift, the sedimentary rocks were tripped off by erosion, exposing very old (Precambrian) igneous and metamorphic rock (Harney Peak, Mt. Rushmore, etc.). The eroded edges of the sedimentary rocks, including the Pahasapa Limestone, were exposed around the perimeter of the Black Hills. Groundwater moved through the rocks in considerable volume, and most of the cave enlargement took place during this time. Again, the enlargement was concentrated along zones of older cave development and alteration. except in a few places the cave does not extent to the top or bottom of the limestone, nor does it extend far below the water table. Evidently the cave is not the product of simple artesian groundwater flow, infiltration from the surface, or hydrothermal water rising from depth. If it were, the cave would be largest where the water first entered the limestone. It is clearly the result of mixing between two or more of these water sources, which produced a zone of solutionally aggressive water. Its main solutional phase was about 60-40 million years ago.

Stage 9: As the water table dropped, weathering of the limestone walls took place and continues today. Most important, the crumbly, altered dolomite of Stage 2 has decomposed into a powdery sand that formed files of sediment on the cave floors and allowed the thin calcite veins (mainly the orange-brown veins of Stage 2) to protrude as boxwork. Bedrock walls have acquired a thin weathering rind of fluffy powder, colored red, yellow, and black from the oxidation of minerals in the rock (such as the pyrite from Stage 2). Rising of moist air from the lower levels causes condensation to occur on the walls of the cooler upper levels, dissolving the limestone into domes and chutes in the ceilings of upper levels. The condensation moisture, saturated with dissolved limestone, seeps through the walls to the water table, although some of it evaporates in the lower levels to produce aragonite frostwork and popcorn. These deposits can also be formed by water seeping from the surface. Active drips fed by infiltration from the surface deposit flowstone, stalactites, etc. In zones of ponding, such as the lakes, a wall crust of calcite has formed, and calcite rafts form at the surface. This water is supersaturated with calcite and cannot dissolve limestone. Older crusts higher in the cave, including dogtooth spar in larger openings (Stage 6), has been shaved off by condensation corrosion and weathering.

Major time indicators:

Orange-brown calcite = older than 320 million years. It is cut across by the red paleofill and never occurs in the paleofill, except as eroded fragments.

Red sediment fill (sand and clay paleofill) = about 300 million years old. This marks the well-known break that separates the mississippian and pennsylvanian rocks throughout the western states. Be careful-many of the dolomite beds weather to red colors too, as in the Post Office. Much of the paleofill in the cave has subsided into lower levels as the cave enlarged, but this can be easily recognized by the lack of (or disruption of) the white calcite coatings and vein fillings (described below).

White calcite veins and including dogtooth spar = between 300 and 70 million years ago, probably toward the younger end. This fills cracks and coats pockets in the red paleofill, so it is definitely younger. It is cut by the present wills and overlain by lack deposits such as wall crusts.

Quartz crystals = about 100-70 million years old. In places they coat the dogtooth spar, especially along faults.

The cave itself has an origin that spans the entire period from about 320 million years ago. The major solutional phase was about 60-40 million years ago, during which time the present topography developed. However, the present cave follows the patterns of the early gypsum and anhydrite zones, as shown by the fact that the orange calcite (originally gypsum) is concentrated only around the present caves. In large breakdown or blasted areas the calcite veins can be seen to die out away from the cave. Therefore, the cave pattern predates the uplift of the Black Hills. The pattern seems well adjusted to the Black Hills uplift, with fractures radiating away from the center of the hills. However, the Black Hills have long been an area of uplift and weakness in the earth's crust, and cracks tend to maintain the same patterns and are repeatedly reactivated.