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But the same water that nourishes hones and plagues appliances boasts another power. It dissolves rock. Limestone is composed of calcium carbonate — the shells of onceliving sea creatures. When rain combines with carbon dioxide, it forms a weak solution of carbonic acid. The acidified water trickles through the porous stone, and, over time, dissolves the stone along its path.
Thus were the great caverns of the Shenandoah Valleyformed. The same process, on a smaller scale, produced the hundreds of sinkholes that dot the area.
Sinkholes, according to Loudoun County hydrogeologist Brutus Cooper, occur when acidified water follows the fractures that slice through limestone bedrock. The water pools in small open spaces in the porous rock. When precipitation is abundant, the level rises, and the acidified water dissolves the rock on the pocket’s ceiling. When the level decreases, the water runs out though a drainage tunnel, taking with it the dissolved stone.
Each time the water table rises, the carbonic acid does its work; when the stone is eaten away, the water washes away the dirt and mud, further enlarging the hole.
Cooper said a sinkhole occurs when the weight of the ceiling col
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