Hart 1997 (5) Flashcards
What does this paper speak of?
his paper brings together the field data from 33 drumlins to show the relationship between drumlin formation and other subglacial deforming bed processes.
The drumlin structure continuum:
(a) depositional - which are similar to flutes, formed by sediment flowing into the low pressure area behind a large obstacle - and these conditions are mostly found within rock-cored drumlins
(b) deformational - which contain different styles of deformation associated with a relatively weak core,
(c) erosional - which consist of either truncated stratified sequences, or homogeneous tills with distinct ice flow patterns.
If more sediment enters the deforming layer than can be removed…
then there will be net subglacial deposition which will result in the build up of a deforming bed till; but if more sediment leaves the deforming layer then net erosion will occur.
What happens when a glacier moves over a potentially deformable bed?
there is a coupling between the glacier and its underlying sediment. This may lead to an increase in velocity of the glacier.
This deformation is known as subglacial glaciotectonic deformation and takes place in the deforming layer beneath the glacier.
Modem theories of drumlin formation can be summarised into two models…
The deformational theory of drumlins
The fluvial theory
What does the deformational hypothesis for drumlin formation of Smalley and Unwin (1968), Menzies (1979) and Boulton suggest?
that drumlins form around a more competent obstruction within the deforming layer. The more competent mass which provides the obstruction for material is known as the core, whilst the surrounding sheath is known as the carapace. The core can be made either of a clast of hard or soft rock, or a more competent mass of till.
What does Boulton (1987) suggest will lead to the development of a sheath fold?
the presence of more competent inhomogeneities within the deforming layer or steps in the dicollement surface (base of the deforming layer).
Boulton’s model shows the following effects within the deforming layer associated with the core:
(i) divergence followed by convergence of the sediment flow around the core; (ii) a low velocity zone immediately up-glacier of the core; (iii) an acceleration of sediment flow around the up-glacier flanks of the core, reaching a maximum at its maximum diameter; (iv) decelerating flow around the down-glacier flanks of the core; and (v) a low velocity area in the lee position.
According to Boulton, what will a drumlin do depending on its strength of core?
He suggests that if the core is weak, the sediment may be deformed into a sheath fold which is typical of subglacial deformation, and thus the resultant drumlin form reflects this fold.
If the core is strong there will be an undeformed core with a thin carapace.
What are the two main problems with Boulton’s theory?
(a) although Boulton (1987) acknowledges these two types of drumlins - deformational and erosional, the former has the most emphasis, which has caused problems since few drumlins have been observed with internal sheath folds; (b) drumlins should occur everywhere that subglacial deformation occurs, but they do not.
Hart (1995a) has suggested that drumlins associated with a deforming bed can be theoretically divided into what three broad classes:
(a) Depositional structure
(b) Deformational structure
(c) Erosional structure
Hart (1995a) has suggested that drumlins associated with a deforming bed can be theoretically divided into three broad classes:
(a) Depositional structure
the presence of a large core will form a cavity or low pressure area into which the deforming bed will flow and stop moving, and thus be ‘deposited’. The resultant drumlin will be similar to a large flute. Till fabric patterns would be expected to show flow into the low pressure area.
Hart (1995a) has suggested that drumlins associated with a deforming bed can be theoretically divided into three broad classes:
(b) Deformational structure
the core will be deformed into the shape of drumlin
Hart (1995a) has suggested that drumlins associated with a deforming bed can be theoretically divided into three broad classes:
(c) Erosional structure
the core will represent an erosional remnant of the deforming layer. If the core is made of till then the till fabric will indicate flow down and glacier, whilst the fabric from the upper carapace till may flow in the shape of the drumlin
