Stac Pollaidh at 612 meters is a mountain in the Northwest Highlands of Scotland. The peak is a colossal ice-smoothed lump with a distinctive spiky-topped ridge. It has been of fascination to geologists for years. Consisting of chocolate-red sandstone, it rests on the Precambrian metamorphic bedrock of Lewisian Gneiss, formed 1,500 million years ago.