Geologic Time

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Jul 16, 2001

The concept of time has always fascinated us. From knowing when the seasons would come, to calculating the length of the day. We ask each other, how old are you? When did that building get torn down? How long ago did you get married? Kids, and scientists, ask how old is that tree? How long has the sun been around? How old is that rock?

As long as the concept of time has been around, people have attempted to calculate the age the earth. In the 5th century BC, Herodotus studied the Nile River and deduced that it must be thousands of years old based on the rates of sedimentation. In the 17th century, the Archbishop James Ussher, using the Bible, declared that the earth had formed on the evening of October 22, 4004 BC. In 1756, Johann Lehmann identified three ages of rocks: Primitive, all crystalline rocks, like granite; Secondary, sedimentary rocks containing fossils; Alluvial, containing all soils and gravels. Giovanni Arduina created a similar scale in 1760. His included the Primitive and Secondary ages, and added Tertiary for all unconsolidated sediments and Volcanics for extrusive igneous rocks.

It wasn't until the early nineteenth century that the modern geologic time scale was developed. Through the use of faunal succession (see my article on Stratigraphy for more on faunal succession) geologists were able to name the different eras and periods of the geologic time scale. The use of fossils in naming the time periods is evident in the names of the geologic eras. The suffix -zoic is from the Greek word for life, and the prefixes Paleo-, Meso-, and Ceno- refer to ancient, middle, and recent respectively. Put together we have the Paleozoic (or ancient life) Era, the Mesozoic (middle life) Era, and the Cenozoic (recent life) Era.

Unfortunately, the naming of the different geologic periods has not followed their actual hierarchy in the time record, or been a smooth process. One example, in the 1830's Adam Sedgwick studied the rocks of northern Wales and eventually described them and proposed that the rocks be named the "Cambrian System" after Cambria the Roman name for Wales. At the same time, Roderick Murchison was studying the rocks in southern Wales and published a description for these rocks, which he named the "Silurian System" after a Welsh tribe, the Silures. Further studies showed that the rocks and fossils of Sedgwick's Upper Cambrian System and the rocks and fossils of Murchison's Lower Silurian System were very similar. A bitter feud developed between the two men as to whose name was to be used for the disputed rocks. This feud was not resolved until 1879 when another geologist, Charles Lapworth, recognized that the overlapped sequences were distinct and named them the "Ordovician System" after another Welsh tribe, the Ordovices. Even though the new Ordovician was recognizable all over the world, the scientific community did not immediately adopt it. The US Geological Survey did not recognize the Ordovician until 1903.