Hutton’s Unconformity on Newton Shore

In 1787 James Hutton visited Arran and studied an outcrop on the very north east of the Island. It was to become the basis of a treatise that he wrote that was entitled the Theory of The Earth. This theory was eventually to change the way scientists thought about the processes that shaped the earth and formed rocks and was later to earn him the accolade of Father of Modern Geology. Hutton observed on the Newton Shore a calciferous sandstone bed dipping gently to the north west, the sandstone laying directly on top of steeply dipping schistose rock, the dip of the schist being approximately 45o south west. On observing the difference in the rock types, their age gap, their composition and the discordant angle of their contact, he had verified his theory.

In his Theory of the Earth he envisaged an ongoing cycle of weathering and erosion of exposed rock surfaces at some points on the Earths surface and parallel mountain renewal through deep crustal processes at other points on the Earths surface. When he saw the unconformity near Lochranza he new he had found the proof of his theory. He was to later find other unconformities of a similar nature at Siccar Point and other locations. Hutton’s Theory of the Earth and Uniformitarianism first published in 1795, was not broadly accepted at the time. Other contending theories about the processes that shaped the Earth were strongly held, such as the theory that all rocks had been deposited by the biblical flood, or by a Neptunian process where all rocks were laid down in the seas.

It also has to be said, that Hutton’s Theory was not well understood by the scientific populus on the whole. However, after Hutton’s death his friend John Playfair published some drawings of the unconformity on Arran and at Siccar point and the eminent geologist Charles Lyell championed Hutton’s ideas in his publication The Principles of Geology. From then and to this day Hutton’s Theory has been widely accepted by geologists. Drawings of the Arran outcrop and cross-section made by Hutton’s friend Sir John Clerk, were included in his treatise the Theory of The Earth. Copies of these diagrams can be viewed in the Geology Section of the Heritage Museum.

With the present techniques in geology and a knowledge of the related geology in Scotland we can assign definte ages to the rocks seen at Hutton’s Unconformity on Arran. The underlying steeply dipping schists belong to the Southern Highland Group in particular the Dalradian Subgroup, they are thought to be about 520 Million years old and originally deposited in the Cambrian age, they were originally deposited as muds and silts on the bottom of a deep ocean. Although there is fossil evidence that similar Southern Highland Group rocks found at Callander in central Scotland, were deposited in the early Cambrian, no such fossils have been found here. It is likely that the Dalradian metasediments found on Arran were deposited during the Cambrian age. The Dalradian sediments were heated and folded to produce schists and metasediments. The overlying sandstones are thought to belong to the Kinnesswood Group of the Lower Carboniferous and are about 360 Million years old, they were deposited in a river system (Image 1.). The missing time between the rocks is about 160 Million years!