Before the late eighteenth century, residents of England didn’t necessarily take much interest in the rocks under their feet. The arrival of the Industrial Revolution, however, drew the attention of the country’s entrepreneurs downward. England’s industrialists began plunging deep underground for coal—the fossilized remains of ancient swamps—and carving canals across the countryside to transport it efficiently. In this environment, a geologist named William Smith managed to marry his knowledge of rock layers with his love of fossils.

The eventual outcome of Smith’s research was A Geological Map of England and Wales and Part of Scotland, first published in 1815. On a scale of 5 miles per inch, the map measured 6 feet by 8 feet 6 inches. It was not the world’s first geologic map, but it was the first to map such a large area in such detail.

In mapping rocks that lay underground, Smith faced a challenge. He had to relay three dimensions of information on a two-dimensional surface. Near the end of the eighteenth century, he had seen an agricultural map that used color coding to indicate soil type, and he borrowed that convention for his map of rock layers, or “strata.” Color gradations showed where one rock formation gave way to the next. Smith supplemented the surface map with a cross section of rock layers (middle right).

To distinguish rock layers, geologists rely on several factors, such as color, hardness, and slope. Smith added an important new criterion: fossils. Because species change over time, the fossils they leave behind also change. Smith did not grasp the concept of evolution, but he did understand that changes in fossil assemblages could be used to order rock layers. His discovery became known as the principle of faunal succession, or fossil succession.

In the more than 150 years that have elapsed since Smith’s death, scientists have developed rock-dating techniques based on the rates of radioactive decay of atoms. Radiometric dating gives absolute dates to some kinds of rocks and organic materials—the age of our planet is estimated at 4.6 billion years—but most fossil-bearing, sedimentary rocks (rocks created by erosion) cannot be dated radiometrically. To estimate the ages of these rocks, geologists and paleontologists continue to rely on Smith’s principle of faunal succession.

To learn more about Smith’s life and the legacy his scientific discoveries, please see the feature article William Smith (1769-1839).

Map reproduced by permission of the British Geological Survey. ©NERC. All rights reserved. IPR/100-28DR. To obtain a copy of the map, visit the BGS online shop.) Caption by Michon Scott.