GEOLOGICAL PERIODS

Rocks deposited over the last 545 million years (the Phanerozoic era) are divided into 11 periods; 7 of these are definitely present on Arran, and a further 2 are possibly present. The periods were originally established largely by the types of fossils present, but subsequently techniques have been developed that allow absolute dates to be obtained. The study of the order or rocks, and their corrrelation from location to location across the world is known as Stratigraphy.

Scotland, shown diagramatically on the left, has rocks from many different geological periods, and Arran straddles the boundary between very different geological blocks (i.e. the Highland Boundary Fault which separates the Highlands from the Lowlands)

There are lots of gaps in the stratigraphic record, and many uncertainties despite modern dating techniques. On Arran there are considerable difficulties over resolving the age of a number of different rocks.

As well as a wide range of Phanerozoic rocks, Arran also has rocks from the Proterozoic Era - elsewhere in the world these rocks, described informally as Precambrian, range in age back to around 2500 million years, but the Arran representatives may have been deposited just before, or even during the early stages of, the Cambrian Periods.