 The culture that built the brochs is unknown , but by the late Iron Age the Northern Isles were part of the Pictish kingdom . The main archaeological relics from these times are symbol stones . One of the best examples is located on the Brough of Birsay ; it shows three warriors with spears and sword scabbards combined with traditional Pictish symbols . The St Ninian 's Isle Treasure was discovered in 1958 . The silver bowls , jewellery and other pieces are believed to date from approximately 800 AD . O 'Dell ( 1959 ) stated that " the treasure is the best survival of Scottish silver metalwork from the period " and that " the brooches show a variety of typical Pictish forms , with both animal @-@ head and lobed geometrical forms of terminal " . 
