Caves and Caverns
It had its beginnings some 60 million years ago, long before two-legged creatures walked on earth. As rains poured down on the bed of limestone and rivers flowed over it, the soft rock was nibbled away by the acid rainwater until hair-thin cracks began to appear. More acid rain fell. The water trickled down, enlarging the cracks. It found new paths between the layers of stone. The paths widened into tunnels. The tunnels crisscrossed and grew into rooms. Over millions of years the rooms grew bigger and bigger. By the time the mazelike network of passageways and rooms was discovered in 1901, the most spectacular system of caverns found in North America -- the Carlsbad Caverns -- had formed.
It is hard to imagine just how big a cave can be. Some of the rooms in this natural wonder are about 1,000 feet (300 meters) under the ground. The cave's largest room could hold ten football fields. In one place its ceiling is as high as a 30-story building. In 1930, this underground wonderland in the foothills of the Guadalupe Mountains, New Mexico, was established as Carlsbad Caverns National Park.
Inside the Cave
With nothing to stop the flow of acid rainwater through the tunnels and rock-walled rooms, huge areas of limestone were devoured. But then the work of the acid rainwater was brought to a halt. The earth's crust wrinkled and folded. The bed of limestone tilted up and became part of a mountain range. The water that etched its way through the limestone rock slowly drained away.
Still, Carlsbad Caverns did not look as it does today. As raindrops trickled down to the empty cave rooms over the centuries, something new began to happen. Water started to decorate the cave.
About 1 million years ago a single drop of rainwater clung to the ceiling of Carlsbad's Big Room. As the water dripped, a tiny ring of lime crystallized on the ceiling. A second drop -- and a third, a fourth, a fifth -- left lime in the same place. As time passed, the rings of lime formed a little stone "icicle."
Another drop of water dripped to the floor of the cave. Again the lime was left behind. As time passed, thousands of drops fell on the same spot. The specks of lime formed what looked like a stubby stone candle.
The icicle of stone on the ceiling is called a stalactite. The stubby candle on the floor is a stalagmite. (Think of the c in stalactite as standing for ceiling and the g in stalagmite as standing for ground. This will help you to remember which is which.) Sometimes the icicle-like stalactites and the stubby stalagmites meet in the middle to form columns.
Stalactites, stalagmites, and columns are not the only cave formations. Lime-laden water covers cave walls with rippling flowstone. It forms curtains of dripstone when it oozes from cracks in the ceiling. It builds a border of rimstone when it evaporates from a pool on the floor. It coats sand grains with layer after layer of lime until each grain is transformed into a cave pearl.
The ceilings of some caves are covered with short, hollow stalactites that look like soda straws. Others have glittering stone needles on their walls or stone pincushions bristling from the floor. There are delicate cave feathers, graceful cave flowers, and a strange stalactite (called a helictite) that grows sideways and up as well as down. The color of cave formations varies also. They can be white, red, brown, or a combination of colors, depending on the minerals that form them.
In Carlsbad these weird and beautiful stone shapes stopped growing when the climate changed. Rains fell less often. Rivers grew shallow. Water seldom reached the rooms deep under the ground. Today scientists speak of the great caves as "dead."
Kinds of Caves
Although no two caves look alike, all of the really big caves in the world were formed in the same way. They were hollowed out of limestone (or related rocks like gypsum and marble) by acid water. They are called solution caves. A more familiar word for them is caverns.
Caverns are not the only kind of cave. For example, sea caves are formed by the steady pounding of waves on the rocky cliffs along the shore. The waves do not dissolve the rock. They dig it out, grinding away at it year after year with pebbles and fine sand. The best-known sea caves are the Blue Grotto in Italy's island of Capri (grotto is still another word for cave) and Fingal's Cave in the Scottish Hebrides.
North America's largest sea cave is Sea Lion Cave in Oregon. Herds of sea lions raise their families in the cave's big room. Other sea caves are scattered along the Atlantic and Pacific coasts, as well as the shores of the Great Lakes and the Bay of Fundy.
In the western part of the United States and on the volcanic islands of Hawaii there are hundreds of lava caves. These formed after hot, melted rock welled up from deep inside the earth. Rivers of the liquid rock, called lava, flowed above ground. Even when the surfaces of the lava rivers cooled, forming a hard crust, their fiery interiors traveled on. The hollow, hardened tubes they left behind are lava caves. Lava caves are usually tunnels only a yard or so under the ground. Lava stalactites hang from their ceilings, and their floors are covered with ripple marks made by the fiery rivers that formed them.
Some lava caves contain huge beds of ice. Ages ago cold air from the surface entered the cave. The tube-shaped cave became a trap for the cold air. When rain or snow carried water to the cave, the water froze, making the porous lava rock still colder. The thicker the ice became, the less likelihood there was of its melting. Even in the desert, there are caves with perpetual ice.
In Canada and the northwestern United States there are also glacial ice caves. Hollowed out of glaciers, these caves have roofs and walls of ice. They grow bigger when warm air reaches them and smaller when it is cold. As they grow, blocks of ice often tumble from ceilings and walls. Glacial caves are usually too dangerous to explore.
During the Ice Age, when great glaciers covered large parts of North America, still another kind of cave was born. In the path of a glacier, boulders were split off from rocky hillsides. After the ice sheet melted, streams tumbled the boulders about and enlarged the openings in the hills. These openings, which never became very big, are splitrock or boulder caves. They are found chiefly in New England.
Life in Caves
Almost every cave is inhabited, but most caves are not suitable places for humans to make their homes. Caves are usually too dark, cold, and wet to live in. In the past, some people, such as Cro-Magnons and Neanderthals, did live in caves. The traces they have left indicate that when they lived in caves, they stayed close to the entrances. If they ventured to the inner parts of caves, it was for special ceremonies. It is possible that in some remote areas of the world, there are people who still live in caves much as the earliest cave dwellers did. However, most caves are populated by animals and plants.
Some animals found in caves do not live there permanently. Birds build nests near the entrance. Skunks use caves as a nursery in which to rear babies. During the winter, caves get new residents as bears, snakes, and many insects settle down to sleep. But these creatures are not true cave dwellers. They take shelter in the cave only when they cannot find any place better.
The real cave dwellers live deep inside the cave. Some, like rats and bats, leave the cave to find food. But there are insects, fishes, and salamanders that spend their whole lives in black cave rooms. A long line of their ancestors lived in the same place. During centuries of darkness these creatures have lost their coloring, becoming white or pale pink or transparent. Many are now blind, making up for eyesight with better hearing or a sharper sense of touch or smell.
The blind and wingless cave beetle is covered with fine hairs. The ghostly blindfish that swim in underground streams are unusually sensitive to vibrations. Blind cave salamanders are rarer than the blindfish. Some kinds can see when they are born, although their eyelids grow together later on.
While these animals are able to adapt to cave life, few plants can. Molds and mushrooms flourish in the damp, dark underground rooms, but green plants cannot live without light. Only in caves that have been wired for electricity will you see green moss and feathery ferns on the rocky ledges.
Uses of Caves
Today, caves in the United States are used mostly for recreation by sightseers and spelunkers, people who explore caves as a hobby (from the Latin word spelunca, which means "cave"). Spelunkers must be specially trained and equipped.
From the early 1800's through the Civil War, caves in the southern United States were important sources of nitrates, an essential ingredient in gunpowder. Caves in the southern United States and Mexico have also been a source of bat guano (manure), which is used as a phosphate fertilizer.
Since the late 1980's, however, the U.S. government has passed laws aimed at protecting caves from human activities, such as mining in areas near caves. Authorities hope to preserve the fragile habitats, valuable artifacts, and water resources of many caves.
Finding a Cave
Out riding one summer afternoon, a New Mexican cowboy named Jim White spotted a dark, funnel-shaped cloud of bats rising from a sandy hillside. He searched the hillside to find where the thousands of bats had come from. His explorations led him from a yawning hole in the ground through tunnels slippery with bat droppings to an enormous room. He had never seen anything like it before -- great stone formations hung from the ceiling and rose from the floor. Searching for the bats' home, White had discovered Carlsbad Caverns!
Most caves are found by accident. However, speleologists, the scientists who study caves, are learning to locate them by examining rock formations and following the paths of underground streams.
There are about 30,000 known caves in North America, 17,000 of which are in the United States. They occur in every state except Rhode Island and Louisiana. More than 130 caves have been opened to the public for study and enjoyment. Of these, 15 are in the national parks or monuments, and 30 are in state parks. The remainder are privately owned and operated.
The best part of a cave tour can come when the guide turns out the lights. The jet blackness is darker than anything you have experienced before. It is a world where the sun never shines. For a few seconds time rolls backward, and you can picture the ancient beginnings of caves and caverns.