Fossils are the stone remains of animals or plants that were once alive. Fossils can be the bones of a dead dinosaur or his big footprints in the sand. Usually only the skeletons of animals are left after millions of years. But sometimes a whole animal, like a woolly mammoth, gets trapped in ice. The ice stays frozen for thousands of years, and paleontologists are lucky to find a whole creature that has barely changed over time. Or an insect gets stuck in tree sap. The sap hardens into a hard, clear material called amber. These creatures look just the way they did when they wandered into the sap millions of years ago.
Let’s look at the way dinosaur bone fossils are made. Most dinosaur fossils are formed by “mold and cast”. Imagine a Stegosaurus drowns in a river and its body sinks to the bottom of the river bed. The flesh of the animal rots away or is eaten by smaller creatures. Eventually, only the bones are left. Mud and sand, called sediment, cover the skeleton over many years. More layers of sediment fall on the skeleton. Over time, the floor of the river sinks from the weight of the sediment. The lower layers of soft mud and sand are pressed into hard rock.
Now the skeleton is completely surrounded by compressed stone. The bone is slowly washed away by little trickles of water seeping through the stone. This water is called ground water. The bones leave an open space in the exact shape of the dinosaur skeleton. This open space is called a natural mold. Now, the ground water brings tiny pieces of rock into those empty spaces. After millions of years, these tiny rock pieces fill the mold. The rock is pressed further and further underground. Over time the entire skeleton mold becomes solid rock. Many years later, the rock surrounding the skeleton rises to the earth’s surface. This can happen during an earthquake, or as mountains rise naturally. The top layers of rock are worn away by wind and rain. Slowly, the wind and rain show the fossil to the outside world. Not all fossils rise to the surface. Many times they remain under layers and layers of rock, and paleontologists have to dig a long time to find them.