Relative Time
Rocks record geologic time. Geologists speak of time in two different ways: relative time and absolute time. The age of different rock layers or strata can be determined relative to each other within what geologists call a stratigraphic sequence. Geologists invoke established principles of stratigraphy to determine the relative age. For example, the oldest rock layer is at the bottom of a sequence of rocks and, like a layer cake, was deposited first: The Principle of Superposition (see below). Relative dating techniques permit geoscientists to compare two or more entities or events to determine which is older and which is younger. People use similar relative dating techniques on a daily basis when they judge ages of persons based on their physical characteristics.
Most of the early work done in geology involved attempts to establish the relative age of different rock layers. As a result, geologists have developed a system that uses specific terms (e.g., Cretaceous) to refer to different periods of time in Earth history. For the most part, these terms come from the geographic location where rocks of that age were first studied and are usually very well exposed. For example, the length of time referred to as the Jurassic Period is named for exposures of rocks in the Jura Mountains in Europe. The Mississippian Period refers to a length of time during which rocks that are particularly well exposed in the state of Mississippi in the United States were formed. A geologic time scale will commonly illustrate all the periods of time from the oldest at the bottom to the youngest at the top.
All rocks around the world that were formed during a specific time period may not look exactly the same. For this reason, geologists working in specific locations assign local names to certain rock layers. These are called formations and are rock units pervasive enough that they can be mapped over a large geographic area. So, one may refer to the Jurassic-age Morrison Formation in Utah, and will be referring to a formally recognized, specific sequence of rocks deposited in Utah during the Jurassic Period. And that sequence of rocks is called the Morrison Formation.
Stratigraphy
Stratigraphy is the description of all rock bodies forming the Earth’s crust and their organization into distinctive, useful, mappable units based on their inherent properties or attributes in order to establish their distribution and relationship in space and their succession in time, and to interpret geologic history. The subject was essentially invented by William Smith in England in the 1790s and early 1800s. Smith, known as the Father of English Geology, created the first geologic map of England and first recognized the significance of strata or rock layering.
The image below shows the stratigraphy of Zion National Park in Utah. You can see the distinct sedimentary rock layers that are visible throughout the park.
Stratigraphic Analysis
How can the relative age of different rock layers be determined convincingly? How can one be certain that rock A is actually older than rock B? As geologists spent more time looking at sequences of rocks outdoors, a number of principles were established. It is these principles that allow relative time to be determined. Learn more about these principles on the next page.