This chapter expands upon the potential of rocks and geology in measuring time in prehistory. In particular, the relative age of stratified sedimentary rocks could be determined from the order of their superposition. In an undisturbed rock sequence, the older rocks would lie below the younger. Tracing them laterally might reveal their relationship to younger or older rocks that might have been preserved. William Smith (1769–1839), an English surveyor and civil engineer, used this principle and developed it in a critical way. Using this approach—walking the outcrops and identifying rocks by their distinctive fossils—other geologists slowly pieced together a geological sequence of rocks. From here, the chapter launches into a still-broader discussion on determining the age of the planet Earth itself.
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