Saturday, January 10, 2015
Onyx Cave, Arkansas
Draperies of flowstone are only one of several types of speleothems, or cave formation, that can be found in Onyx Cave, near Eureka Springs, AR. The term 'onyx' is not a mineral name, but rather a commercial one. It has been used to refer to a number of different rocks or gemstones over history, but most often refers, today, to a dark or black agate or to travertine calcite. The onyx of Onyx Cave is travertine calcite, or limestone, and the formations were formed by slow precipitation of the mineral from groundwater.
Onyx cave is a publically accessible tour cave. Several other caves that are open for tours are also found in the same region, and this isn't a coincidence. The southern Ozark Plateau has one of the densest populations of caves and karst formations on the planet. The region is underpinned by up to 1500 feet of soluble limestone and dolostone, and is very old. Most of the sedimentary rocks are 340 million to half a billion years old, and they have been heavily fractured at several points in geologic history. Great age, soluble rock, fractures, and groundwater means caves - lots of caves.