<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2013) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> U-shaped valleys, trough valleys or glacial troughs, are formed by the process of glaciation . They are characteristic of mountain glaciation in particular . They have a characteristic U shape, with steep, straight sides and a flat or rounded bottom (by contrast, valleys carved by rivers tend to be V - shaped in cross-section). Glaciated valleys are formed when a glacier travels across and down a slope, carving the valley by the action of scouring . When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice, called glacial till or glacial erratic . </P> <P> Examples of U-valleys are found in mountainous regions like the Alps, Himalaya, Rocky mountains, Scottish Highlands, Scandinavia, New Zealand and Canada . A classic glacial trough is in Glacier National Park in Montana, USA in which the St. Mary River runs . Another famous U-shaped valley is the Nant Ffrancon Valley in Snowdonia, Wales . </P>

What type of glacier forms a u shaped valley