Fjords

Fjords are spectacular examples of glacial erosion by ice flowing through major rock channels and thereby producing landscapes of selective linear erosion. A fjord is essentially a glaciated valley that is now flooded by the sea, to form a long, narrow, steep-sided coastal inlet. Since fjords are essentially drowned troughs, the literature on trough formation is relevant to fjord evolution and so this review does not isolate the two forms. The cross-sectional profiles of troughs and fjords are often referred to as U-shaped, but in fact are best approximated by the formula for a parabola:

Vd= awb where w is the valley half width, Vd is valley depth, and a and b are constants. However, true cross profiles deviate from this mathematical parabola largely due to the production of breaks in slope by pulsed erosion through time. These effects have been modeled by imparting a valley glacier on a fluvial, V-shaped valley. Ice velocities at the base of the glacier are highest partway up the valley sides and lowest below the glacier margins and center line. By assuming that the erosion rates are proportional to the sliding velocity, the greatest erosion occurs on the valley sides, thereby causing broadening and steepening of the valley. The development of the steep sides of troughs and fjords may be aided by pressure release or dilatation in the bedrock.

The long profiles, or longitudinal section along the valley, of troughs and fjords are different from those of river valleys in that they possess overdeepened basins on their floors at points where glacial erosion is concentrated. This occurs at the junctions of tributary valleys or at constrictions in the valley cross profile where ice discharges increase. Each basin terminates at a sill or threshold where the ice flow becomes less constrained and velocities decrease. This gives rise to long profiles that are typically stepped. In addition to changing glacier ice discharges, spatial variations in bedrock lithology and structure and preexisting physiographic features may also dictate the location of overdeepenings and thresholds. Trough and fjord long profiles have long been employed as a means of classification. Specifically, a fourfold classification recognizes: (1) alpine types, cut by valley glaciers emanating from high ground; (2) Icelandic types, with closed trough heads at their upper ends, having been cut by ice spilling over from surrounding plateaux; (3) composite types, which are through-troughs or through-valleys open at both ends and cut beneath an ice sheet; and (4) intrusive or inverse types, which are cut against the regional slope. The shallowing of fjords and the occurrence of thresholds reflect the increased buoyancy and eventual flotation of the glacier down flowline. Glacier flow will also diverge where the glacier emerges into the lower and less constricted topography of the coast, giving rise to a reduction in ero-sional capacity. As isostatic rebound of the land takes place during deglaciation, fjord thresholds may emerge to form skerries or numerous low-lying bedrock islands and strandflats. The locations of fjords reflect the present and past distributions of low-altitude outlet glaciers and occur along the coasts of Greenland, Alaska,

The steep sides of Kjosnesfjord, seen from Jolstravatnet, Norway. Copyright Bryan and Cherry Alexander Photography

Norway, many of the islands of the Canadian Arctic archipelago, and the Svalbard archipelago. The longest fjords in the Arctic are Greely Fjord (Nansen Sound, Nunavut) at 400 km and Nordvestfjord (Ittoqqortoormiit (Scoresbysund), Greenland) at 300 km.

There are a variety of fjord plan forms ranging from sinuous to rectilinear. Sinuous or meandering forms are often regarded as preglacial legacies, where glacier ice has excavated and accentuated preexisting (preglacial) fluvial valleys. Where fjord alignments can be linked to bedrock structure or lineaments, it is often suggested that glacier ice has exploited lithological weaknesses rather than preexisting fluvial valleys. Rectilinear fjord plan forms have been linked to regional patterns of intersecting fractures or faults. There is undoubtedly a continuum of forms ranging from tectonically controlled grabens through glacially modified river and fault systems to entirely glacially eroded fjords. An excellent example of the juxtaposition of different fjord plan forms occurs on northern Ellesmere Island, where the occurrence of both rectilinearity and sinuosity attests to the long-term impact of a combination of factors in the glacial erosion of fjords.

David J.A. Evans

See also Ellesmere Island; Glacial Erosion; Ittoqqortoormiit (Scoresbysund)

Further Reading

Benn, D.I. & D.J.A. Evans, Glaciers and Glaciation, London: Arnold, 1998

Dowdeswell, E.K. & J.T. Andrews, "The Fjords of Baffin Island, Description and Classification." In Quaternary Environments: Eastern Canadian Arctic, Baffin Bay and West Greenland, edited by J.T. Andrews, London: Allen and Unwin, 1985

Harbor, J.M., "Numerical modelling of the development of U-shaped valleys by glacial erosion." Geological Society of America Bulletin, 104(1992): 1364-1375 Loken, O.H. & D.A. Hodgson, "On the submarine geomor-phology along the east coast of Baffin Island." Canadian Journal of Earth Sciences, 8 (1971): 185-195 Syvitski, J.P.M., D.C. Burrell & J.M. Skei, Fjords: Processes and Products, New York: Springer, 1987

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