The geological map shows that metamorphic rocks cover the greater part of Shetland. These rocks are represented by the clayslates and schists that extend from Fitful Head to the Mull of Eswick; and by the gneiss found from Scalloway to Delting, and also on Burra Isle and Trondra, Whalsay and Skerries, Yell, the west side of Unst and of Fetlar, the east side of Northmavine, and the north sea-board of the Sandness-Aithsting peninsula. Associated with the two series of metamorphic rock are bands of quartzite; while at Fladdabister, Tingwall, Whiteness, Weisdale, Northmavine and Unst are beds of limestone.

The geological formation of Unst is interesting. The gneiss on the west is succeeded by a band of mica, chlorite and graphite schists. Next come zones of serpentine and gabbro, to be followed by schistose rocks at Muness. Fetlar shows a similar formation.

Of the igneous rocks we may mention first the intrusive granites of Northmavine, Muckle Roe, Vementry, Papa Stour, Melby and South Sandsting. A bed of diorite extends from Ronas Voe to Olnafirth. Part of Muckle Roe also shows diorite. Syenite occurs round Loch Spiggie and is traced northward through Oxna and Hildasay to Bixter and Aith. Lava, tuff, and other volcanic materials appear in various parts of the islands, as between Stennis and Ockran Head, in Northmavine, Papa Stour, the Holm of Melby, Vementry and Bressay.

The predominating sedimentary rocks in Shetland belong to the Old Red Sandstone formation. These are found from Sumburgh Head to Rova Head, and in Fair Isle, Mousa and Bressay. The fault or boundary line between them and the metamorphic rocks is clearly traceable at various points along the east of the Mainland. Foula, Sandness and Papa Stour, on the west, show remnants of Old Red Sandstone. This formation is believed to have once covered the whole area now represented by the Shetlands. To the same geological period the hardened sandstone is considered to belong, which occurs in the western peninsula, covering most of the district of Sandness, Walls, Sandsting and Aithsting.

Glacial phenomena—striae, moraines, boulders—are interesting features in the geology of Shetland. Over Unst, Fetlar, Yell and the north of Mainland the movement of the ice had been uniformly from east to west. Over the middle and southern districts, the glaciers had curved in a north-westerly direction.

The variety of rocks accounts for the variety of soils in the islands. There are many fertile regions of sandy and loamy soil. Perhaps the best is the limestone district of Tingwall. Most of the county, however, is covered with peat moss and peaty soil.