Chapter 4 Measuring Geological Time
If you read any book or article about geology, you are confronted with strange names of the geologic past such as Phanerozoic, Jurassic, Holocene, Hadean, and so on. These words come from the various Eon, Eras, Epochs, or Periods of the geologic times scale. The geologic timescale is a representation of time based on the rock record of Earth (e.g., the timing and relationships of events in geologic history). It is the history of our Earth, in chronological order. It was first developed in the 18th century, built by using the principles of relative age dating you read about in the previous section of this chapter. The geologic time scale was completed long before scientists were ever able to assign numerical ages to events in the geologic past, and remains largely unchanged. The main difference is that we can now know the time at which the events occurred.
Development of the geologic timescale begins with the work of William Smith. Smith worked as a surveyor in the coal-mining and canal-building industries in southwestern England in the late 1700s and early 1800s. While doing his work, he had many opportunities to look at the Paleozoic and Mesozoic sedimentary rocks of the region, and he did so in a way that few had done before. Smith noticed the textural similarities and differences between rocks in different locations, and more importantly, he discovered that fossils could be used to correlate rocks of the same age. Smith is credited with formulating the principle of faunal succession (the concept that specific types of organisms lived during different time intervals, the types of organisms on Earth have changed and evolved through geologic time, and once an organism goes extinct it is no longer observed in younger layers in the rock record), and he used it to great effect in his monumental project to create a geological map of England and Wales, published in 1815. For more on William Smith, including a large-scale digital copy of the famous map, see the William Smith Wikipedia page. The principle of faunal succession is also the geologic foundation for the Theory of Evolution.
Inset into Smith’s great geological map is a small diagram showing a schematic geological cross-section extending from the Thames estuary of eastern England all the way to the west coast of Wales. Smith shows the sequence of rocks, from the Paleozoic rocks of Wales and western England, through the Mesozoic rocks of central England, to the Cenozoic rocks of the area around London (Figure 4.3.1). Although Smith did not put any dates on these—because he didn’t know them—he was aware of the principle of superposition (young sedimentary rocks form on top of older ones), and so he knew that this diagram represented a stratigraphic column. And because almost every period of the Phanerozoic is represented along that section through Wales and England, it is a primitive geological time scale that records the geologic history of Great Britain.
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