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 , wide, and 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 and linear incisions.
Significance
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 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.
Characteristics
Buried, open and partially filled 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.Vincent Gaffney, Kenneth Thomson, Simon Finch, Mapping Doggerland: The Mesolithic Landscapes of the Southern North Sea, University of Birmingham, 2007
Dimensions They vary in channel depth and width; Danish examples run from wide and from deep. They vary in depth along their course, exhibiting overdeepening; overdeepened sections cut into bedrock and typically are significantly deeper than either upstream or downstream sections of the same tunnel valley. They have steep sides which are frequently asymmetric.
Tunnel valleys frequently include relatively straight individual segments parallel to and independent of one another. Tunnel valley courses may be periodically interrupted; the interruption may include a stretch of elevated esker, indicating the channel ran through ice for a distance. The below-grade sections typically run in length; in some cases the sections form a larger pattern of an interrupted channel composed of strings of depressions which can extend from .
Structure The upstream portion – that section furthest into the glacier – consists of a branching system forming a network, similar to the anastomostic branching patterns of the upper reaches of a river (as contrasted with dendritic patterns). They typically exhibit the largest cross-sectional area in the center of the course and terminate over a relatively short distance in elevated outwash fans at the ice-margin.
Tunnel valleys are found to cross the regional gradient – as a result they may be crosscut by modern stream networks. In one example, tributaries of the Kalamazoo River cut at nearly right angles across buried tunnel channel filled with ice and debris. They frequently terminate at a recessional moraine. Tunnel valleys from successive glaciations may crosscut one another.“Principles of glacier mechanics”; Roger LeB. Hooke; 2nd Edition; 2005; Cambridge
Tunnel valleys frequently run along roughly parallel courses. They originate in and run through regions which include clear evidence of glacial erosion through abrasion and may exhibit striations and roche moutonnée. Depositional forms such as terminal moraines and outwash fans are found at their terminal end. In Michigan tunnel valley channels have been observed to diverge slightly with an average spacing between the channels of and a standard deviation of .
Tunnel valley channels often start or stop abruptly. They have convex-up longitudinal profiles. They are often occupied by elongated lakes of underfit streams. They frequently show signs of subsequent depositions such as eskers.
Evidence of erosion mechanisms Evidence suggests that erosion in a tunnel valley is primarily the result of water flow. They erode by meltwater, which it has been argued, episodically drains in repeated jökulhlaups from subglacial lakes and reservoirs; examples of such motion have been observed in Antarctica. Although there is evidence of ice erosion such as linear striations in the bedrock, these are observed only in the widest valleys, and are believed to have played a secondary role.
The subglacial layout of valley tunnels is predominantly oriented parallel to glacial ice flow lines – essentially they stretch from areas of thicker sheet ice toward areas of thinner sheet ice. They can exhibit reverse gradients, which result when pressurized meltwater flows over obstacles such as ridges or hills along the glacier bed.
Tunnel valleys can be formed under extremely thick glacial ice – examples have been observed on the bottom of Lake Superior and in the oceans offshore in Antarctica. The course of a tunnel valley typically runs from thickest glacial ice to the glacier margin; as a result the glacial ice pressurizes the water such that it runs uphill toward its end.
Formation of tunnel valleys Although there is agreement on the role of meltwater in creation of tunnel valleys, several theories are still under consideration for the role of that meltwater: Steady state theory – Boulton and Hindmarsh propose a steady state theory. They suggest tunnel valleys form in unconsolidated sediment when meltwater flows under pressure through an initially narrow subglacial conduit. With progressive removal of sediment by meltwater, ice deforms under its own weight into the cavity to create a tunnel valley through a positive feedback mechanism. Jökulhlaup driven erosion – Piotrowski argues that ice sheets may, in some instances, be cold-based; that is they contact land that is frozen (permafrost) and they freeze to the permafrost. Meltwater builds up behind this frozen ice terminus until it generates sufficient pressure to lift the ice and break the bond, with a catastrophic meltwater release such as is seen with the Icelandic jökulhlaup. As a consequence of this jökulhlaup a tunnel valley is formed. Upglacier erosion – Wingfield proposes that tunnel valleys form gradually, with the valley head cutting progressively back toward the source up-glacier during deglaciation.Wingfield R.; The origin of major incisions within the Pleistocene deposits of the North Sea (1990) Marine Geology, 91 (1–2), pp. 31–52.
Periodic outbursts of subglacial water have been observed moving subglacial water between subglacial lakes beneath the East Antarctic Ice Sheet. Satellite data recorded a subglacial discharge totaling traveling ~ over a period of less than a year. As the flow subsided, the weight of ice closed the tunnel and sealed the lake again. The water flow was modeled satisfactorily with channeling in ice and in sediment. The analytic model shows that over some regions, the ice-bedrock geometry included sections which would have frozen, blocking off flow, unless erosion of the sedimentary substrate was the means of creating a channel and sustaining the discharge. Hence, combining this data and analysis with Icelandic jökulhlaup observations, there is experimental evidence that some form of the jökulhlaup hypothesis with features of the steady state model is correct.
Common features of tunnel valley theories
Subglacial meltwater flow is common to all theories; hence a key to understanding channel formation is an understanding of subglacial meltwater flow. Meltwater may be produced on the glacier surface (supraglacially), below the glacier (basally) or both. Meltwater may flow either supraglacially or basally as well; the signatures of supraglacial and basal water flow differ with the passage zone. Supraglacial flow is similar to stream flow in all surface environments – water flows from higher areas to lower areas under the influence of gravity. Basal flow exhibits significant differences. In basal flow the water, either produced by melting at the base or drawn downward from the surface by gravity, collects at the base of the glacier in ponds and lakes in a pocket overlain by hundreds of meters of ice. If there is no surface drainage path, water from surface melting will flow downward and collect in crevices in the ice, while water from basal melting will collect under the glacier; either source will form a subglacial lake. The hydraulic head of the water collected in a basal lake will increase as water drains through the ice until the pressure grows high enough to either develop a path through the ice or to float the ice above it.
Steady state theory Sources of water and water drainage routes through and below temperate and sub-polar glaciers are reasonably well understood and provide a basis for understanding tunnel valleys. For these glaciers, supraglacial water ponds or moves in rivers across the surface of the glacier until it drops down a vertical crevice (a moulin) in the glacier. There it joins subglacial water created by geothermal heat; some portion of the water drains into aquifers below the glacier. Excess subglacial water that cannot drain through sediment or impermeable bedrock as groundwater, moves either through channels eroded into the bed of sediment below the glacier (called Nye channelsThe smaller channels, known as Nye channels, are named after British physicist John Nye.) or through channels upward into the glacial ice (called Rothlisberger channels), eventually flowing out at the ice margin. On the simplest level, the tunnel valley can be considered a larger-scale version of these phenomena.
Tunnel valleys or tunnel channels are produced by meltwater flows beneath glacial ice. Tunnel valleys are often buried or partially buried by sediment accumulation during periods of ice advance and retreat.