Earth Geologic Time

Until the mid of 1800's most of people thought that the Earth was only a few thousand of years old. But today there is more scientific evidence that some of Earth rocks are billion of years old. But how we can predict the age of an rock? Finding the age of a rock is based on two ideas:

• Superposition: this idea is a simply theory that says that the older layer is at the bottom and the youngest one is on the top.
• Original horizontaly: this other idea recall that sedimentary rocks are form in horizontal layers one in the top of  the other.

Scientists can use this two ideas to tell the relative age of a rock. The relative age of a rock is the age of a rock compared with other rock. This scientific experiments permit to know which layer is older.