The island is composed of a variety of volcanic and sedimentary rock formations from the Devonian period . At that time the Scottish landmass formed part of the Old Red Sandstone Continent and lay some 10 @-@ 25 degrees south of the equator . The accumulations of Old Red Sandstone , laid down from 408 to 370 million years ago , were created as earlier Silurian rocks , uplifted by the formation of Pangaea , eroded and then were deposited into river deltas . The freshwater Lake Orcadie existed on the edges of the eroding mountains , stretching from Shetland to the southern Moray Firth . The structure of Papa Stour is largely made up of ashes and lavas from volcanic activity associated with this period , including bands of solidified volcanic ash and lava ( rhyolite ) , but it is also a Devonian fish bed at Lamba Banks . It are numerous large boulders deposited by Pleistocene glaciation .