During the past 2.5 million years the world has experienced a series of ice ages interspersed with warmer interglacial periods. We are currently enjoying an interglacial. However, 100,000 years ago ice from Scandinavia covered Shetland and 25,000 years ago a more localised icecap covered the central ridge of the islands.
Ice sheets scoured the surface of the land, deepening existing channels between the islands and peninsulas, smoothing the harder uplands of granite and creating roche moutonnées - tear-drop-shaped mounds of bedrock that taper in the direction the ice travelled. In some cases, valleys were carved out by sub-glacial drainage channels – water flowing beneath the ice sheets themselves and glacial moraine (debris) was deposited on the valley floors.
