Structural mapping

The structure of Haut Glacier d’Arolla was determined by field and aerial photograph observations, from which a series of structural maps were produced. Field observations were made over three seasons between 1999 and 2001.

Sedimentary descriptions and clast analysis

Sedimentary descriptions were made in the field based on Reference HambreyHambrey’s (1994) modification of Reference MoncrieffMoncrieff’s (1989) nongenetic classification of poorly sorted sediments. Clast analysis is used to differentiate between sediment sources, transport histories and depositional environments in the glacial setting (Reference BoultonBoulton, 1978; Reference Benn and BallantyneBenn and Ballantyne, 1994). The approach used in this study is to compare samples of known transport or known depositional environments with samples of unknown or uncertain transport or depositional environments.

Thirty-eight sediment samples of 50 clasts each were collected from a variety of locations on and around Haut Glacier d’Arolla (Fig. 1) and classified into groups according to where the sample was collected. For each clast the following were recorded: (i) lithology, (ii) shape (relative lengths of the a (long) axis, b (intermediate) axis and c (short) axis), (iii) roundness (smoothness or angularity of the edges), and (iv) texture (surface roundness such as faceting and striations). Clast sizes were recorded using the Reference WentworthWentworth (1922) scale of grain-size. Clast roundness was estimated visually using the six roundness classes according to Reference KrumbeinKrumbein (1941) and Reference PowersPowers (1953). Clast data are presented in a modified RA/C₄₀ plot (Reference Benn and BallantyneBenn and Ballantyne, 1994), which is a covariant plot of the RA index (modified in this study to be the percentage of very angular, angular and subangular clasts) and the C₄₀ index (percentage of clasts with a c: a axial ratio of ≤ 0.4).

Medial moraines

The medial moraines of Haut Glacier d’Arolla were investigated by Reference Small, Clark and CawseSmall and others (1979), Reference Gomez and SmallGomez and Small (1985) and Reference Small, Gurnell and ClarkSmall (1987) when the glacier was considerably larger. The observations made in this section are independent of this previous work, but comparisons are made in the discussion.

Haut Glacier d’Arolla has two main medial moraines: an eastern and a western (Fig. 1). The relationship between ice structure and medial moraine formation is different at each of the moraines. The point of emergence of the eastern medial moraine has moved up-glacier, in step with the rapid ablation and lowering of the surface of the glacier. The eastern medial moraine consists of a series of elliptical or V- shaped lenses, orientated roughly parallel to primary stratification and longitudinal foliation. At some locations, debris emerges along planes parallel to primary stratification (Fig. 3a). Debris in each lens is of a distinct lithology, a variety of either meta-pelite or meta-granite. The a axis of clasts emerging from the primary stratification tends to be parallel to fold axes measured along the plane of emergence (Fig. 3b). Once debris has emerged from the ice, it creates a variable debris cover ranging from ~20% to 100%. Down- glacier, the lenses become one joined-up mass, with denser debris cover creating an ice-cored ridge.

Fig. 3. Debris emerging to form the eastern medial moraine. (a) Debris emerging from primary stratification at the top of the glacier tongue to form one lens of the eastern medial moraine. Iceaxe right of centre for scale. Ice flow top left to bottom right. (b) Rose diagram displaying the a axis (long) orientation of clasts emerging in situ from the ice, forming one lens of the eastern moraine. The orientation of fold axes measured along the plane of emergence is 004°, dipping at 11° N. The a axis of the majority of clasts is orientated at a similar angle.

The western medial moraine is situated on the boundary between the main Haut Glacier d’Arolla flow unit and the La Vierge tributary glacier. In some places, the ice core of the western medial moraine is exposed by cross-cutting crevasses. Within the crevasse walls, steeply dipping foliated ice containing occasional debris-rich layers is exposed (Fig. 4a). The debris-rich layers vary in width from 30 to 1000 mm. Debris also emerges from less-steeply dipping (50-54° to east) debris-rich septa on the eastern margin of the western medial moraine. The a axis of clasts from within the debris- rich ice exposed in the medial moraine is parallel to the trace of the surrounding longitudinal foliation (Fig. 4a and b).

Fig. 4. Debris in the western medial moraine. (a) Two debris bands exposed by a crevasse cutting the western medial moraine. The long axis of many clasts is near-vertical, parallel to longitudinal foliation. Ice-axe for scale in centre of panel. Ice flow is into page. (b) Rose diagram displaying the a-axis (long) orientation of clasts emerging from foliation within the walls of a crevasse cutting the western moraine. The orientation of foliation measured within the crevasse is 170°, dipping at 72° W. The a axis of the majority of clasts is orientated at a similar angle.

Sediment samples were collected from the western and eastern medial moraines; they range from cobble-gravels to boulder-gravels and were dominated by clasts with high RA values (100%) and moderate to high C₄₀ values (44-90%) (Fig. 5).

Fig. 5. Results of clast analysis from Haut Glacier d’Arolla, presented as RA/C₄₀ plot. RA index has been modified in this study to be the percentage of very angular, angular and subangular clasts. Small numbers next to each data point correspond to sample numbers provided in Figure 1.

Crevasses and crevasse-traces debris

Crevasse traces, found near active crevasse fields near the top of the glacier, commonly (~80%) have debris associated with them (Fig. 6). In contrast, crevasse traces that have travelled a long distance from their source do not. The debris is brown or black clay, silt and sand. It has a high organic content with fragments of grass, moss, pine needles and leaves. The clay and silt commonly smells organic (foetid). Some deposits also contain angular to subangular rock fragments up to 5 mm in diameter, but insufficient gravelsized clasts for clast morphological analysis. Deposits are occasionally wider and deeper (up to 80mm) along some irregularly shaped crevasse traces with associated crystal quirks (ice structures representative of former englacial streams and moulins;Reference StenborgStenborg, 1968).

Fig. 6. A crevasse trace within an area of active crevassing picked out by a line of mud and silt. Ice flow is right to left.

Englacial debris layers

Within the walls and ceiling of ice caves at the snout of Haut Glacier d’Arolla are occasional discrete debris accumulations, termed englacial debris layers in this paper (Fig. 7). They are uncommon; fewer than ten were observed in each of the three field seasons between 1999 and 2001. Similar englacial debris layers are observable at the glacier surface, where they provide a source for dirt cones (see below).

Fig. 7. Englacial debris layers exposed in ice cave wall under snout of glacier. Thin debris layer concordant to foliation (two arrows) has been offset by a near-vertical, discordant debris layer (horizontal arrow). Offset direction indicated by small arrows. Ice flow is right to left.

Englacial debris layers vary from very thin (<10mm) dispersed silt and clay layers to ~0.5m thick. Their thickness varies along the length of the exposed layer, typically between 0.2 and 0.3m. All debris layers have a sharp boundary with the adjacent debris-free ice. The englacial debris layers usually extend for 5-20m, before disappearing into the roof of a cave or cave walls. The semitransparent nature of the ice indicates the layers continue within the ice, but at an angle to the ice cave wall. No debris layer was actually observed to start or finish; instead they change orientation or disappear into the debris-covered cave floor.

The material observed within the debris layers varies. It is always frozen with interstitial ice within the matrix. Some thin layers contain only clay, silt and fine sand, whilst thicker layers typically contain clast-rich intermediate diamicton. Englacial debris layers sometimes contain massive, unsorted sediments, whilst others contain bedded sedimentary sequences in which beds can be traced along the debris layer for ~1 m. Where a layer contains clasts within a finer matrix, the clasts show a strong fabric, parallel to the edges of the debris layers (i.e. perpendicular to debris layer strike). All sediment samples were clast-rich sandy diamicton with up to 10% faceted clasts and 2% striated clasts. Clasts were dominantly subangular and subrounded, with moderate to high RA values (24-74%) and low C₄₀ values (22-32%), similar to samples collected from dirt cones and subglacial sediments (Fig. 5).

Dirt cones

‘Dirt cones’ are ice-cored, debris-mantled mounds formed by differential ablation rather than mechanical processes. In August 2001, ten main dirt cones were recognized at Haut Glacier d’Arolla (Fig. 8a and b), similar to the elongated ridges described on glaciers in Svalbard by Reference BoultonBoulton (1967), Greenland by Reference DrewryDrewry (1972), and Iceland and New Zealand by Reference Kirkbride and SpeddingKirkbride and Spedding (1996).

Fig. 8. Dirt cones near the snout of Haut Glacier d’Arolla. (a) Dirt cone with long axis trending parallel to ice flow, created from an englacial debris layer emerging parallel to longitudinal foliation. Ice flow into page. (b) The exposed ice core of a dirt cone. Ice flow right to left. Debris on the up-glacier (right) side of the dirt cone is frozen, and is fed by an englacial debris band which is concordant with local transverse foliation orientation.

The debris that forms dirt cones emerges as englacial debris layers from within the ice, either as wide sheets or linear trains of debris. Debris emerging in linear trains of sediment emerges roughly parallel to longitudinal foliation (Fig. 8a), whereas debris emerging in sheets emerges along reactivated crevasse traces (thrust faults) orientated perpendicular to the ice-movement direction (Fig. 8b).

Occasionally, bedded sedimentary sequences are exposed within the debris forming dirt cones. Five sediment samples were collected; all are sandy-pebble gravels with up to 26% faceted and 4% striated clasts. All samples had low to moderate RA values (0-38%) and low to moderate C₄₀ values (6-44%) and plot close to glaciofluvial, subglacial and englacial sediments on a RA/C₄₀ plot (Fig. 5).

Medial moraine formation

The eastern medial moraine of Haut Glacier d’Arolla forms from rockfalls in the southeastern accumulation area of the glacier (Fig. 9). The distinct lithology (varieties of meta-pelite or meta-granite) of each lens suggests point sources for the rockfalls. Once buried in an englacial position, the debris is reorientated during the folding of the surrounding primary stratification in a manner similar to that proposed for polythermal glaciers in Svalbard (Reference Hambrey and GlasserHambrey and Glasser, 2003). In some cases, debris is exposed along a single limb of a fold within the open to isoclinal folding of primary stratification (Fig. 2a and b), or it may be exposed along both limbs of the fold, forming a V-shaped moraine (Fig. 9). The eastern moraine forms where it is because (1) it is fed by regular rockfalls from an area of relatively weak rock (the weathered contact between meta-pelites and meta-granites) which exists at Col de Mont Brule and (2) it marks the junction between two minor flow units (schematically represented in Fig. 2), where tighter folding concentrates the debris.

Fig. 9. Schematic summary of contemporary debris transport processes at Haut Glacier d’Arolla. Aerial view shows main inputs and transport processes occurring at the glacier surface. Cross-sections 1-3 summarize moraine formation. Cross-section 4 runs parallel to flow direction through the snout of the glacier.

Previous authors (Reference Small, Clark and CawseSmall and others, 1979; Reference Gomez and SmallGomez and Small, 1985; Reference Small, Gurnell and ClarkSmall, 1987) suggested that the eastern medial moraine is formed by the emergence of coarse angular clasts from a series of short longitudinal debris bands, identical to the debris emerging along planes of primary stratification described here. Introducing the concept of folding of rockfall-bearing primary stratification into the hypothesis of medial moraine formation explains the intermittent nature of the moraine lenses and the V-shape of some moraines (noted in Fig. 9 here and Fig. 2 in Reference Gomez and SmallGomez and Small, 1985).

The western medial moraine of Haut Glacier d’Arolla is interpreted as being directly fed from rockfall material from La Vierge, as well as internal rockfall-derived debris layers concentrated along this flow unit boundary as a result of tighter folding at the boundary (Fig. 9). Reference Gomez and SmallGomez and Small (1985) suggested that the western medial moraine is formed by direct rockfall from La Vierge and additionally nourished by two near-vertical englacial debris layers, in turn derived from rockfall and supraglacial clasts, which have been eroded along the side-wall/glacier interface. In contrast, Figure 9 explains the presence of multiple englacial debris layers that feed the western medial moraine, rather than just the two observed by Reference Gomez and SmallGomez and Small (1985). No evidence of the comminution of supraglacial clasts was observed during data collection for this study, so abrasion at the side- wall/ice interface (suggested by Reference Gomez and SmallGomez and Small, 1985) is not included in Figure 9.

Debris associated with crevasse traces

The high organic content of the debris associated with crevasse traces suggests the majority of this debris originates on the surface of the glacier. Debris may have been transported into open crevasses by the wind or surface meltwater. Once closed, crevasse traces create negative- relief features, which may act as a sediment trap for debris being washed or blown over the glacier surface.

Englacial debris layers

Two main hypotheses are suggested here for the origin of englacial debris layers at Haut Glacier d’Arolla. The first is that they are infilled englacial water channels. In this hypothesis, debris is transported by water between foliae (cf. Reference Kirkbride and SpeddingKirkbride and Spedding, 1996) or along thrust faults (cf. Reference Glasser, Bennett and HuddartGlasser and others, 1999). Sediment-rich water forces its way between foliae, forming englacial debris layers concordant with local foliation, and exploiting thrust faults or reactivated crevasse traces, creating englacial debris layers discordant to local foliation. Infilled foliation-parallel conduits crop out as linear trains of debris. In contrast, thrust faults may encourage sheet-flow of water and wide englacial debris layers.

Either water pressure must have been very high to allow up-slope propagation of water along structural weaknesses, or the discordant englacial debris layers have been subsequently back-tilted by compression since deposition, or a combination of the two. High water pressure may occurearly in the melt season, before the subglacial drainage system is fully developed and when the snout is still frozen to its bed. High basal water pressure is suggested by Reference Ensminger, Alley, Evenson, Lawson and LarsonEnsminger and others (2001) for the injection of turbid meltwaters into basal crevasses, forming laminated debris bands at Matanuska Glacier, Alaska, USA. An alternative mechanism is provided by Reference Kirkbride and SpeddingKirkbride and Spedding (1996) and Reference Spedding and EvansSpedding and Evans (2002), who have suggested that subglacial streams can become englacial when a subglacial deepening is present near the glacier snout. The presence of such an overdeepening near the snout of Haut Glacier d’Arolla, although small, was suggested by Reference SharpSharp and others (1993, Fig. 4).

The second hypothesis for the formation of englacial debris layers is that they are debris-laden thrust faults. This hypothesis requires that up-glacier ice overrides down- glacier ice via thrust faults, which also incorporates and transports basal sediments to a higher position. If thrusting has created the englacial debris layers, the bedded sedimentary sequences within the layers suggest unaltered subglacial sediment has been frozen-on and incorporated into the thrust in a manner similar to that described on polythermal glaciers in Svalbard (Reference Hambrey, Dowdeswell, Murray and PorterHambrey and others, 1996; Reference Murray, Gooch and StuartMurray and others, 1997) and on temperate glaciers in Chile (Reference Glasser and HambreyGlasser and Hambrey, 2002). However, thrusting does not explain the linear nature of debris layers emerging parallel to longitudinal foliation.

The similarity of sediment samples from englacial channels and subglacial sediments suggests that the former may be derived from the latter. Either hypothesis could account for the transport of subglacial clasts into an englacial position, and it is also possible that both processes operate in tandem.

Formation of dirt cones

The textural similarity between the sediment in englacial debris layers and the sediment forming dirt cones at Haut Glacier d’Arolla and the similarity in clast morphology (moderate to high RA values and low C₄₀ values) (Fig. 5) suggests that the latter form as a result of differential ablation in locations where englacial debris layers crop out at the glacier surface. Englacial debris layers orientated parallel to longitudinal foliation form dirt cones with ridges orientated parallel to longitudinal foliation. In contrast, dirt cones fed by englacial debris layers emerging from thrust faults are elongated perpendicular to ice flow.

Clast morphology data (Fig. 5) indicate some dirt-cone samples have a similarity with glaciofluvial sediments whilst others are similar to subglacial sediments, further suggesting the englacial debris layers that form the dirt cones are polygenetic.