James Hutton was one of the great thinkers of the 18th century, living at the time of the Scottish Enlightenment alongside other famous Scots such as Adam Smith, Joseph Black and James Watt. Often called the 'Father of Modern Geology', Hutton carried out pioneering research on the formation of rocks, and it was this research that began our understanding of the age of the Earth. His observations of certain rock outcrops around the country led to his understanding of the immensity of geological time, most especially in the creation of unconformities. An unconformity is a geological feature where rocks of differing ages are found one above the other today, often with different angles of bedding (or layering). Hutton recognised that for an unconformity to form, there must have been a large gap in time between the formation of the oldest rock and the youngest rock. Siccar Point in Berwickshire is perhaps his most famous unconformity, well known to geologists around the world.

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He also demonstrated that igneous rocks were once molten. Through his study of Salisbury Crags, a Carboniferous igneous intrusion (sill) in Edinburgh, he demonstrated how the molten rock had forced its way between layers of sandstone before cooling to form rock. He published the results of his studies in his 'Theory of the Earth' in 1795.