Travertine is a common feature in the northern Ozarks and along the Buffalo River due to the abundance of soluble limestone there. Common in caves (stalactites, stalagmites), travertine forms by the precipitation of minerals from ground water. In the example above, it formed on the face of a bluff, giving the bluff a melted appearance.
Like limestone, travertine is composed of the mineral calcite which dissolves if exposed to acid. When rain falls, it picks up CO2 from the atmosphere and soil, and becomes slightly acidic. It then flows underground through the bedrock dissolving some limestone along the way. When the groundwater re-surfaces at a spring or seep, The pressure drops, forcing the CO2 out of the water. The loss of CO2 lowers the waters acidity; It can no longer hold the calcite in solution, and calcite precipitates as the sedimentary rock travertine.
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Above are speleothems- they’re also called dripstones, stalagmites and stalactites, or other names depending on their shape. They can be seen when touring Blanchard Springs Caverns located about 10 miles north of Mountain View, Arkansas. The cave is full of breathtaking features like these which form by precipitation from ground water that drips from the cave ceiling. The groundwater, which has traveled through rock and soil before reaching the cave, is saturated with minerals it dissolved along it’s path. The principal mineral that forms the dripstones is calcite. As the groundwater enters the open air of the cave a pressure drop causes the calcite to come out of solution and precipitate a tiny coating on the outside of the growing speleothem. Over time some of them can reach from the roof to the floor of the cave and elaborate, and marvelous shapes develop.