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.