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On Murchison's recommendation, Peach then joined the Survey in 1862, moving to the Scottish branch in 1867. Alongside James Geikie, Peach helped to map the Midland Valley, with palaeontology being his main focus.

However, it is his work on the Northwest Highlands and Southern Uplands that Peach is most famous for. His skill lay in being able to 'picture' or understand the internal structure of a mountain and how it related to the rocks seen on the surface - an understanding needed if the processes that led to the formation of the mountains were to be worked out. He was also an excellent artist, and used this skill to put onto paper the structures he interpreted in his mind. His written work and organisation skills were not so well accomplished, but these tasks were generally undertaken by his ever-present friend and colleague John Horne.

It was in the Northwest Highlands that Peach and Horne established the structural formation of the area, putting an end to the Highlands Controversy that had previously raged between Sir Roderick Murchison and James Nicol (1810 - 1879). Their work also established some of the major principles of structural geology, most notably in their research on the Moine Thrust. (However, the simultaneous work of Charles Lapworth (1842 - 1920) on the area should not be forgotten as he too, independently and simultaneously solved the "secret of the Highlands" during his own research.)

Further Reading:

Flett, Sir John Smith. 1937. The First Hundred Years of the Geological Survey of Great Britain. His Majesty's Stationery Office, London.

Oldroyd, D. R. 1990. The Highlands Controversy. Constructing geological knowledge through fieldwork in nineteenth-century Britain. University of Chicago Press, Chicago and London.