The locations of the fossils in the rock layers reveal the ancient history of the earth
According to geologists, there are three types of rocks which are igneous, sedimentary and metamorphic rocks. Igneous rocks are formed when soft molten lava from volcanoes solidifies to form hard and brittle rocks. As the igneous rocks get eroded by wind, water and ice, they become powdered. The powdery rocks settle at the river beds after getting carried by river currents. The powder settles in layers to form sedimentary rocks which are soft. Sedimentary rocks harden and become denser due to pressure of overlying rocks in the ground and temperature to form metamorphic rocks which are changed rocks.
The earth is always in slow but continuous change due to the dynamic forces of nature. Rocks in the earth’s crust undergo stratification as older layers are covered by new ones. Rocks accumulate from bottom up. The oldest layers are the bottom most and newest ones are the topmost. Each layer is called a stratum, plural being strata. This is known as the law of superposition.
All these layers were once on the surface and organisms, animals, plants lived on them. As stratification occurred, fossils or relics of living things were buried between or imprinted on the layers. A fossil of an organism found at a lower layer is more ancient and one found near the surface is more recent. Paleontology is the study of fossils.
Animals and plants die and their bodies rest in sediment which protects it. Hard body parts like bones or teeth are preserved for thousands of years. However, soft organs are eaten or perish fast. We have dinosaur bones but not their internal organs; hence we have to make assumptions about their internal parts.
Paleontologists study fossils of organisms like dinosaurs and fish to know about the prehistoric creatures of earth. Fossils are an evidence of past history of earth and a record of biological life of the planet and hence are very integral in scientific research.
A fish in the ocean maybe buried by layers of rocks as rocks are deposited from the bottom up. The ocean may get covered by sand and rock to form lowland where reptiles may come to live next. But once they die, they are buried under deposition of rocks on top of which other animals will come to live, they will die too and get covered by rock strata. Scientists have discovered whale bones in the Andes Mountains which indicate that it was once under the ocean. Fossils of ancient trees in the Mojave desert reveal that the land was once a jungle.
Archaeopteryx is a small and special dinosaur with feathers whose fossil has been found. From this fossil paleontologists could know that the dinosaur was half bird too. From this they could conjecture that birds came from dinosaurs.
Fossils are always found in between sedimentary rocks because they undergo stratification as previously explained. Trapped by these rocks and due to pressure and temperature, the bacteria turn to petroleum oil and natural gas, dead trees turn to coal etc and hence we get the reserves of fossil fuels from prehistory. We extract these from coal and gas mines.
Charles Darwin noticed that the fossils in the bottom layers were unconceivable. He could not recognize them. They were very different from the organisms found at the newer layers where the organisms were more familiar and complex. He concluded that organisms have evolved over time and did not remain the same over the ages. He also realized that newer species arrived to take over the old ones which become extinct.
Now, let’s look at the different ways fossils are preserved in rocks.
1. Fossils maybe frozen dry in ice when the whole body is preserved as being mummified. Mammoths in northern Siberia were accidentally preserved in ice during the Ice Age and human bodies may be preserved for thousands of years.
2. There maybe imprints like seal on mud. For instance, footsteps. The imprints may solidify to stone in a process called ossification.
3. When granules of earth seep into the pores of bones, the granules mix with water and create the shape of skeleton. The bones may perish or turn to rock or stone due to sand granules in them. But due to pressure the sand grains create the shape of bones although the actual bones get destroyed. The process of bones turning to stone is also called ossification.
The Grand Canyon, which was created by the stratification of sedimentary rocks, displays a history of the earth that goes back to 2 billion years. For millions of years the earth’s crust was carved by the Colorado River which caused layers of sedimentary rocks to pile up on one another. Many layers of sedimentary rock make up the mile high Grand Canyon which serves as a history of earth’s geology.
Sharktooth Hill in California, which is 100 miles away from the ocean, tells a history of the earth’s past. Millions of years ago the Sharktooth hill was under the ocean. Fossils of marine life are found surrounded by silt. Silt is sand, clay and other earth granules carried by the current of the running river water and deposited at the river’s bottom or an ocean’s bottom. The remains of whales, sharks, dolphins and turtles have been found 30 feet deep into the ground.
Paleontologists have found that the area was once an ocean that had a river flowing into it that deposited sediment, consisting of sand and clay, at the bottom. When animals died they were buried in the sediment. One day the river stopped flowing and lots of animals collected in the sediment at the bottom. When the river started to flow again, it buried all the bones as well.
After millions of years today, the area is no longer an ocean and the bones are preserved as layer of fossils called a bone bed. A bone bed is a layer in the earth’s rocks containing large amounts of fossilized plant and animal remains. Today Sharktooth hill is on dry land.
