Ness of Burgi

Between 394 and 384 million years ago during the Devonian Period, rift basins formed in and around the Caledonian Mountains as the landmass of Euramerica was pulled apart by tensional forces. Sediments eroding from the mountains built up within these basins. Three distinct groups of sedimentary rocks are recognised in Shetland. They formed at different times, in different sedimentary environments and lay tens of kilometres apart within the Orcadian basin. They were brought to their present relative positions by horizontal movements along strike-slip faults. The sedimentary rocks of the South Mainland are the youngest of the three groups. Boulders, pebbles, and finer sediments were washed down the mountains by fast-flowing rivers. When they reached the plain, the larger stones were dumped at the foot of the mountains in fan-shaped deposits called alluvial fans. Lighter sediments flowed across the fans in floods of sand-charged water - a process known as sheet flood deposition. The alluvial fans were reworked as rivers flowed across them, transporting and redepositing the sediments. At Ness of Burgi layers of conglomerate alternate with layers of finer sediment. These rocks help us to understand more about alluvial processes and give an insight into a long vanished environment.