Stone icicles

Many a cave is filled with what look like giant stone icicles. Some of these "icicles" hang down from the cave's roof. Others stick up from the floor. They are often as thick as tree trunks. Sometimes, the ones on the ceiling and the ones on the floor meet to form thick pillars. These strange-looking stone "icicles" are usually found in limestone caves.

The "icicles" that hang from the ceiling are called stalactites. They are made by water trickling through cracks in a cave's limestone roof. The water carries tiny bits of the mineral called calcite with it. As some of the water dries, it leaves bits of calcite stuck to the ceiling. Each drop of water adds more calcite. Slowly, as more bits of calcite are added, the stalactite grows longer and longer.

Some of the water drips to the cave floor. This water also has bits of calcite in it. As the water dries up, bits of calcite are left on the floor. Slowly, the calcite piles grow higher and higher. These "icicles" sticking up from the floor of a cave are called stalagmites.

Often, water drips off a stalactite onto the top of a stalagmite below it. Slowly, the two grow toward one another until they finally join.

Many people get stalactites and stalagmite mixed up. They can't remember which hangs from the ceiling and which sticks up from the ground. But there is an easy way to remember. Just remember that ceiling begin with c, and stalactite has a c in it. And ground begins with g, and stalagmite has a g in it. Then you'll always know that stalactites hang from the ceiling and stalagmites stick up from the ground.

Holes in rock

A den for bears and a bedroom for bats. A home for prehistoric people. A dark place of gloom and mystery. A sparkling wonderland of strangely shaped rocks. That's a cave !

A cave is simply a hole in the earth. It may be a small hole, not much bigger than a room in a house. It may be a place of long, twisting tunnels and huge caverns. It may be high up on a mountainside or it may be in the ground.

Most caves are in the kind of rock called limestone. These limestone caves are made by water. Water seeps through cracks in the rock. It trickles downward, carrying dissolved limestone with it. Slowly, the cracks grow wider as more and more limestone is carried away. After many thousands of years a cave may have a great many winding passages and broad rooms, all made by the slow trickle of water.

Layer cakes of rock

The upper part of the earth's rocky crust is like a layer cake. It is made up of many layers of different kinds of rock, one on top of the other. These layers were built up slowly, one after the other. Here is how this might have happened in one place.

At first, there was only the top of the bare rock crust. Volcanoes erupted, pouring out ash and lava that hardened into a layer of black rock called basalt.

Earthquakes caused this part of the land to sink. Ocean water flowed in to form a smaller body of water called a sea. Rivers dumped tons of sand into the sea for thousands of years. The sand sank down and formed a thick layer on top of the basalt.

Snails, clams, and other little shelled animals moved in from the ocean to live in the sea. When they died, their bodies sank down onto the sand. Their soft parts rotted away and only the shells were left. For millions of years, these shells piled up until they formed a thick layer.

The weight of the shells and the water squeezed the sand together until it became a layer of the rock called sandstone. Squeezed together by their own weight and the weight of the water, the shells were crushed into powder. Minerals in the water glued the powder together. After millions of years, the powdered shells were a layer of the kind of rock called limestone. The layer of limestone was on top of the layer of sandstone, which was on top of the layer of basalt.

This kind of building up of layers of rock is still going on. The layer of mud that now lies at the bottom of a sea will some day be a layer of rock !

Pictures in stone

Tucked away in the earth's rocks there are often "pictures" from the past.

Many times, in ages past, animals walked through mud and left footprints. Slowly, over thousands of years, the mud hardened into rock. The footprints were preserved forever. We have found rocks in which there are footprints of dinosaurs and other animals. We have even found rocks with the footprints of prehistoric people.

Leaves of ancient plants and feathers from the first kinds of birds also fell into the mud. They, too, left prints that were preserved when the mud hardened into rock.

Ancient snails, clams, and other shelled sea creatures died and were covered up with mud. The mud hardened around them. When their soft parts rotted away, and their shells dissolved, a hole was left in the rock - a hole that exact shape of the dead animal. This hole was like a mold. Slowly, it filled up with minerals that hardened into stone. The stone was exact shape the dead animal had been.

These pictures and shapes in stone are called fossils. They tell us about the plants and animals of millions of years ago

Bones in rock

A giant dinosaur, with jaws shaped like the bill of a duck, plodded along the shore of a lake in search of food. Sighting a large bunch of cattails growing in shallow water, it waded out toward them.

Suddenly, the big dinosaur stepped into a wide, deep hole filled with soft, watery mud. At once, the animal began to sink.

It thrashed about wildly, trying to find solid ground under its feet. The weight of its own body dragged it down. The mud rose to its chest, then to its neck. Slowly, its head went under. Mud filled its mouth and nostrils and it died. This was eighty million years ago.

The dinosaur's body sank to the bottom of the hole. Slowly, the soft parts of the dinosaur rotted away. Only its bones were left.

Over many years, the mud around the bones was packed tightly together. In time, it became clay. Over many more years, the clay turned to rock. And, as still more years passed, the lake dried up. The rock with the dinosaur bones in it was now the side of a cliff.

During all those years, water filled with dissolved minerals often got into the bones. Slowly, all the hollow places in the bones were filled with dissolved minerals that then hardened.

As millions of years passed, wind and rain slowly wore away the side of the cliff. Finally, so much rock was worn away that some of the dinosaur bones, preserved in the rock all those years, were sticking out.

One day, eighty million years after the dinosaur had died, a group of scientists saw the bones sticking out of the cliff. With pickaxes and crowbars, the scientists removed the bones from the rock.

The scientists packed the bones carefully and sent them to a museum. At the museum, the bones were put together. People could then see the skeleton of the giant creature that had lived so long ago.

Most of the skeletons of prehistoric animals that you see in museums were preserved in rock in this way. Such preserved bones are called fossils. Sometimes, all of the original material is replaced with minerals. Then the fossil is said to be petrified, or turned to stone.

The magic metal

Have you ever watched someone wrap a sandwich or cover a bowl of food with what looks like a sheet of shining, silver paper ? It isn't paper at all - it's a thin sheet of the wonderful metal called aluminum.

Aluminum is a light, silvery metal. It is never found all by itself, as a pure metal. It is always mixed with other minerals. Most aluminum comes from a rock that looks like a bunch of brown pebbles mixed into a piece of gray concrete. This rock is called bauxite.

Aluminum is often called the "magic" metal. It is light, very strong, and won't rust. It can be stretched and rolled into almost any shape. So aluminum is used to make everything from airplanes to pots and pans to chewing-gum wrappers.

This metal has two names. People in the United States call it aluminum (uh Loo muh nuhm). But people in Great Britain, Canada, Australia, and many other countries call it aluminium (al yuh MIHN ee uhm).

A rock we eat !

If someone at the dinner table said, "Pass the rocks, please," what would you give him ?

Salt, of course !

The salt we use to make food taste better is a mineral called halite. Big lumps of halite are found in the earth's crust. The halite is dug out in chunks and crushed up small enough to fit through the hole in a saltshaker. Halite forms in square crystals. No matter how small it crushed, it nearly always breaks into the shape of a cube !

Sometimes, a hole is dug down to salt that is in the earth. Water is forced down into the salt, then pumped back up with salt in it. The water is then heated. When it dries up, crystals of pure salt are left. Salt is removed from seawater in much the same way.

We need salt to keep healthy, and, of course, salt makes our food taste better. So salt has always been very important to people. In ancient times, salt was so precious and hard to get that it was used as money ! The soldiers of ancient Rome were given salt as part of their pay. This part of their pay was called the salarium. Our word salary, which is another word for pay, comes from the Latin word salarium. A man who was not a very good soldier was "not worth his salt." We still say this today about people who don't do a good job for the money they are paid.