Sandur are found in glaciated areas, such as Svalbard, Kerguelen, and Iceland. Glaciers and icecaps contain large amounts of silt and sediment, picked up as they erode the underlying rocks when they move slowly downhill, and at the snout of the glacier, meltwater can carry this sediment away from the glacier and deposit it on a broad plain. The material in the outwash plain is often size-sorted by the water runoff of the melting glacier with the finest materials, like silt, being the most distantly re-deposited, whereas larger boulders are the closest to the original terminus of the glacier.
An outwash plain might contain surface meandering streams that rework the original deposits. They may also contain kettle lakes, locations where blocks of ice have melted, leaving a depression that fills with water. The flow pattern of glacial rivers across sandar is typically diffuse and unchannelized, but in situations where the glacial snout has retreated from the terminal moraine, the flow is more channelized.
Sandur are most common in Iceland, where geothermal activity beneath ice caps speeds up the deposition of sediment by meltwater. As well as regular geothermal activity, volcanic activity gives rise to large glacial bursts several times a century, which carry down large volumes of sediment.
The Appalachian peninsula that makes up the essential part of southern Québec (Lower St-Lawrence and Gaspé areas) also contains several example of paleo-sandar, dating from the Pleistocene ice melt.
The prototype sandur
The original sandur from which the general name is derived is Skeiðarársandur, a broad sandy wasteland along Iceland's south-eastern coast, between the Vatnajökull icecap and the sea. Skeiðarársandur is the largest sandur in the world, covering an area of 1300 km², and volcanic eruptions under the icecap have given rise to many glacial bursts (jökulhlaups in Icelandic), most recently in 1996. The peak flow of the 1996 jökulhlaup was estimated to be 50,000 m³/s, compared to the normal summer peak flow of 200-400 m³/s, and the net deposition of sediment was estimated to be 12.8 million cubic metres. The level of the sandur was raised by up to 10 metres in places.
Between glacial bursts, the sandur is usually criss-crossed by glacial rivers in normal flow. The Ring Road which encircles Iceland was completed in 1974 with the stretch across the sandur. It was washed away by the 1996 jökulhlaup, which was caused by the eruption of the Grímsvötn volcano, but has since been repaired.