A geologist stared at a cliff of tilted rock and found 'deep time'

In 1788, James Hutton examined an outcrop at Siccar Point, Scotland, where near-vertical rock layers met flat layers above them, proof of two rock formations separated by an immense stretch of time. He concluded the Earth had to be far older than the roughly 6,000 years then widely assumed, slowly shaped by the same processes still at work today. His idea, uniformitarianism, became the foundation of modern geology and later shaped Darwin's thinking.

— James Hutton, Theory of the Earth — 1788, Transactions of the Royal Society of Edinburgh