Most people have seen a representation of the geologic time scale and know that it shows an ordered arrangement of life: the ages of rocks and fossils. Students are sometimes required to memorize eras, periods and epochs, and their approximate dates. But for most of us the geologic time scale is a confusing and slightly mysterious diagram. What exactly does it show and how was it devised?

What it shows primarily is relative age, not absolute age of the Earth and its inhabitants. Absolute age is a specific number of years, which can sometimes be determined by radioactive dating methods. These dating methods were developed around 1900 but only recently have become much more reliable. Relative age, on the other hand, is the order of rock layers or strata from most ancient to present. Geologists have been working on a relative scale for centuries. The geologic time scale shows the relationships between various rock strata and fossils. Some of these materials can also be dated by radioactive methods, which then provides approximate dates, but they are two separate ways of looking at the Earth's age. Usually both relative ages and absolute dates appear on the same chart so that, for instance, we can tell that the Mesozoic era began about 225 million years ago.

Sir Charles Lyell, celebrated English geologist of the 19th century, first formalized the relative time scale, but the idea and some of the divisions were developed much earlier. Lyell is often considered the father of modern geology, as his classic work, "Principles of Geology," influenced countless natural scientists. It was this book that Charles Darwin read while on the Beagle, and it was Lyell who later encouraged Darwin's studies. "After my return to England," wrote Darwin, "it appeared to me that by following the example of Lyell in Geology, and by collecting all facts which bore in any way on the variation of animals and plants under domestication and nature, some light might perhaps be thrown on the whole subject...."

The facts that Lyell had collected allowed him to set up a time scale in 1839 that consisted of three Eras (Primary, Secondary and Tertiary) divided into 16 epochs and formations. Our current scale contains many more epochs and periods, as later geologists have gained more knowledge of the Earth's history.

Freaks of Nature

People have been trying to determine the age of the Earth for centuries and have been puzzling over fossils for just as long. Leonardo da Vinci in 1508, recognized fossils as the remains of once living creatures, but his views were not shared by a majority. As late as 1691 biologist John Ray denied such an idea. Various theories abounded, such as the idea that these odd formations, which resembled actual plants and animals, were created by emanations from the stars. They were also described as freaks of nature, or as products of the rocks themselves while in a softened state. Some people thought fossils were placed on the Earth by the Devil to confound humans. Finally, when so many fossils had been found which were clearly identical to each other and living organisms that it became impossible to deny their organic origin, fossils were then accepted as relics of the Biblical flood. This at least placed them in the realm of the once-living and gave geologists a basis on which to work, although there was little acceptance of the idea of extinct species. People believed that if all species had been individually created by God, extinction was impossible.

Baron Georges Cuvier (1769-1832), French paleontologist and zoologist, showed that many fossil vertebrates are extinct species and that fossils from younger deposits are more similar to living forms than those from older deposits. He noted many breaks in the fossil record and postulated that many catastrophes had produced such breaks. After each worldwide catastrophe, new flora and fauna were created and then wiped out by the next catastrophe; the Biblical flood being the most recent of these cataclysmic events. This theory has been called "catastrophism," to describe the numerous catastrophes and creations which were supposed to have occurred.

Like many early scientists, Cuvier is remembered both for the advancement of his field, as in the recognition that some species have become extinct, and for theories such as catastrophism, which have since been discredited.

4004 B.C.

While the debate continued about the causes and composition of fossils, there was relative agreement about the age of the Earth. Christian scholars, studying old Hebrew writings, calculated that the Earth was about 6,000 years old. To be even more precise, Archbishop James Ussher, in the middle of the 17th century, used Biblical data to conclude that the Earth had been created in 4004 B.C. on the night of October 22.

These few thousands of years did not satisfy everyone however, especially geologists. While the church was regarded as the ultimate authority by many, scientific thinkers looked for facts to resolve such questions. Towards the end of the 18th century, James Hutton (1726-1797) realized that the processes of soil erosion, deposition and volcanic activity going on in the present could have produced all of the Earth's features; catastrophes were not necessary. Those processes are slow, however, and 6,000 years was simply not enough time. Hutton's theory, which earned the title "uniformitarianism," for the uniform processes of the Earth, was expounded in the 1795 book Theory of the Earth. The book was not widely read, due to its style, but in 1802 Hutton's friend, Edinburgh professor John Playfair, produced a more readable explanation of the idea: Illustrations of the Huttonian Theory of the Earth.
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Playfair wrote, "It is admitted... that the Scriptures are not intended to resolve physical questions... and if, in consequence of this principle, a considerable latitude of interpretation were not allowed, we should continue at this moment to believe, that the Earth is flat; that the Sun moves around the Earth; and that the circumference of a circle is no more than three times its diameter. No ingenuity has been able to reconcile the natural history of the globe with the opinion of its recent origin...."

Hutton's uniformitarianism, or theory of gradual change rather than cataclysmic upheavals, was disseminated more widely by Playfair and was further supported and established by Lyell. The concept of unlimited time allowed others to develop more realistic biological theories.

Strata Smith

The man who developed stratigraphy, the study of rock layers, and found the tool for determining relative ages of strata, was not one of Europe's famous teachers or geologists. He was an engineer and surveyor whose life work was planning canals, draining swamps and reporting on coal deposits in rural England. William Smith (1769-1839), nicknamed "Strata Smith" for his ability to identify strata by their fossils, used his knowledge every day but it was difficult to persuade him to put pen to paper. By 1799, however, an unpublished manuscript identifying strata and fossils near Bath, England was circulating Smith's ideas. Finally in 1815 he published the first geological map of England and two years later a paper appeared: "The stratigraphical system of organized fossils." His principal contribution to geology, along with his geological maps, was the revolutionary idea that rock strata could be identified by the fossils found within them. It sounds obvious now but only because so much information has been compiled since then. In 1831, eight years before his death, the Geological Society of London awarded Smith the first Wollaston Medal for his contributions to the field of geology.

With this beginning, those who followed Smith were able to develop a classification of formations. Then, a formation consisted of the rocks deposited while particular species lived. Later it became clearer to specify that periods did not begin and end because of the fossils but the other way around. For example, the Cretaceous was a time of widespread oceans, low-lying land, and progressive upheavals in certain areas, and the fossils from this period reflect the conditions on Earth at that time.

Radioactivity

Geologists continued on their two-part search -- to determine the age of the Earth and the relationships between various rock strata and their diagnostic fossils. 

Various methods have been used for computing the age of the Earth's crust. Measuring the rate of change in the salinity of the seas was one proposition, principally endorsed by Edmund Halley (of Halley's Comet), and abandoned when it became apparent that too many factors were involved. The rate of deposition of sediment was also investigated, but there is no way to determine if such rates have been stable throughout time. British physicist, Lord William Thomson Kelvin, tried to estimate the age of the Sun and Earth by calculating the rate of cooling of the Sun. Assuming that both were originally molten and are cooling at a stable rate, he estimated that the Earth's crust was solidified between 20 and 40 million years ago. Other calculations using all of these methods set the age of the Earth anywhere from 17 to 1,584 million years, but most estimates were in the 75 to 100 million year range.

Just before the turn of the century, it was discovered that radioactive materials in the Earth's core are continually producing heat, which effectively destroyed the idea that the Earth and Sun are cooling at a specific rate. Using calculations based on the half-life of uranium decaying to lead, new estimates of the Earth's age came to over 1,000 million years.

Radioactive isotope dating methods require the use of igneous rocks containing radioactive materials and, generally, from very old formations. Marine sediments can sometimes be used for dates in the Pleistocene, and for even more recent material, such elements as Carbon-14 are used. In addition, layers of sediments from lakes in glaciated regions may be counted, rather like tree rings, to obtain relative age calculations. When such dating methods overlap it is possible to verify estimates and compile data on similar fossils and formations. So far, the oldest known rocks have been dated at 3.68 billion years; it's probable that even older material will be discovered.

Introduction of the Scale

Once geologists were allowed almost unlimited time, and having acquired the tool (fossils) for determining the relative ages of strata, it remained to study more of the Earth's surface and to categorize the formations found there.

Lyell's early geologic time scale consisted of several major divisions which had already been described and named, although not placed in chart form. The terms "Cambrian" and "Silurian" for instance, were named in 1835 for ancient Welsh tribes by British geologists studying formations in Wales. A Belgian geologist working on strata around the Paris Basin coined the term "Cretaceous" in 1822. "Cretaceous" is derived from the Latin word for chalk, although other rocks appear in this formation, not simply chalk.

The names of formations are based on the locality where first identified. Rather like the type specimens of plants and animals, geologic systems are defined from a "type section" is found in the "type area." Most of these systems were defined in Europe and later traced to other localities, but periods such as the "Mississippian" and "Pennsylvanian" are obviously of American origin.

As in any science as diversified as geology, workers in the field have not always agreed with each other, whether studying systems or accepting someone else's name. Geologists in parts of Europe use "Gotlandian" instead of "Silurian," or "Holocene" instead of "Recent." In general, however, the use of these terms makes communication between geologists all over the world much easier. The chart itself is a vital tool for studying the Earth and its inhabitants. It allows us to analyze the relationships between species and trace the myriad courses of evolution. The time scale is still far from complete accuracy but it lays the foundation for further work in the geological sciences.

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