 The superficial rock of Orkney is almost entirely Old Red Sandstone , mostly of Middle Devonian age . As in the neighbouring mainland county of Caithness , this sandstone rests upon the metamorphic rocks of the Moine series , as may be seen on the Orkney Mainland , where a narrow strip of the older rock is exposed between Stromness and Inganess , and again on the small island of Graemsay . 
