Geologists have developed a set of principles to compare the age of rock layers.

Relative age is the age of rocks and geologic features compared with other rocks and features nearby. There is order in rock formations.

This principles states that in undisturbed rock layers, the oldest rocks are on the bottom. Unless some force disturbs the layers after they were deposited, each layer of rock is younger than the layer below it.

Most rock-forming materials are deposited in horizontal layers. Sometimes rock layers are deformed or disturbed after they form. Even though they might be tilted, all the layers were originally deposited horizontally.

This principle of relative-age says that sediments are deposited in large, continuous sheets in all lateral directions. The sheets continue until they thin out or meet a barrier.

This is a piece of an older rock that becomes part of a new rock is called an inclusion. In the following picture magma intrudes into the rock layers forming a dike. The dike contains inclusions from the rock layers. The dike is younger than the rock layers. The fault line is younger than the dike.

Sometimes forces within Earth’s crust cause rock formations to break or fracture. If one geologic feature cuts across another feature, the feature that does the cutting is younger, and the feature being cut is older.

An unconformity is a surface where rock has eroded away; producing a break, or gap, in the rock record. An unconformity indicates that the sediment deposition was not continuous. An unconformity does represent a gap in time. It could represent a few hundred years, a million years, or even billions of years.

Matching rocks and fossils from separate locations is called CORRELATION The rock layers correlate if there are similarities in the rock types or similarities in the fossils they contain.

Index fossils represent species that existed on Earth for a short length of time, were abundant, and inhabited many locations. Fossils that lived on Earth for long periods of time are not helpful. The most useful fossils represent species, like certain trilobites, that existed for only a short time in many different areas on Earth.