General Geology


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Pre-Cambrian formations (Tour Map symbols 'Fe', 'Gf', 'Rt')

Events which uplifted the Black Hills (Space Shuttle photo) as a dome structure (see also 3-D topographic map) occurred in several episodes. The crystalline materials forming the central core consist of granites, Pre-Cambrian shists, volcanic basalts, rhyollites, and pegmatites. Bear Butte, located northeast of the Black Hills, was formed as an extension of the same volcanic intrusion events which affected the Black Hills. The granite has weathered into spectacular spires and needles in some locations. Mount Rushmore memorial and Crazy Horse monument (under construction) are carved out of this resistant granite.

 

Paleozoic/Mesozoic Formations (Tour Map symbol 'De')
The uplift dome occurring during the Tertiary period caused numerous faults, tipped the rock at very high angles, deformed strata, and formed several geomorphological oddities. After erosion, the outcrops of sedimentary strata lie at an angle along the dome edges. Traveling from the central core in any direction, one encounters progressively younger strata until encountering the Cretaceous and Tertiary strata of the Great Plains which lies nearly horizontal to the land surface. Erosion has eroded all the strata from areas above the crystalline core whose elevation is estimated to have been twice the height as in the present day. The stratigraphic column reveals deposits ranging from the early Paleozoic to middle Mesozoic eras, but not every period is represented.

 

Selective Formations (Tour Map symbol 'Rt')
The Deadwood formation overlies the Pre-Cambrian shist as an angular unconformity (gap in a depositional sequence). It also forms a cliff that towers above the city of Deadwood. The Deadwood members are conglomerates, sandstones, siltstones, and mudstones deposited in a marine environment.

Limestone formations are numerous in the Black Hills. The Madison formation (limestone) forms the gray cliffs above many of the canyon roads (Tour Map symbols 'Cl') in the northern Black Hills. The carbonates in these formations have solubilized underground in many locations to form caves in the east and southern 'Hills. Along highways, one can observe streams disappearing underground (Tour Map symbol 'Fe') only to reappear later downstream or as a spring in another creek nearby. The Jewel Cave National Monument and The Wind Cave National Park as well as numerous commercial ventures offer cave touring in the eastern and southern 'Hills.

Several red shale beds are visible while traveling around the 'Hills. The most prominent of these is the Spearfish formation known as the 'racetrack' because it is exposed as an irregular oval around the entire Black Hills. The highly erodible material is most visible in the northern 'Hills upon which I-90 is built from Rapid City to Wyoming. The red shale is capped in many locations by gypsum.

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