My Science Box

Teacher Background
Geologists organize time, not on a calendar, but on a geologic time scale. This is the principal vocabulary shared by geologists and paleontologists so that when one scientist talks about such-and-such period or such-and-such epoch, others know what general time frame he or she is talking about. While it is not necessary for middle school students to memorize the names of the various eons, eras and periods, it is important for them to know how to read and find information on a geologic time scale.

The geologic time scale is made possible by Nicolas Steno (see detailed information about him in the Background section of the Layers Upon Layers lesson). With Steno’s law of superposition, geologists could identify the relative age of various rock layers, and therefore, the relative ages of the fossils contained in the rocks.

After Steno, a major advance in geology came from William Smith (1769-1839), a surveyor and amateur geologist. In the process of his work as a surveyor, he carefully observed rock layers all across England. He noticed that the fossils not only differed from one rock layer to the next, but that the same sequence of fossils appeared wherever he looked. His observation came to be known as the principle of faunal succession – since layers of sedimentary rock contain fossils in a specific sequence, and since the relative age of rock layers can be determined by superposition, rock layers may be correlated in time by the fossils they contain. In one series of rock layers, fossils A, B, C, D, and E could be found from bottom to top. Elsewhere in England, fossils D, E, F, G, and H were found in sequence. Thus, rocks containing fossil G and H are younger rocks containing fossil A, even though they aren’t found in the same place.

This use of fossils to identify the order of rock layers in the overall stratigraphic column allowed Smith to create a complete geologic map of England. Moreover, the principle of faunal succession allowed geologists worldwide to organize rocks and fossils into their relative temporal order. Based on these observations, major differences in the types of fossilized organisms found led scientists to organize the time scale into broad categories called eons, then subcategories called eras, and so on through finer and finer divisions of time. Only recently, since the 1950’s with the invention of the mass spectrometer and its application towards the radiometric dating of rocks, has it been possible to determine the actual ages of rocks and the fossils they contain.

Student Prerequisites
None, although exposure to Steno’s law of superposition and experience relating rock layers to relative time will help students understand why they are doing this activity in the context of geology.