Salisbury Crags in the context of Ladies' Scottish Climbing Club

In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that it does not cut across preexisting rock beds. Stacking of sills builds a sill complex and a large magma chamber at high magma flux. In contrast, a dike is a discordant intrusive sheet, which does cut across older rocks.

James Hutton FRSE ( /ˈhʌtən/; 3 June O.S. 1726 – 26 March 1797) was a Scottish geologist, agriculturalist, chemical manufacturer, naturalist and physician. Often referred to as the "Father of Modern Geology," he played a key role in establishing geology as a modern science.

Hutton advanced the idea that the physical world's remote history can be inferred from evidence in present-day rocks. Through his study of features in the landscape and coastlines of his native Scottish Lowlands, such as Salisbury Crags or Siccar Point, he developed the theory that geological features could not be static but underwent continuing transformation over indefinitely long periods of time. From this he argued, in agreement with many other early geologists, that the Earth could not be young. He was one of the earliest proponents of what in the 1830s became known as uniformitarianism, the science which explains features of the Earth's crust as the outcome of continuing natural processes over the long geologic time scale. Hutton also put forward a thesis for a 'system of the habitable Earth' proposed as a deistic mechanism designed to keep the world eternally suitable for humans, an early attempt to formulate what today might be called one kind of anthropic principle.