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Mojave River Valley Museum
Geology : Paleontology
FOSSILS AND ROCKS
To tell the age of most layered
rocks, scientists study the fossils these rocks contain. Fossils provide
important evidence to help determine what happened in Earth history and when it
In the late eighteenth and early nineteenth centuries, the English geologist and engineer William Smith and the French paleontologists Georges Cuvier and Alexandre Brongniart discovered that rocks of the same age may contain the same fossils even when the rocks are separated by long distances. They published the first geologic maps of large areas on which rocks containing similar fossils were shown. By careful observation of the rocks and their fossils, these men and other geologists were able to recognize rocks of the same age on opposite sides of the English Channel.
William Smith was able to apply his knowledge of fossils in a very practical way. He was an engineer building canals in England, which has lots of vegetation and few surface exposures of rock. He needed to know what rocks he could expect to find on the hills through which he had to build a canal. Often he could tell what kind of rock was likely to be below the surface by examining the fossils that had eroded from the rocks of the hillside or by digging a small hole to find fossils. Knowing what rocks to expect allowed Smith to estimate costs and determine what tools were needed for the job.
Smith and others knew that the succession of life forms preserved as fossils is useful for understanding how and when the rocks formed. Only later did scientists develop a theory to explain that succession.
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Fossils, Rocks, and Time
By Lucy E. Edwards and John Pojeta, Jr.
Contents & Introduction
Putting Events in Order
The Relative Time Scale
Rocks and Layers
Fossils and Rocks
The Numeric Time Scale