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National Geographic : 1964 Jun
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"The topographic map showed a hollow halfway up a mountain that led to a cave opening with a stream tumbling into it," Bill said. Members of the Huntsville Grotto of the National Speleological Society followed the stream 400 feet underground, then stopped -the water plunged into a huge black well. "There was so much mist we couldn't see bottom. The real puzzler, though-there was no roar as the water hit. We threw in some rocks. Not a sound! Finally we flung down a forty-pound boulder. Four or five seconds later we heard a thud. Light Vanishes Into Abyss "The next Friday we returned with a mag nesium flare, the kind made to light up the landscape for night aerial photography. We tied it to a line, pulled the pin, and tossed it over the lip. "When the flare exploded, the flash was blinding. Then we saw the light falling, through the cascading water, lots of smoke and spray, and finally the light went out. "How deep the cave was we still didn't know. But we lowered 375 feet of calibrated cord with a rock tied to the end. The stone snagged on a ledge, and when we tried to jerk it free, the cord broke." The pit of Fern Cave, as finally plumbed, Water and Acid Fashion a Cave; Nature's Work Takes Millenniums As geologists interpret evidence written in the rocks, this is how limestone caves form: 1. Isolated pockets occur where slightly acid water, seeping through fissures, dis solves and carries away soluble minerals. 2. In the second stage, stream erosion deepens the valley, bringing the river's ac tion into play. Underground water now flows faster and hastens cavern building. Pockets enlarge and connect; the cave takes shape. Sinkholes dot the surface. 3. In the final stage, the water table drops below the cave, leaving it dry. Water now has a new task: interior decoration. Trick ling down into the air-filled cave over thou sands of years, it gradually deposits minerals on ceilings (stalactites) and on floors (sta lagmites). Now an ornamented wonderland, the cavern attracts explorers and visitors. This is a living cave-one with water. When dripping ceases, the cave is defined as dead. Surface erosion ultimately will destroy it-eventual fate of most caverns. A pit in the floor drops into a younger cave, a water-filled sub-basement. 810 DIAGRAMSBY JAMESCOOPER© N.G.S. 810
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