Geologic Time

In this topic I learn about how the history of Earth is studied.
One way of finding the age of sedimentary rocks is called superposition. Superposition means that in a series of rock layers the bottom layer is the oldest and the top layer is the youngest. Using this idea, scientists are able to determine the relative age of a rock. The age of a rock compared with another rock is called the relative age of a rock. In a given place, they can tell which layers are older than others.
 By comparing rock layers from around Earth, scientists have put together a geologic column. This is a listing of Earth's rock layers in order from oldest to youngest. It is miles thick. 
Fossils are often found inside the rock layers. A fossil is any trace, imprint, or remains of a living thing preserved in Earth's crust. Fossils provide clues to a rock's relative age. A number of fossils called index fossils are the remains of living things that were widespread but only lived for a short part of Earth's history.  Many rocks contain radioactive elements. Certain radioactive elements break apart, or decay, into other elements. This happens at a constant rate which is now known. The rate is called half-life. Half-life is the time it takes for half the mass of an original element to change into a new product, the decay product.
By comparing the amount of original element to the amount of decay product, we can determine how long this process has been going on. This is how scientists tell the absolute age of a rock. The absolute age is the age of a rock in years.
Scientists are able to assign actual ages to the rocks in the geologic column. Scientists use these ages to put together a history of Earth. One of the long stretches of time that Earth's history is divided into is called an era. We now live in the Cenozoic era, which began about 70 million years ago.
And This is what I know about Geologic Time .