Jewel Cave National Monument contains Jewel Cave, currently the third longest cave in the world, with 181.89 miles of mapped passageways. It is located approximately 13 mi west of the town of Custer in South Dakota's Black Hills.
P3290001 P3290002 P3290003 P3290004  During the Mississippian time period, between 345 and 360 million years ago, a shallow sea covered the area. The sea advanced and receded several times. Sediment and calcium carbonate shells accumulated at the bottom of the sea, and over time, were compressed to form the Pahasapa Limestone (regionally known as the Madison Formation). The shells that formed the limestone came from ancient marine animals such as brachiopods.
P3290007  The blunt nailhead spar crystals (the "Jewels" that sparkle in the light)  that line most of the cave’s walls are not forming today. They formed when the cave was still completely or partially filled with water. As acidic water dissolved the limestone and created the cave, it became saturated with calcite. Some of this calcite was re-deposited underwater on the walls of the cave, in the form of spar.  Pockets of dogtooth spar, which are sharp-ended crystals, formed when the limestone was still deeply buried under younger rocks. They once lined the openings of early caves that were not completely filled with sediment from deposition of the Minnelusa Formation. P3290013 P3290014 P3290020
P3290022 P3290024  Gypsum speleothems form because water seeping into the cave often contains small amounts of gypsum (calcium sulfate, CaSO4) picked up from the limestone or overlying sandstone. When this water evaporates in the cave, it deposits gypsum in the form of needles, beards, flowers, or spiders. Gypsum formations are found only in dry parts of the cave. P3290025  Hydromagnesite speleothems are often the by-product of frostwork or popcorn formation. When calcite and aragonite crystallize out of water seeping from the cave walls, magnesium becomes more concentrated than calcium in the remaining water. In areas of very high evaporation, the magnesium will precipitate out as hydromagnesite. Hydromagnesite often appears on the walls as small white clumps resembling chalky cottage cheese. P3290028
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