Tunnel valley

A tunnel valley is a U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages. They can be as long as 100 km (62 mi), 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep.

Tunnel valleys were formed by subglacial erosion by water and served as subglacial drainage pathways carrying large volumes of meltwater. Their cross-sections often exhibit steep-sided flanks similar to fjord walls. They presently appear as dry valleys, lakes, seabed depressions, and as areas filled with sediment. If they are filled with sediment, their lower layers are filled primarily with glacial, glaciofluvial or glaciolacustrine sediment, supplemented by upper layers of temperate infill. They can be found in areas formerly covered by glacial ice sheets including Africa, Asia, North America, Europe, Australia and offshore in the North Sea, the Atlantic and in waters near Antarctica.

Tunnel valleys appear in the technical literature under several terms, including tunnel channels, subglacial valleys, iceways, snake coils and linear incisions.

Tunnel valleys play a role in identifying oil-rich areas in Arabia and North Africa. The Upper Ordovician–Lower Silurian materials there contain a roughly 20 m (66 ft) thick, carbon-rich layer of black shale. Approximately 30% of the world's oil is found in these shale deposits. Although the origin of these deposits is still under study, it has been established that the shale routinely overlies glacial and glacio-marine sediment deposited ~445 million years before the present by the Hirnantian glaciation. The shale has been linked to glacial meltwater nutrient enrichment of the shallow marine environment. Hence the presence of tunnel valleys is an indicator of the presence of oil in these areas.

Tunnel valleys represent a substantial fraction of all meltwater drainage from glaciers. Meltwater drainage influences the flow of glacial ice, which is important in understanding of the duration of glacial–interglacial periods, and aids in identifying glacial cyclicity, a problem that is important to palaeoenvironmental investigations.

Tunnel valleys are typically eroded into bedrock and filled with glacial debris of varying sizes. This configuration makes them excellent at capturing and storing water. Hence they serve an important role as aquifers across much of Northern Europe, Canada and the United States. Examples include Oak Ridges Moraine Aquifer, Spokane Valley-Rathdrum Prairie Aquifer, Mahomet Aquifer, the Saginaw Lobe Aquifer, and the Corning Aquifer.

Tunnel valleys have been observed as open valleys and as partially or totally buried valleys. If buried they may be partially or totally filled with glacial outwash or other debris. The valleys may be incised in bedrock, sand, silt, or clay.

A part of a tunnel valley may go uphill: water can flow uphill if it is under pressure in an enclosed pipe: for example in Doggerland (submerged land which is now part of the bed of the North Sea) are some infilled tunnel valleys that flowed from north to south across the hollow of the Outer Silver Pit.