Geological map of the British Isles

On this day in 1815, one of the most significant geological maps in history was published for the first time. Designed by geologist William Smith it was the first detailed survey of the British Isles that looked at the underlying rock structure, not just the surface layer of the soil. His trick, which he picked up from an earlier soil map he’d seen, was to use color coding for the rock layers. Color gradations showed where one rock formation gave way to the next. As this description of the map on NASA’s web site says (http://earthobservatory.nasa.gov/IOTD/view.php?id=8733) “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.” My thanks to Ted Nield for the tip! Image credit wikimedia.org.

Date in History: 17 January 1815