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Knowing that a certain rock was deposited in the “Pennsylvanian”, for example, may help the PG to interpret rocks in a certain area.Also, petroleum geologists are mainly interested in rocks from the Mesozoic and Paleozoic Eras. How can you tell the age of a rock or to which geologic time period it belongs?One way is to look at any fossils the rock may contain. When you find layers of rocks in a cliff or hillside, younger rocks are on top of older rocks.This is to say that in a given location, the geologic record can be and is quite often interrupted as the ancient local environment was converted by geological forces into new landforms and features.Sediment core data at the mouths of large riverine drainage basins, some of which go 7 miles (11 km) deep thoroughly support the law of superposition.

The scale shows how old certain rocks are in millions of years. You will see the Jurassic rocks were deposited about 138-205 million years ago.

This is because almost all of the oil and gas found so far is contained within these rocks.

These rocks are 60 to 600 million years old, approximately.

If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period. But these two methods only give the relative age of rocks--which are younger and which are older. Or how do we know how long ago a particular group of fossilized creatures lived?

The age of a rock in years is called its absolute age.