C2S e

Figure 2.4 The main modes of push moraine formation. A) Subglacial deformation and ice-marginal squeezing. (After Price, 1970; Sharp, 1984; Johnson and Hansel, 1999). The subglacial deforming layer is extruded or squeezed out at the glacier margin as depicted by the arrow. B) 'Double-layer annual melt-out' model of Matthews et al. (1995). Although the example shows the production of a large moraine by a stationary glacier snout, individual recessional moraines could also be produced in this way. C) Pushing of a frozen till slab (From Krüger, 1993, 1994a, 1996): I = glacier ice, 2 = debris-rich glacier ice, 3 = clast paved lodgement till, 4 = 0 °C isotherm, 5 = thrust plane, 6 = mass movement deposits.

possessed a two-tiered structure (Boulton and Dent, 1974; Boulton, 1979; Boulton and Hindmarsh, 1987; Boulton and Dobbie, 1998; Fig. 2.8). This structure is thought to be the product of ductile flow of an upper dilatant layer (A-horizon) and brittle or brittle-ductile shearing of a lower stiff layer (B-horizon). The two-tiered structure has been observed in Icelandic tills by Dowdeswell and Sharp (1986) and Benn (1995). More recent process experiments conducted by Boulton et al. (2001a) suggest that tills may undergo a variety of responses to stress, these being driven by temporal variations in water pressure/effective pressure and concomitant vertical variations in the locus of plastic failure. The result would be cumulative, distributed net strain in response to localized failure events. They also suggest that the emplacement of tills by deformation is not necessarily an instantaneous process, but rather, cumulative whereby till is lost from the base of the deforming horizon. Specifically, the

dD

Alluvium

»»> Eskers

Till

Flutings and drumlin crests

Glacifluvial deposits

Moraine ridges

I ""i I

Hummocky moraine

Lateral meltwater channels

Overridden (fluted) moraines

Kettle holes

1 Bid 1

Glacilacustrine deposits

-s?^ Major terraces

nn

Water bodies

^¿VN Major abandoned channels

KJ O

I al | Alluvium

I | Glacifluvial deposits

I hm | Hummocky moraine

I | Overridden (fluted) moraines

I gid | Glacilacustrine deposits

I I Water bodies

Flutings and drumlin crests ^^ Moraine ridges ^^ Lateral meltwater channels Kettle holes Major terraces Major abandoned channels

HeinabergsjokullHeinabergsjokullHeinabergsjokull

Figure 2.5 A) Portions of an aerial photograph (Landm^lingar Islands and University of Glasgow, 1998) and surficial geology and geomorphology map of BreiSamerkurjokull, Iceland (from Evans and Twigg, 2000) showing overridden push moraines and their adornments of flutings and recessional push moraines. B) Portion of an aerial photograph (Landm^lingar Islands, 1989) of the foreland of Skalafellsjokull (S) and Heinabergsjokull (H), Iceland, showing overridden push moraines (o) and superimposed recessional push moraines (p) and flutings.

Figure 2.5 A) Portions of an aerial photograph (Landm^lingar Islands and University of Glasgow, 1998) and surficial geology and geomorphology map of BreiSamerkurjokull, Iceland (from Evans and Twigg, 2000) showing overridden push moraines and their adornments of flutings and recessional push moraines. B) Portion of an aerial photograph (Landm^lingar Islands, 1989) of the foreland of Skalafellsjokull (S) and Heinabergsjokull (H), Iceland, showing overridden push moraines (o) and superimposed recessional push moraines (p) and flutings.

cessation of deformation produces a single deformation till horizon but in most instances deformation tills probably accumulate by increments as material is lost from the base of the deforming horizon to produce 'tectonic/depositional slices'. Later-stage deformation features may also be superimposed on earlier structures, although this would be difficult to detect at higher strains. Changes in subglacial water pressure and associated stick-slip motions measured at the base of BreiSamerkurjokull are explained by Boulton et al. (2001a) as the result of a change from basal sliding to subglacial deformation (Fig. 2.9).

The numerous flutings that usually characterize the till surfaces of the forelands of active temperate glaciers (Figs. 2.3 and 2.10) have been explained as the products of till squeezing into cavities that develop on the down-glacier sides of lodged and striated boulders (Boulton, 1976;

Figure 2.6 Bouldery veneer draping the foreland of BreiSamerkurjokull, Iceland, and documenting the lowering of the medial moraine visible on the glacier snout in the distance.

Benn, 1994). Such boulders are commonly observed at the heads of flutings in temperate glacier forelands. Therefore, the occurrence of lodged clasts and their associated flutings, composed of deformed sediment and ice-flow parallel clast fabrics, represent landform-sediment evidence for subglacial deformation. However, recent proposals that fluted surfaces are the product of

Dust Bowl 1935
Figure 2.7 The wind-deflated surface of a formerly thin till cover overlying striated bedrock. Southeast of @orisjokull, Iceland.

Figure 2.8 Subsole deformation and the occurrence of a two-tiered till at the base of BreiSamerkurjokull based upon the displacement of rods placed in the deforming layer. Although the convex upward displacement profile has been interpreted as the product of pervasive, viscoplastic deformation in the 'A' horizon it may also be reflective of distributed shear in a Coulomb plastic, as demonstrated by Iverson and Iverson (2001). (After Boulton and Hindmarsh, 1987).

Figure 2.8 Subsole deformation and the occurrence of a two-tiered till at the base of BreiSamerkurjokull based upon the displacement of rods placed in the deforming layer. Although the convex upward displacement profile has been interpreted as the product of pervasive, viscoplastic deformation in the 'A' horizon it may also be reflective of distributed shear in a Coulomb plastic, as demonstrated by Iverson and Iverson (2001). (After Boulton and Hindmarsh, 1987).

ploughing of soft sediment beds by rough glacier soles may prove to be equally valid for the evolution of the subglacial bedforms of active temperate snouts (Tulaczyk et al., 2001). This process can be equally as effective at advecting deformable sediment to the glacier margin as the deformation processes envisaged by Boulton and Hindmarsh (1987), Boulton and Dobbie (1998) and Boulton et al. (2001).

Exposures through the subglacial sediments of Icelandic glacier forelands typically reveal a sequence of glacitectonized pre-advance materials overlain by till, although localized outcrops

Sliding -"■■■• Sediment deformation -'.'■■■ No deformation

Figure 2.9 The relationship between water pressure cycles on sliding and deformation (stickslip cycles) postulated by Boulton et al. (2001a) based upon data collected from Breiôamerkurjôkull.

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