Crystal Grottoes
Coordinates: 39°29′53″N 77°40′37″W / 39.498°N 77.677°W
Crystal Grottoes is the only show cave in the U.S. state of Maryland. It is located on Maryland Route 34 between Boonsboro and Antietam National Battlefield.
Crystal Grottoes was discovered in 1920 in the course of quarrying operations by a road construction crew trying to obtain gravel. A drill bit (for placing explosives) disappeared into a hole, and the crew realized they had found a cave. Blasting produced the entrance which is still used, and in 4/2/1922 the cave was opened to the public. A mapping operation in 1968 revealed about a half mile of passages, but only about one third of the cave is accessed during the tour, which takes about 30-35 minutes.
Crystal Grottoes is an example of a solutional cave, formed in a karst environment within a synclinal occurrence of Tomstown Dolomite. The cave is essentially horizontal throughout its extent and the passages are typically high and narrow. A great deal of brown and red clay fills most of the cave to a considerable extent; the commercial tour routes involved the removal of up to 4–6 feet (1.2–1.8 m) of this clay in 900 feet (270 m) of the passages. Sediment fills many of the undeveloped passages to within a foot or two of the ceiling, thereby making many crawlways. No streams exist in the cave, although a small "lake" or pool is maintained by drip water.
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