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 there is also a Devonian fish bed at Lamba Banks. There are numerous large boulders deposited by Pleistocene glaciation.