FORMATION
Similar to theories regarding drumlin formation, there are many active hypothesis directed around the formation of flutes. While theoretically no one method stands above the rest, some theories are more widely accepted than others. These landforms have been thought to form by:
Historically, flute formation has been linked to erosional, depositional or a combination of both processes. Recent literature is now directed towards the subglacial deformation and the meltwater hypotheses (Hubbard & Reid, 2006).
Figure 1 - Large boulder at the head of a flute, Saskatchewan Glacier, Alberta, Canada.
http://tvl1.geo.uc.edu/ice/image/subland/493-19.html
Shaw & Freschauf (1973) characterize the appearance of flutes in crystalline bedrock solely to net erosion from grooves. They further attribute this formation mechanism to flutings in surficial or drift material, where erosion has occurred in the groove, leaving behind parallel streamlined ridges (Shaw & Freschauf, 1973). This theory implies preferential erosion of the underlying substrate to produce a series of sinuous hummocks.
Figure 2 - Formation of fluted terrain by erosion (Shaw & Freschauf, 1973).
If the eroded material remains behind as linear bedforms in the basal part of the glacier, the flutings are said to be of purely depositional origin (Shaw & Freschauf, 1973).
Figure 3 - Formation of fluted terrain by erosion and subsequent lateral deposition (Shaw & Freschauf, 1973).
One of the most widely accepted explanations for the formation of flutes is through the deformation of saturated subglacial sediment. As the glacier passes over obstacles, a cavity is created on the lee-ward side. This creates a low pressure zone, from which the underlying saturated sediment can flow into. If the basal shear stress is greater than the sediment strength, sediment deformation will occur and it will flow into the cavity. As the glacier continues to move forward, the flute tail elongates down glacier leaving behind a streamlined highly directional till fabric. Till fabric patterns are expected to show flow into low pressure areas, as flow diverges around the obstruction (Hart, 1997).
Problems with the theory arise when there is no obstruction at the head of a flute. If this is the case, the Erosional and Subglacial Meltwater hypotheses can better explain the formation mechanism. Alternatively, Boulton (1976) argues how glacial retreats/advances can effectively sweep away the emplaced boulders leaving behind flutes with no apparent obstructions.
Figure 4 - Depiction of flute formation through subglacial deformation of sediment into low pressure lee-ward cavity (Benn, 1994).
For further information regarding Subglacial Deformation theory relative to drumlin formation, see Subglacial Till Deformation Theory
Subglacial Meltwater Erosion/Deposition
Shaw has been the primary advocate for subglacial meltwater outburts in forming both flutes and drumlins. In this theory, it is hypothesized that a single meltwater flood swept through the entire area preferentially eroding away the subglacial till or erosion followed by the infilling of cavities in the ice bed (Shaw et al., 2000). Evidence for this formation mechanism can be found in subglacial fluvial deposits (gravel lag) and a lack of shear deformation in the flute. Since it is assuming a massive uniform meltwater flux, the morphology and spacing of the flutes should also be coherent.
Problems with this theory arise in locating the transported material. As well, a large volume of water is needed so there must be evidence for subglacial storage of this water.
Figure 5 - Model for meltwater flow pattern on glacial bed leading to flute formation (Munro-Stasiuk & Shaw, 2001).
For further information regarding Meltwater theory relative to drumlin formation, see The Meltwater Theory
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