It are good examples of horizontal water mills , also known as Norse or Clack Mills , around Dutch Loch . Originally these were two story buildings with turf roofs , built into banks to give access to the upper floor where the mill @-@ stone was sited . Inside the building it was a fixed lower millstone , and a rotating upper millstone driven by the water falling onto the paddles below . Some were still in use on Papa Stour in the early years of the 20th century , and it is still a working example of one of these mills on the Burn of Clumlie , at Troswick in the south Mainland of Shetland .