Scapa Flow’s Geology
The rocks that can be seen around the edges of Scapa Flow more often than not have a layered appearance. These are sedimentary rocks that comprise much of Orkney and also extend beneath the sea of Scapa Flow. These rocks were formed around 390 million years ago when Scotland was located south of the Equator and had a hot arid climate.
The landscape 390 million years ago, during what is known as the Devonian geological period, was completely unlike that of today. At that time there was no North Sea and no Atlantic Ocean. The area that we know today as the Moray Firth, Caithness, Orkney and Shetland was a huge low-lying valley or basin, surrounded by mountains, which geologists have named the ‘Orcadian Basin’.
Cliffs on Hoy
The high mountains were continually being weathered and eroded. The resulting sediment consisting of pebbles, gravel, sand, and silt was carried by streams and rivers into the basin, where it was deposited on extensive floodplains. At the deepest parts of the basin there were lakes and on occasion, under wetter climatic conditions these would have joined up to form a huge lake, the ‘Orcadian Basin Lake’. These lakes also received sediment, particularly finer sand and silt.
The lakes and rivers provided a habitat for populations of fish, and their fossil remains are to be found in the lake and river deposits. Sometimes the fossil remains of plants can be found, which indicates that the areas around the lakes and the river margins were, to a certain extent, vegetated. But any vegetation would have been sparse given the climate and the fact that plants were only becoming established on land at that time.
As time passed huge quantities of sediment accumulated layer-upon-layer in the lakes and floodplains producing a sequence of sedimentary rocks several kilometres thick. Eventually the mountains were worn down, the basin was filled in and the area was affected by tectonic processes, which folded and faulted the rock layer sequence. These processes also stretched the crust forming a depression in the crust to form the early North Sea to the east. To the west the Atlantic Ocean formed.
Harabrough, South Ronaldsay
Like the mountains from which they were derived, the thick sequence of Devonian sedimentary rocks were weathered and eroded. Many millions of years of erosion, most recently involving the action of the sea, have produced the landscape we see today.
Colin MacFadyen, Geologist, Scottish Natural Heritage