Maryland Department of Natural Resources

Crystal Grottoes, Washington County

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Crystal Grottoes in Boonsboro, the only commercial cave now open in Maryland, is also one of the largest in the state. Nearly one half mile of passage was mapped in 1968. The cave was discovered in 1920 as a result of quarry operations for road material. Drills penetrated the passages near the present entrance, and an opening was effected by blasting. Because of their beauty and commercial possibilities the caverns were spared in the quarrying: and, in 1922, after clearing and installation of electrical equipment, the caverns were opened to the public. The entrance house was originally a wooden structure. It was replaced in 1942 by the modern stone structure.

Stalactites in Crystal Grottoes
Stalactites in Crystal Grottoes
Cave Entrance


Crystal Grottoes is developed on the east side of a broad subordinate anticline in the Tomstown Dolomite with a dip of 20º E and a strike of N 30º E. The pattern of the cave is strongly controlled by three sets of master joints. The major influence is from those striking N-S, with subordinate passages reflecting joints trending N 30º E and N 30º W. 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 6 feet of this clay in places. 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.


Entrance to the grottoes is by a stairway in the entrance house. The first room is 8 feet below the entrance house and is oblate in shape, being 30 feet long, 10 feet wide, and 15 feet high. At each end it is pinched out by mud flows and narrowing of the passage. This room originally contained a considerable number of formations, but quarrying operations and clearing of passages have removed them except along the west wall where flowstone and stalactites are abundant. Connecting the entrance room with the rest of the cave is a small tunnel 6 feet high that is reached by stairs going down 6 feet. From this point on the cave is a series of fissure-like passages of unsurpassed beauty. The fissures are uniformly 4 to 6 feet wide with ceiling heights varying from 60 to 40 feet. The floor shows little change in elevation throughout the cave. With the exception of the passage from the Blanket Room to the exit, the passages are continuously lined or covered by formations. Delicate drape-like stalactites, bacon rinds, and stout columns predominate. The colors are generally pure white or buff with occasional deeper tints. The passages forming Fairyland are studded with stalactites and stalagmites of a delicate light blue hue.

Detail of stalactites in Crystal Grottoes
Detail of stalactites in Crystal Grottoes

The passages between Fairyland and the Blanket Room are dense with beautiful formations which are striking because of their clean white surfaces. In this area, where formations are occasionally absent, the ceiling and walls are covered by a green-gray residual clay, which combined with the graceful contours of the ceiling rivals the formations in beauty. This clay is an integral part of the dolomite and remains a residual product upon the dissolving of the carbonates. It is porous and retains the structure of the original beds. When dry it is quite strong although the surface is powdery, but when wet it is soft and weak. The Blanket Room is the largest room in the caverns, being 30 feet and 20 feet wide. Large sheets of stalactites and bacon rind hang in clusters from the ceiling, which is here 20 feet high.

The passage leading to the Golden Lake is profuse with formations and in part is bridged by flat-lying travertine, a condition that is found in many of the passages not open to the public. The Golden Lake is a small pool fed by water dripping from the ceiling. In wet seasons the water accumulates at a rate necessitating periodic bailing. The remainder of the cave is along a passageway similar in size to those already described except that formations are sparse. The walls are sooty brown in color and of a nodular texture. The last 40 feet of the cave is through a stone-walled passage leading up a gentle grade to the exit. The passages not open to the public are similar to those already described except they are constricted at many points. Orange-brown clay that covers the floor and lower walls of these passages is often overlain by calcareous formations. With one exception these passages are developed at the same level as the main cave. The exception is the passage at the extreme end of the Flowstone Way that descends about 15 feet below the general level. The commercial tour route covers approximately one third of the known cave.


Maryland's only public showcave is a one hour drive from Baltimore and Washington, DC, through Maryland's scenic Frederick Valley and South Mountain countryside. Crystal Grottoes is located 1.5 miles outside of Boonsboro, MD, on State Rt 34 in Washington County, just 6 miles from Antietam Battlefield and 18 miles from Harper's Ferry. From Baltimore and Washington, take US Rt. 70 west to US Alt 40, toward Boonsboro. You will pass over South Mountain near the Washington Monument State Park, and descend to Boonsboro. Turn left at Rt 34 (Shepherdstown Pike) and drive 1.5 miles to Crystal Grottoes.