8.1 The Geological Time ScalePreviousNextWilliam “Strata” Smith worked as a surveyor in the coal-miningand canal-building industries in southwestern England in the late1700s and early 1800s. While doing his work, he had manyopportunities to look at the Paleozoic and Mesozoic sedimentaryrocks of the region, and he did so in a way that few had donebefore. Smith noticed the textural similarities and differencesbetween rocks in different locations, and more importantly, hediscovered that fossils could be used to correlate rocks of thesame age. Smith is credited with formulating theprinciple offaunal succession(the concept that specific types of organismslived during different time intervals), and he used it to greateffect in his monumental project to create a geological map ofEngland and Wales, published in 1815. (For more on WilliamSmith, including a large-scale digital copy of the famous map,see%29.)Inset into Smith’s great geological map is a small diagramshowing a schematic geological cross-section extending from theThames estuary of eastern England all the way to the west coastof Wales. Smith shows the sequence of rocks, from the Paleozoicrocks of Wales and western England, through the Mesozoic rocksof central England, to the Cenozoic rocks of the area aroundLondon (Figure 8.2). Although Smith did not put any dates onthese — because he didn’t know them — he was aware oftheprinciple of superposition(the idea, developed muchearlier by the Danish theologian and scientist Nicholas Steno,that young sedimentary rocks form on top of older ones), and sohe knew that this diagram represented a stratigraphic column.
And because almost every period of the Phanerozoic isrepresented along that section through Wales and England, it is aprimitive geological time scale.Figure 8.2 William Smith’s “Sketch of the succession of strata and their relativealtitudes,” an inset on his geological map of England and Wales (with era namesadded). [SE after:ession_of_strata.jpg]Smith’s work set the stage for the naming and ordering of thegeological periods, which was initiated around 1820, first byBritish geologists, and later by other European geologists. Manyof the periods are named for places where rocks of that age arefound in Europe, such as Cambrian for Cambria (Wales),Devonian for Devon in England, Jurassic for the Jura Mountainsin France and Switzerland, and Permian for the Perm region ofRussia. Some are named for the type of rock that is commonduring that age, such as Carboniferous for the coal- andcarbonate-bearing rocks of England, and Cretaceous for thechalks of England and France.