Black Hills Speleogenesis
The Black Hills uplift contains a truly world class karstic terrain, which boasts the second and sixth largest caves on the planet as well as numerous other large and complex caves.
The karst features of this area are decidedly unique, however, in that they typically represent one end member of the hypogene (phreatic) to supergene (vadose) origin cycle for caves.
Much if not most major cave development and enlargement in the Black Hills region is thought to have taken place in the phreatic zone below the water table, with secondary enlargement and modification by vadose processes taking a much smaller role than in other typical karst areas.
Geologically speaking, much of the soluble bedrock in this uplift has until quite recently been covered by one or more hydrologically confining rock strata, and has most likely been under at least moderate thicknesses of cover during much of the speleogenetic process.
As a result, karst features may be evidenced only in the subsurface and the more typical expressions like sinkholes lapies and grikes may be totally absent in the surrounding terrain.
Large phreatic passage
Holloch Cave System, Switzerland
Keyhole Shaped Phreatic Tube with vadose "canyon" notch in base
Elephant Cave, Wales
Exposure of the upper portion of the Pahasapa Limestone in Little Elk Creek canyon showing its' mega-porous character