The
Landscape
The
landscape of Wind Cave National Park is formed by the rock types,
their
structure, and how they were weathered and eroded. Gently tilting layers
of sedimentary rocks lie under most of the park.
The Oldest Rocks
The oldest rocks are
exposed in the northwest part of the park. These are schists and pegmatites.
The schists are metamorphic rocks which formed under heat and intense
pressure during an early episode of mountain building, about 2 billion
years ago. They have almost parallel bands, or foliation, caused by the
growth of mica crystals under pressure.
Pegmatites are made
of large crystals of glassy-gray quartz, pink feldspar, silvery micas,
and shiny black tourmaline. Pegmatite is an igneous rock, similar to granite.
It hardened from magma and hot fluids. In places, the pegmatite intruded
into the schists. This proves the pegmatite is younger than the schists,
but still very old at 1.7 billion years. The emplacement of the pegmatite
probably occurred during another mountain building event.
Sedimentary Rocks
To the southeast of
the igneous and metamorphic rocks, progressively younger layers of sedimentary
rocks are at the surface. They span a time from the origin of abundant
sea life, about 600 million years ago, to the end of the age of dinosaurs,
about 60 million years ago. During these years, seas advanced and retreated
over this region many times. Periods of deposition of sediments alternated
with periods of erosion. About 65 million years ago forces within the
earth produced another period of mountain building, raising the "modern"
Black Hills.
Shaping the Landscape
Since that uplift,
weathering, erosion and minor uplifting have been shaping the Black Hills.
Sediments produced by the erosion filled some valleys within the park
and spilled outside the Hills to the east, forming the layers now visible
at Badlands National Park. Rocks more resistant to weathering and erosion,
like pegmatite, limestone, and sandstone, form ridges or plateaus. Weaker
rocks, like schists and shales, form valleys. Examples of limestone, sandstone
and shale are visible in Beaver Creek Canyon, Wind Cave Canyon, and Red
Valley. Schists and pegmatites are visible along State Route 87.
The
Pahasapa Limestone
Wind Cave formed in
the Pahasapa Limestone. This limestone was deposited in a warm shallow
sea about 350 million years ago and is composed mostly of fragments of
calcium carbonate sea shells. Coinciding with the accumulation of limestone,
bodies of gypsum (calcium sulfate) crystallized from the sea water, when
arid conditions caused evaporation. The gypsum formed irregular shaped
masses within the limestone.
The Role of
Gypsum
The gypsum masses
were unstable. Their volumes increased and decreased as they absorbed
and expelled water. This caused fracturing to occur within the gypsum
and in the surrounding limestone. Like thick toothpaste, some gypsum squeezed
into these cracks and crystallized. At a later time, water rich in carbonate
ions converted all of the gypsum to calcite (calcium carbonate). This
set the stage for the cave and boxwork to form.
The
Cave Develops
Since acid-rich water
dissolves limestone, a chemical change in the groundwater had to occur
for the cave to form. The oceans receded allowing fresh water into the
region. As gypsum was converted to limestone, sulfur was chemically freed
to form either sulfuric or sulfurous acid. These acids dissolved the limestone
to form the first cave passageways some 320 million years ago.
After the first period of cave formation, seas
again advanced over this area. About 300 million years ago, layers of
red clay, sandstone and limestone of the Minnelusa Formation were deposited
above the Pahasapa Limestone. Some of this sediment washed into and filled
early-formed cave passageways. These "paleofills", are visible
in higher levels of the cave, near the Garden of Eden and Fairgrounds
rooms.
A
Complex Cave
Seas continued to
advance and retreat over the area for the next 240 million years. Deposition
of sediment alternated with erosion. Development of the cave was probably
slow until the most recent Black Hills uplift about 65 million years ago.
This opened more fractures in the limestone allowing more cave to form.
The waters that made the cave probably sat in the limestone for long periods
of time. Water did not flow through the cave like a river. The water had
plenty of time to dissolve passageways along the many small cracks, thus
developing the complex maze-like pattern.
Boxwork
Slow moving water
was also important in exposing boxwork. At the edges of the former gypsum
masses where the expansions had formed cracks, limestone was dissolved.
This dissolving of the surrounding limestone, left the previously deposited
crack fillings standing in relief. These exposed crystal fins are called
boxwork. A river might have eroded boxwork from the cave.
Where is the Water Now?
Geologists believe
that the water began slowly draining from the cave 40 to 50 million years
ago. Today the water level is about 500 feet below the surface at an area
named "the Lakes". Water, however, is still changing the cave.
Slow seepage of water produces frostwork and popcorn on cave walls and
ceilings. Formations that need more water, like flowstone or dripstone
deposits (stalactites and stalagmites), are rare in Wind Cave and are
limited by the dry climate and semi-permeable clay beds above the cave.
A
Very Unusual Cave
Wind Cave is over
300 million years old, making it one of the oldest in the world. Besides
extreme age, other features make Wind Cave unique. The cave is large and
extremely complex, the
113.05
miles (181.936
kilometers) of known cave fit under just over one square mile of land.
The boxwork is rare and found in few other caves. Wind Cave has undergone
many geological changes and the processes continue. Geologists have many
questions yet to answer before we can fully understand the rich, incredible
world below our feet.
See Palmer
and Palmer 1999 for the latest theory on the Origin of Wind Cave.
Wind
Cave National Park Nature & Science
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