Tunnel valley

A tunnel valley is a large, long, 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.

A tunnel valley can be as long as 100 km (62 mi), 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep (its depth may vary along its length).

Tunnel valleys were formed by subglacial erosion by water and served as subglacial drainage pathways carrying large volumes of melt water. Their cross-sections exhibit steep-sided flanks similar to fjord walls, and their flat bottoms are typical of subglacial glacial erosion.

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.

Understanding tunnel valleys is important because:

Tunnel valleys play a useful 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.

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