Portrait of the British geologist William Smith, 1769-1839. The son of a blacksmith, Smith became a canal surveyor, which allowed him to study the varyied geology of England and Wales. He discovered that geological strata could be reliably identified at different places on the basis of the fossils they contained. He also proposed the principle of superposition, that if a strata overlays another then it was laid down at a later time. In 1815 he published the first geological map of England and Wales. Today he is considered to be the father of English geology.