The Principal Cave & Karst bearing formation in the Black Hills, and the one that will be primarily described and discussed in the following section is the:
Pahasapa (Madison) Limestone
Areas of karst development are widespread in the overlying Minnelusa, Minnekahta and Spearfish Formations, however. Although caves do occur in all of these formations, the largest and most complexly developed systems are found within the Pahasapa Limestone and that is the formation upon which the majority of current speleological study and exploration is being centered .
The Madison group, whose Black Hills outcrop pattern is shown in red on the adjacent map, forms part of the nations largest confined aquifer and is far & away one of the most distinctive and unusual karst aquifers in North America. It covers large portions of Montana, Wyoming, Southern Saskatchewan and North and South Dakota.
The Pahasapa and upper Englewood Limestones form the Madison Group within the Black Hills uplift area.
The Madison Group outcrops as a nearly continuous ring around the Black Hills uplift, as can be seen on the adjacent map and the maps that follow.
This formation is locally 300 to 600 ft. thick, and composed of massively bedded, cliff forming limestone and dolomite that is notably flinty and cavernous in it's upper portions.
This highly karstic formation has localized stream sinks on most every drainage that it crosses on it's journey outwards from central part of the uplift. As such, the Black Hills constitutes one of the principal areas of recharge for this highly important and heavily utilized aquifer. The need to study, understand and protect this critical water bearing resource cannot be overstated.
Most of the Pahasapa Limestone's karst features are found in the formation's upper one half to two thirds and nearest to it's contact with the overlying Minnelusa Formation.
Karstic features have been recognized in this same horizon in deep drill holes well out into the surrounding plains, which suggest that there was at least one other period of karstic development prior to the the most recent period. The high permeability and transmissivity of the zone outside of the zone where active speleogenesis might be expected also argues for an earlier episode, as well.
Although parts of the Minnelusa Formation are also an aquifer, on the whole the formation acts as an effective barrier to vertical water flow into the underlying limestones. It is, in hydrological terminology, an aquaclude, and limits water inflow to the Madison except in those areas where it is not covered by the formation.
The pattern of the Madison Group's outcrops are shown in the sky-blue pattern on the geologic map of the Black Hills Uplift shown below.
It's outcrop pattern covers almost 500 square miles of Black Hill's terrain.
The narrow nature of the exposures along the east and north sides of the Hills is due to the steep dip of the beds in these areas, where prominent faults and folds are also found.
The broader pattern of Madison outcrops along the southern and western sides of the uplift are due to the opposite situation.
In these areas, the gentler dipping beds are crossed by fewer faults and folds, although some of these features are locally quite prominently developed.
Many of the larger caves in the Hills are found in close proximity to such major structures and it can be argued that there is a genetic relationship between the caves and these structures.
This is most particularly the case with the larger Labyrinth style systems like Jewel, Wind, Stagebarn, Reed's and Bethlehem caves.
Directly overlying the Madison group limestones and dolomites is a broad outcrop area of Minnelusa Formation sediments shown in light blue. This unit is a mix of sandstone, siltstone, shale and limestone and does not contain numerous caves, but it does display widespread development of karst features, particularly within it's basal portion.
The Minnelusa Formation contains varying amounts of gypsum, and dissolution of that mineral has led to widespread karst collapses and other features within the unit.
Karstic features in the Minnelusa are often a indicator of major dissolution in the underlying Madison Group.
Minnelusa derived karstic fill is not infrequently found in the caves in the underlying limestones, particularity where they are located just below the contact with this unit.
The Minnekahta and Spearfish Formations are typically exposed at the outermost edges of the central Black Hills uplift. They are shown on the map above as the peripheral rings of highlighted bright and darkest green respectively. Both of these formations host karst features, but are particularly well developed within the Spearfish Formation
Caves within the Minnekahta tend to be smaller and restricted to a relatively planar geometry, as the unit is at most 60' thick, but they have many interesting features and have been found throughout the Hills region.
The Spearfish Formation contains several thick layers of gypsum and has stringers of the same throughout its' 400' thickness. Gypsum's high solubility has resulted in some fairly major and impressive sinkholes, dolines, collapse features and caves. Some of these features are expressions of long term ongoing solution in the underlying formations working its way upward, and others are the result of near surface processes more closely tied to climatic conditions.