Article from the August 2005 issue of
ON THE SURFACE
Drumlins
(By Paul De Schutter)
Drumlins (Gaelic druim, the crest of a hill) are a typical subglacial landform.
The most identifying characteristic of drumlins are their shape, resembling an inverted spoon with its steep slope facing the direction from which the ice advances (the stoss side). Drumlins can be up to 7 km in length, 2 km in width and 30 m in height. There is no single composition typical of a drumlin, but most have a carapace of lodgement till. Some drumlins have a core of hard rock, or more resistant sediment, but some have no core at all.
Current theories of drumlin formation can be divided into two models: the deformational theory of drumlin formation, and the fluvial theory.
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Drumlins, Kejimkujik National Park, Canada. The
low tapering tail points in the direction of ice flow; their trend records
the main southeastward course of the ice sheet across Nova Scotia. |
The fluvial theory, as proposed mainly by Shaw and Cox, attributes drumlin formation either to catastrophic flooding due to the release of meltwater that is believed to have accumulated beneath melting ice sheets, or to floods caused by regional uplift due to tectonic movements.
However, the deformational theory for drumlin formation seems to be more widely accepted. Especially work by Boulton has shown that when a glacier moves over a potentially deformable bed there is a coupling between the glacier and its underlying sediment. This is called subglacial glaciotectonic deformation and takes place in the deforming layer beneath the glacier. Deformation is an intrinsic part of ice sheet flow, as well as of subglacial erosional and depositional processes.
Drumlins associated with a deforming bed can be broadly divided into three categories: depositional, deformational and erosional.
Drumlins with a depositional structure have a large resistant core which formed a cavity (or low pressure area) into which the deforming bed will move and be deposited. Such a rock-cored drumlin is actually quite similar to a large flute. Depending on the subglacial water pressure, the cavity may be filled with either deforming bed till (high pressure) or melt-out sediments or fluvial sediments (low pressure).
Erosional drumlins on the other hand are composed entirely of homogeneous (pre-existing) till with till fabrics oriented in the ice-flow direction (and thus not associated with the 3- dimensional drumlin form). Differential erosion, caused by a locally more resistant part of the till initiated drumlin formation.
There are many types of deformation associated with drumlins, giving rise to a number of drumlin forms intermediate between totally erosional and depositional ones.
Stoss-side deformation (i.e. deformation along the leading edge of the drumlin) is the simplest style of deformation associated with drumlins. This can consist of clasts stacked up on the stoss-side of the drumlin, stoss-side large thrust blocks, or ductile subglacial deformation. In this case a drumlin can be quite similar in appearance to a push moraine. Stoss-side deformation may also be accompanied by lee-side and core deformation.
If the core of the drumlin is relatively weak, it may become overturned during drumlinisation, forming a subglacial fold. This is called recumbent folded core deformation.
Concentric drumlins (where the internal structure of the drumlin mirrors its external form) are probably formed by subglacial extension acting on layers of different competencies, after the deposition of the till. Due to this extension these layers are eventually broken up into boudins, which give rise to the concentric internal structure of the resulting drumlins.
Although the individual structures of drumlin cores are formed by different processes, the overall drumlin shape is thought to result from net subglacial deforming bed erosion. If more sediment enters a subglacial area than is removed (negative sediment flux) the sediment will build up and a subglacial till sequence – but no drumlins - will develop. In contrast, if more sediment is removed than enters in a subglacial area (positive sediment flux), then any resistant cores will be left behind as erosional remnants. A typical drumlinforming sequence would thus be 1) the net subglacial deposition of a deforming bed till, 2) a subsequent change in subglacial conditions (a reduction in sediment supply or an increase in glacier velocity) leading to a net subglacial erosion, where any inhomogeneities in the drift will act as cores for drumlinisation.
Thus, although there is a continuum in drumlin structure (from erosional over deformational to depositional, mainly dependent upon the characteristics of the coreforming material), the ultimate shape of a drumlin is related to erosional subglacial processes. It has been suggested that the elongation of a drumlin can be related to the velocity of the ice sheet and shear strain, while its height might be due to the thickness of the deforming layer. Therefore, the shape and structure of drumlins can give important information about past glacier dynamics.
Sources :
J. K. Hart: The relationship between drumlins and other forms of subglacial glaciotectonic deformation, Quarternary Science Reviews Vol. 16 (1997) pp. 93-107
J. menzies et al: Evidence, from microstructures, of deformable bed conditions within drumlins, Chimney Bluffs, New York State, Sedimentary Geology, 111 (1997) pp. 161-175
D. E. Cox: Drumlins and subglacial meltwater floods ( http://www.sentex.net/~tcc/sgfcrit.html )
Image credit: Natural Resources Canada ( http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=220 )
By Paul De Schutter
If you have any comments or additional information you want to share about the topics covered in this column, you are welcome to send them to paulds@pi.be.
Other on the surface articles by the same columnist.
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Last modified on Thursday, September 01, 2005 at 15:07