Tunnel valley

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A tunnel valley is a deep but narrow valley with a 'U' shaped cross-section and frequently a 'U' shaped plan which is usually found filled with glacial till. It is formed when the edge of an ice sheet advances up a slope. Its length may be appropriately measured in centimetres or kilometres, depending on the circumstances of formation. The small ones are known as nye channels. The larger ones may be a hundred kilometres long and are known also as a rinnental or tunnel-dale.

They are found for example, in north Germany where the thick ice sheet of the Weichsel and earlier Glaciations, having flowed down from the mountains of Scandinavia, began to rise up the north-European slope, driven by the altitude of its surface over Scandinavia. Their alignment indicates the direction of ice flow at the time of their formation.

The thickness of the ice means that melt water lying in crevasses can have a head of perhaps hundreds of metres. This applies great hydrostatic pressure at the bottom of the ice where the melting is assisted by geothermal heat. Where at the ice margin, the land surface rises away from the ice, the water escapes with a helical flow and forms tunnels, partly in the ice and partly in the rock below. When the ice melts, the lower part of the tunnel remains but at this stage, is usually filled by till or the sediment in a Proglacial lake.

The physics of their formation means that they are frequently double, with two branches extending uphill to the former ice margin from a single seat of origin. The branches may have branched again before the open air was reached. They therefore give the impression of the effect of a normal river flow in the opposite direction.