William 'Strata' Smith (23 March 1769 – 28 August 1839) was an English geologist, credited with creating the first detailed, nationwide geological map of any country. At the time his map was first published he was overlooked by the scientific community; his relatively humble education and family connections prevented him from mixing easily in learned society. Financially ruined, Smith spent time in debtors' prison. It was only late in his life that Smith received recognition for his accomplishments, and became known as the "father of English geology".

Smith was born on 23 March 1769, in Churchill, Oxfordshire, the son of John Smith (1735–1777), the village blacksmith, and his wife Ann (née Smith; 1745–1807). His father died when Smith was eight years old, and he and his siblings were raised by his uncle, a farmer also named William Smith. Largely self-educated, Smith was intelligent and observant, read widely from an early age, and showed an aptitude for mathematics and drawing. In 1787, he met and found work as an assistant for Edward Webb of Stow-on-the-Wold, Gloucestershire, a surveyor. He was quick to learn and soon became proficient at the trade.

In 1791, Smith travelled to Somerset to make a valuation survey of the Sutton Court estate, and building on earlier work in the same area by John Strachey. He stayed in the area for the next eight years, working first for Webb and later for the Somersetshire Coal Canal Company, living at Rugborne Farm in High Littleton. During this period, Smith inspected coal mines in the area, where he first observed and recorded the various layers of rock and coal exposed by the mining. Smith's coal mine studies, combined with his subsequent observations of the strata exposed by canal excavations, proved crucial to the formation of his theories of stratigraphy.

Smith worked at one of the estate's older mines, the Mearns Pit at High Littleton, part of the Somerset coalfield and the Somerset Coal Canal. As he observed the rock layers (or strata) at the pit, he realised that they were arranged in a predictable pattern and that the various strata could always be found in the same relative positions. Additionally, each particular stratum could be identified by the fossils it contained, and the same succession of fossil groups from older to younger rocks could be found in many parts of England. Furthermore, he noticed an easterly dip of the beds of rock—low near the surface (about three degrees), then higher after the Triassic rocks. This gave Smith a testable hypothesis, which he termed the principle of faunal succession, and he began his search to determine if the relationships between the strata and their characteristics were consistent throughout the country. During subsequent travels, first as a surveyor (appointed by noted engineer John Rennie) for the canal company until 1799 when he was dismissed, and later, he was continually taking samples and mapping the locations of the various strata, and displaying the vertical extent of the strata, and drawing cross-sections and tables of what he saw. This would earn him the name "Strata Smith". As a natural consequence, Smith amassed a large and valuable collection of fossils of the strata he had examined himself from exposures in canals, road and railway cuttings, quarries and escarpments across the country. He also developed methods for the identification of deposits of Fuller's earth to the south of Bath, Somerset.

He published his findings with many pictures from his fossil collection, enabling others to investigate their distribution and test his theories. His collection is especially good on Jurassic fossils he collected from the Cornbrash, Kimmeridge Clay, Oxford Clay, Oolitic limestone and other horizons in the sequence. They included many types of brachiopods, ammonites and molluscs characteristic of the shallow seas in which they were deposited. Some of the names he coined (like Cornbrash) are still used today for this formation.

It could be seen from Smith's findings that the deeper – and therefore older – the strata were, the more the fossilised species within them differed from living organisms. This gave great support and impetus to the hypothesis of biological evolution (which pre-dated the work of Charles Darwin).

In 1799, Smith produced the first large-scale geological map of the area around Bath. Previously, he only knew how to draw the vertical extent of the rocks, but not how to display them horizontally. However, in the Somerset County Agricultural Society, he found a map showing the types of soils and vegetation around Bath and their geographical extent. Importantly, the differing types were coloured. Using this technique, Smith could draw a geological map from his observations showing the outcrops of the rocks. He took a few rock types, each with its own colour. Then he estimated the boundaries of each of the outcrops of rock, filled them in with colour and ended up with a crude geological map.

In 1801, he drew a rough sketch of what would become the first geological map of most of Great Britain. In the same year he claimed that a book he proposed to publish would provide geological information to enable the canal engineer to "choose his stratum, find the most appropriate materials, avoid slippery ground, or remedy the evil". The book was never published however. Smith travelled extensively across Great Britain working as a mineral surveyor allowing him to meet prominent people such as Thomas Coke, 1st Earl of Leicester, and the Duke of Bedford.