In common with most of the Orkney isles, Mainland rests almost entirely on a bedrock of Old Red Sandstone, which is about 400 million years old and was laid down in the Devonian period.
These thick deposits accumulated as earlier Silurian rocks, uplifted by the formation of Pangaea, eroded and then deposited into river deltas.
The freshwater Lake Orcadie existed on the edges of these eroding mountains, stretching from Shetland to the southern Moray Firth.
As in nearby Caithness, these rocks rest upon the metamorphic rocks of the eastern schists, and in Mainland where a narrow strip is exposed between Stromness and Inganess, they are represented by grey gneiss and granite.

The Lower Old Red Sandstone is represented by well-bedded flagstones over most of the islands; in the south of Mainland these are faulted against an overlying series of massive red sandstones.

Many indications of glacial action exist in the form of striated surfaces in Kirkwall Bay, with boulder clay with marine shells, and many boulders of rocks foreign to the islands made of chalk, oolitic limestone, flint, &c. Local moraines are found in some of the valleys.

The soil generally is a sandy loam or a strong but friable clay, and very fertile.
Large quantities of seaweed as well as lime and marl are available for manure.
In common with most of the Orkney isles, Mainland rests almost entirely on a bedrock of Old Red Sandstone, which is about 400 million years old and was laid down in the Devonian period. These thick deposits accumulated as earlier Silurian rocks, uplifted by the formation of Pangaea, eroded and then deposited into river deltas. The freshwater Lake Orcadie existed on the edges of these eroding mountains, stretching from Shetland to the southern Moray Firth. As in nearby Caithness, these rocks rest upon the metamorphic rocks of the eastern schists, and in Mainland where a narrow strip is exposed between Stromness and Inganess, they are represented by grey gneiss and granite.
The Lower Old Red Sandstone is represented by well-bedded flagstones over most of the islands; in the south of Mainland these are faulted against an overlying series of massive red sandstones.
Many indications of glacial action exist in the form of striated surfaces in Kirkwall Bay, with boulder clay with marine shells, and many boulders of rocks foreign to the islands made of chalk, oolitic limestone, flint, &c. Local moraines are found in some of the valleys.
The soil generally is a sandy loam or a strong but friable clay, and very fertile. Large quantities of seaweed as well as lime and marl are available for manure.
Mainland, Orkney