Arran – Hutton’s Unconformity

wee_map_midlandThe Isle of Arran has been a Mecca for geologists for many years due to its wonderfully varied geology. Even in the early days of geological exploration, the likes of James Hutton and Sir Archibald Geikie were attracted to its rugged slopes and coastline. It was here at Lochranza in particular that Hutton first found evidence that supported his ‘Uniformitarianism’ theory – that the Earth was much older than previously thought. Here, gently dipping layers of sedimentary rocks are seen to overlie steeply dipping metamorphic rocks. To Hutton’s eyes, the basal rocks had a history much older than the overlying rocks. He proposed that they had been created, deformed, uplifted and eroded, whereupon they were then overlain by sediments deposited in the sea to create the younger rocks. This whole unit must then have been uplifted to its current position for the process of erosion to begin again. Hutton correctly perceived that this could only have happened over an incredibly long period of time. This became one of several unconformities that Hutton identified. Siccar Point is perhaps his most famous.

cs_hutton_unconformHutton’s Unconformity – Laggan Site of Special Scientific Interest on the north coast of Arran is the site of another unconformity discovered by James Hutton. © Scottish Natural Heritage.

The basal metamorphic rocks consist of Dalradian schists that dip steeply to the southeast. These schists are Precambrian in age. The unconformably overlying sedimentary rocks come from a thick sequence of cornstones in the Kinnesswood Formation of the Inverclyde Group (Lower Carboniferous). The beds of this formation dip gently to the northwest.

Further reading:

Arkley, S.L.B. (2005) North Newton Shore, Isle of Arran. In The Old Red Sandstone of Great Britain (Barclay, W.J., Browne, M.A.E., McMillan, A.A., Pickett, E.A., Stone, P. and Wilby, P.R.), Geological Conservation Review Series, No. 31, Joint Nature Conservation Committee, Peterborough, pp. 192-195.

McKerrow, W.S. & Atkins, F.B. 1989. Isle of Arran – A Field Guide for Students of Geology. 2nd edn. The Geologists’ Association, London.