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Correlation Using Relative Ages

A distinctive rock unit or index fossil can be correlated across a distance and help to piece together a geologic history for the region.

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Correlation Using Relative Ages

What is rock matching?

If we want to understand the geological history of a location, we need to look at the rocks in that location. But if we want to understand a region, we need to correlate the rocks between different locations so that we can meld the individual histories of the different locations into one regional history.

Matching Up Rock Layers

Superposition and cross-cutting are helpful when rocks are touching one another and lateral continuity helps match up rock layers that are nearby. To match up rocks that are further apart we need the process of correlation. How do geologists correlate rock layers that are separated by greater distances? There are three kinds of clues:

Distinctive Rock Formations

1. Distinctive rock formations may be recognizable across large regions (Figure below).

The famous White Cliffs of Dover in southwest England can be matched to similar white cliffs in Denmark and Germany.

Index Fossils

2. Two separated rock units with the same index fossil are of very similar age. What traits do you think an index fossil should have? To become an index fossil the organism must have (1) been widespread so that it is useful for identifying rock layers over large areas and (2) existed for a relatively brief period of time so that the approximate age of the rock layer is immediately known.

Many fossils may qualify as index fossils (Figure below). Ammonites, trilobites, and graptolites are often used as index fossilsMicrofossils, which are fossils of microscopic organisms, are also useful index fossils. Fossils of animals that drifted in the upper layers of the ocean are particularly useful as index fossils, since they may be distributed over very large areas.

A biostratigraphic unit, or biozone, is a geological rock layer that is defined by a single index fossil or a fossil assemblage. A biozone can also be used to identify rock layers across distances.

Key Beds

3. A key bed can be used like an index fossil since a key bed is a distinctive layer of rock that can be recognized across a large area. A volcanic ash unit could be a good key bed. One famous key bed is the clay layer at the boundary between the Cretaceous Period and the Tertiary Period, the time that the dinosaurs went extinct (Figure below). This widespread thin clay contains a high concentration of iridium, an element that is rare on Earth but common in asteroids. In 1980, the father-son team of Luis and Walter Alvarez proposed that a huge asteroid struck Earth 66 million years ago and caused the mass extinction.

The white clay is a key bed that marks the Cretaceous-Tertiary Boundary

The white clay is a key bed that marks the Cretaceous-Tertiary Boundary.


  • A single rock unit contains the story of the geology of that location. To understand the geology of a region, scientists use correlation.
  • To correlate rock units, something distinctive must be present in each. This can include an index fossil, a unique rock type, a key bed, or a unique sequence of rocks.
  • A key bed can be global. An example is the iridium layer that was deposited at the time of the Cretaceous-Tertiary extinctions.


  1. What features must the iridium layer that dates to around 66 million years ago have to be a key bed?
  2. Why are microfossils especially useful as index fossils?
  3. What is the process of correlation?

Explore More

Use the resource below to answer the questions that follow.

  1. What does the rock that forms the hoodoos of Bryce Canyon tell geologists about the environment at the time the sediments were deposited?
  2. Why are the rocks at Bryce Canyon orange?
  3. What happened when the Colorado Plateau rose?
  4. How were the hoodoos created for the most part?
  5. What is the rock that creates the white cliffs at Zion? Why are the sands at angles?
  6. When was that portion of Utah a giant sand dune? Why are there dunes stacked on top of dunes?

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A rock unit that is defined by a characteristic index fossil or fossil assemblage.

key bed

A distinctive, widespread rock layer that formed at a single time.


A fossil that must be studied with the aid of a microscope.


Methods for establishing the age relationships of rock units that are not in the same locality.

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