Devonian 410-362 Ma ( Arran at the Equator)

Palaeogeographical reconstructions suggest that during Devonian times Scotland was close to the equator, and the typically oxidized, red, terrigenous sandstones and conglomerates of Devonian age that appear on Arran would confirm such a position within the tropical rain belt. High precipitation rates would be necessary to create the great thicknesses of sedimentary rocks known as the Old Red Sandstone. By the earliest Devonian the closure of the Iapetus Ocean was already complete and the resulting Caledonian Orogeny had assembled the terranes which make up the British Isles.

The resulting scenario of rising mountains with intermontaine basins is suggested by the alluvial fan breccias and fluvial sandstones, overbank silts and conglomerates of the Old Red. These rocks form a belt that sweeps across the middle of Arran, from Sannox Bay in the northeast to between Dougarie and Machrie in the west.

There are fine outcrops of clast supported conglomerates in Sannox Burn, whereas the outcrops in Sannox Bay are sandstones interbedded with matrix supported conglomerates. Igneous activity also occurred during the Devonian and is represented by basic lavas interbedded with the sedimentary rocks.