How did William Smith use fossils to map rock layers in Britain?

William Smith, often called the "Founder of Stratigraphy," revolutionized geology by recognizing that specific fossils are consistently associated with particular rock layers.12

While working as a surveyor and engineer in the 1790s, Smith observed that rock strata in regions like Somerset dipped regularly and could be traced across vast distances. By examining fresh excavations from canal projects, he discovered that each stratum contained "fossils peculiar to itself". He utilized both the lithologic character of the rocks and their unique fossil content to identify the successional position of different layers.21

This insight allowed Smith to perform stratigraphic correlation between widely separated areas, confirming that faunal succession follows a predictable, orderly progression. He synthesized these observations into his landmark 1815 "Geologic Map of England, Wales and Part of Scotland", which established the standard for modern geologic mapping. His techniques remain fundamental to the science of stratigraphy today.132