 Middle Devonian basaltic volcanic rocks are found on western Hoy , on Deerness in eastern Mainland and on Shapinsay . Correlation between the Hoy volcanics and the other two exposures has been proposed , but differences in chemistry means this remains uncertain . Lamprophyre dykes of Late Permian age are found throughout Orkney . Glacial striation and the presence of chalk and flint erratics that originated from the bed of the North Sea demonstrate the influence of ice action on the geomorphology of the islands . Boulder clay is also abundant and moraines cover substantial areas . 
