Giant seals from over 15 million years ago are found in rock layers at Sharktooth Hill. By knowing the age of the fossils, scientists can also know the age of the rock in which it is found.
The oldest rock layers are at the bottom and the newest layers are at the top.
We can tell about the Earth’s history by looking at the different layers. The upper layers, those that are closest to the surface of the Earth, are the newest layers to be laid down.
Layers below are older. Since sedimentary rocks form on top of each other, it is very easy to see Earth’s geologic history in areas where this type of rock is found.
The Grand Canyon is an excellent example. For millions of years, the Earth’s surface was carved by the Colorado River. Many layers of sedimentary rock make up the mile-high walls of the canyon. The walls display a history of the Earth’s surface that dates back about 2 billion years.