Science File -

Hutton's unconformity backed up this idea that the earth must be far, far older. Our explanation for unconformities like this are that the schists were metamorphosed from earlier rocks by heat and pressure far beneath the surface of the Earth. They were then subjected to earth movements that tilted them and they became exposed at the surface of the earth by the weathering and erosion of the rocks above them – or perhaps the exposure happened before the folding. Once at the surface, they must themselves have been eroded to provide the flattish surface on which the sandstone was subsequently formed. Some sandstones are formed on the surface from desert dunes, but others are formed on and beneath the sea bed when sediments are deposited. The rocks on Arran must therefore have sunk beneath the sea again (or the sea rose to cover them), which allowed sand to be deposited, buried and cemented to form rock. As we now see these rocks just above sea level, further earth movements must have taken place to uplift them and to erode the rocks or sediments that must once have been above them. The two unconformable layers of rock therefore suggest a lengthy history indeed. 

All was not quite so straightforward for Hutton, however. At the time it was not even known that rocks such as sandstone were formed from sediments. Many other observations made by Hutton helped to formulate our current ideas of rock formation, with granites and similar rocks being the frozen remains of molten bodies of rock, and sedimentary rocks being laid down usually beneath water. He developed the 'uniformitarian' idea, which held that the processes we see on the surface of the Earth today are the same ones that were responsible for shaping the rocks and the landscape – the unending cycles of erosion, consolidation and uplift. The opposing view was that changes to the Earth were all caused by 'catastrophes' such as earthquakes and volcanoes. Hutton had developed these ideas over some years, and the unconformity near Lochranza was one further piece of evidence that his ideas could explain how our current landscape was formed.