Introduction

Rock glaciers are significant mass-wasting landforms in the Pyrenees, but they are mainly relict, showing in most cases no signs of present-day activity (Chueca, 1991, 1994). A first analysis of the spatial distribution of rock glaciers in the Central Pyrenees was made by Chueca (1992), who identified 170 rock glaciers in the Spanish (southern) Pyrenean area and determined that lithology and fracturing density were important distribution controls. Recently, Serrano et al. (1999) classified 13 active rock glaciers located both in the Spanish and French sides of the Pyrenees in morphogenetical terms, using two categories typically mentioned in the literature: talus rock glaciers (periglacial origin) and debris rock glaciers (glacigenic origin, linked to morainic deposits incorporated by glaciers) (cf. Barsch, 1996; Clark et al., 1998). In this paper, we present a 10-yr geodetic survey on Besiberris glacigenic rock glacier, one of the morphologies mentioned by Serrano et al. (1999).

Velocity measurements by geodetic techniques have been carried out in a reasonable number of rock glaciers located around the world, mainly in alpine environments, but few observations cover long, decadal periods (Potter, 1972; Benedict et al., 1986; Francou and Reynaud, 1992; Gorbunov et al., 1992; Sollid and Sorbel, 1992; Whalley et al., 1995; Barsch, 1996; Potter et al., 1998; Sloan and Dyke, 1998; Koning and Smith, 1999). More recently, photogrammetry and digital photogrammetry have also been applied to determine surface displacements using multiyear aerial photographs (Kääb et al., 1997, 2002; Kaufmann, 1998; Kaufmann and Ladstädter, 2003). In the Pyrenees, only the Argualas rock glacier, of periglacial origin and placed in the Panticosa massif, has been surveyed using geodetic techniques during a 3-yr period (Serrano et al., 1995).

Besiberris rock glacier first descriptions were made by Serrano et al. (1991) and Chueca et al. (1992). The existence of activity in the rock glacier was inferred from several facts: (1) absence of vegetation and limited lichen growth; (2) marked unstability of the frontal talus, with evidences of rock slides and micro-debris-flow processes; (3) presence of an apron of fresh coarse boulders at the foot of the fronto-lateral slopes. The geodetic measurements began in 1993. Movement data from several traverse sections allow a better evaluation and understanding of rock glacier velocity and dynamics than a single line or isolated points; therefore, three traverse lines that control the rock glacier dynamics in its head, central, and toe sectors were established. In 2003 displacements were measured, obtaining data for the three transects. The aim of this work is twofold: (1) to present the results of surface velocities and vertical displacements from 1993–2003; (2) to identify differential spatial dynamics of the rock glacier.

Study Area

Besiberris rock glacier is located in the eastern-central Spanish Pyrenees (Fig. 1). The Besiberris mountains are composed of weathering-resistant and highly fractured granites and granodiorites. They are mainly aligned in a north–south trend, acting as a drainage divide between Noguera Ribagorzana and Noguera de Tor river valleys (Besiberri N, 3014 m; Besiberri del Mig, 3002 m; Besiberri S, 3030 m). Minor, relatively low ridges stretch east–west from this principal ridge (Tuc Comtessa, 2760 m; Tossal de Mola Gran, 2885 m; Pic d'Avellaners, 2965 m; Pic de la Torreta, 2951 m; Pic d'Escobedieso, 2800 m). The landscape was glaciated during the Pleistocene. Minor transformation of these glacial landforms took place during the Holocene, mainly by periglacial and mass-wasting processes. Besiberris is a medium size overdeepened glacial valley (length 5 km; width 3.5 km; maximum altitude, 3030 m; minimum altitude, 1500 m) with high rockwalls and strong altitudinal differences between crests and valley bottoms (Fig. 2). Above the treeline (2100–2200 m), slopes of the geoecological alpine and nival stages are covered with rocky debris in their lower sectors and mantled with moraine material in some areas. The mean annual air temperature (MAAT) of this area varies from −0.5°C at 3000 m to +6.8°C at 1500 m; however, coldest month average temperatures reaches −4.3°C at 3000 m and +0.3°C at 1500 m. The annual precipitation varies from 2600 mm (3000 m) to 1400 mm (1500 m). Discontinuous and sporadic permafrost has been reported in the nearby Maladeta massif, but detailed studies have not been carried out in the Besiberris area (Chueca and Julián, 1994).

Besiberris rock glacier occupies the floor of a small glacial cirque situated in the head of the Besiberris valley, facing northwest (42°35′48″N, 0°49′20″E) (Figs. 3, 4A and B). The front slope is at 2570 m a.s.l., while its head lies at 2780 m. The rock glacier occupies an elongated area left by the retreat of Besiberris glacier since the Little Ice Age (LIA). It has a maximum length of 680 m, and an average width of 250 m (380 m, maximum width). The overall slope of the rock glacier is 17° (11° head sector; 21° center sector; 26° toe sector), and the frontal slope (20–25 m high) varies between 35° and 43°. Its surface is composed of angular granites and granodiorites boulders that are very unstable with an interstitial void ratio up to 50% (Fig. 4C).

Morphologically, the rock glacier is categorized as tongue-shaped, with a surface topography that shows both extension (longitudinal furrows and ridges lined up along the central axis) and compression (in the toe area and in a small lateral body with marked and well-developed traverse furrows) (Fig. 3). In morphogenetical terms, the rock glacier is a debris rock glacier of glacigenic origin, linked to the ice and the morainic deposits generated by the former Besiberris glacier. Talus materials are also incorporated by avalanche processes. Two geoelectrical soundings carried out in the middle portion of the rock glacier (García et al., 1998) suggest the existence of an ice core (8–18 m thick and partially visible in several exposures located in its head section) covered by a coarse-debris layer (1–1.5 m thick) (Fig. 5). During the last decade, most of the previous year's winter snow melted by the end of the summer season; accumulation of snow in the cirque was reduced to small patches located against the cirque headwalls in the contact with the avalanche cones (2750–2800 m a.s.l.). The amount of ice added to the rock glacier under today's climatic conditions is negligible.

Methods

In August 1993 the Besiberris rock glacier surface was first surveyed in detail. Geodetic triangulation (Wild T2 theodolite) was utilized to determine the three-dimensional positions of 42 target points (boulders) distributed on three traverse lines which crossed over the rock glacier in its head, center, and toe sectors (18 points on A–A′; 14 points on B–B′; 10 points on C–C′) (Fig. 6). In order to provide a reliable measure of rock glacier movement, boulders that were well interlocked, stable, medium sized, and large sized (lengths range from 0.8 to 3 m) were selected A steel bolt was drilled into each boulder and used as target reflector. Fixed stations were established on both sides of the rock glacier and on the walls of the surrounding cirque in bedrock surfaces. The accuracy of the procedure was better than ±1.0 cm. In order to measure surface displacement over a 10-yr period, the same survey protocol was conducted (Leica Total Station) in September 2003. Boulder positions on the three traverse lines were remeasured (four target points, placed on the A and C lines were not located, probably due to sliding of the blocks). Targets were located with positional accuracy of better than ±0.5 cm.

Results and Discussion

The average downvalley movement of surface boulders during the 10-yr measurement period shows distinct differences between traverse lines, with an increase in displacement rates from the head to the toe sectors of the rock glacier (line A = 8.72 cm yr⁻¹; line B = 10.65 cm yr⁻¹; line C = 13.35 cm yr⁻¹) (Table 1; Fig. 6). The range of displacements varies from 3.0–16.5 cm yr⁻¹ in traverse line A, 3.2–21.8 cm yr⁻¹ in traverse line B and 5.3–24.6 cm yr⁻¹ in traverse line C. In all three cases, however, velocities are higher at the axis of the rock glacier than toward the lateral margins (Figs. 7, 8). This fastest flow in the unstable central section is also evidenced by the observation of lichen growth, which is almost imperceptible in comparison with the amount of Rhizocarpon geographicum detected in the stable perimeter area of the rock glacier. The increase in velocity from head to toe sectors shows that the main rock glacier body is undergoing extensive flow, a possible origin of the observed longitudinal furrows and ridges parallel to the central axis. Considering the surface slope measured in the head, center, and toe areas, a relationship between that parameter and the observed velocity (mean, maximum, and minimum values) seems to exist within the Besiberris rock glacier. The lack of detailed knowledge on its internal structure makes difficult to evaluate the contribution of other factors (thickness, density, temperature; Paterson, 1994) in producing speed differences.

The average velocity of the Besiberris rock glacier was calculated using a model from Barsch (1996), which compares the height of a rock glacier talus apron to the average surface velocity. Average velocity (V_m) of the entire rock glacier is calculated as:

In terms of vertical movement, the rock glacier is showing clear subsidence. In all three traverse lines surface lowering has been detected (average values: line A = 5.00 cm yr⁻¹; line B = 7.10 cm yr⁻¹; line C = 5.27 cm yr⁻¹). The subsidence ranges are 1.1–13.8 cm yr⁻¹ in line A; 1.5–12.0 cm yr⁻¹ in line B; and 2.1–8.1 cm yr⁻¹ in line C (Table 1; Fig. 6). The total ablation is greater in the central traverse line (71.0 cm) than in the head (50.0 cm) and toe (52.7 cm) traverses and is also much more distinct near the axis than in the outermost ridges. The pattern is very similar to the observed at the glacigenic Galena Creek rock glacier (Konrad et al., 1999) and is probably linked to the existence of an increase in insulating surface debris that reduces ablation rates both toward the rock glacier terminus and toward the head: in the first case due to the advance of the rock glacier and, in the second, in relation to the continuous incorporation of talus materials by avalanche processes.

The velocity measurements presented here are consistent with those cited in the literature, commonly placed between 0.1 and 1.0 m yr⁻¹ (Whalley and Martin, 1992; Barsch, 1996; Sloan and Dyke, 1998; Shroder et al., 2000). Comparison with the only previous detailed study of rock glacier dynamics in the Pyrenees on Argualas rock glacier (Serrano et al., 1995) shows that Besiberris rock glacier has a slightly lower rate of horizontal displacement. During the period 1991–1994, Argualas rock glacier moved with an average velocity of 14.5 cm yr⁻¹ (head area), 16.62 cm yr⁻¹ (center area), and 26.83 cm yr⁻¹ (toe area), and the average surface velocity was estimated at 22.3 cm yr⁻¹ (Serrano et al., 1995). However, while Besiberris is a glacier-derived form, Argualas rock glacier has been classified as of periglacial origin. Geoelectrical soundings have shown the absence of a massive ice core, but instead detected a permafrost-rich layer of 10 to 20 m depth beneath a 2- to 4-m-deep ice-free surface layer (Fabre et al., 1995). The morphogenetical disparity (and associated parameters such as density, thickness, or temperature) makes it difficult to directly compare displacement rates from both rock glaciers. Additionally, factors as surface slope, aspect, or lithology of the area are also different. It is important to note that velocity values for glacigenic derived rock glaciers were probably much higher at the LIA maximum, when ice thickness was greater (Whalley et al., 1995). After decades of subsidence processes linked to ice degradation, flow velocities on Besiberris rock glacier have been reduced to the current low rates.

To complement the analysis of the movement of the rock glacier, its basal shear stress τ has been calculated following the standard equation for laminar flow:

Studies by Chueca et al. (1998) and Chueca and Julián (2002) have shown significant losses in surface, thickness and volume of all Pyrenean glaciers since the LIA maximum (1820–1830). Those are linked to the climatic amelioration (higher temperature; drier conditions, and reduced snowfall) observed in the Pyrenean region since the end of the LIA and accentuated in the last few decades (Saz and Creus, 2001; Saz, 2003). Even if protected by the layer of insulating debris, the probable reduction of thickness in Besiberris rock glacier ice-core by this atmospheric warming must have contributed in recent decades to a lowering in the basal shear stress values, which are now near the limit of ice deformation and help explain the low flow rates observed.

Conclusions

Displacements for the Besiberris rock glacier are consistent with those obtained for other rock glaciers. Average horizontal flow rates from 8.72 to 13.35 cm yr⁻¹ and subsidence values from 5.00 to 7.10 cm yr⁻¹ have been obtained over a 10-yr period. Horizontal velocity increases from the head to the toe sector of the rock glacier. Extensional flow dominates as inferred from increasing surface velocities from head to toe. Instability along the central axis and more stable lateral margins suggest deterioration of an ice-core.

If higher temperature, drier conditions, and reduced snowfall continue, ice-core ablation could lead toward a reduction of Besiberris rock glacier flow rate. As Potter et al. (1998) suggested, rock glaciers of glacigenic origin placed in an adverse climatic situation are starved systems in which the small accumulation of snow and ice is kept in balance only by the debris mantle that reduces ablation beneath it. Whalley et al. (1995) have also indicated that with this configuration, the progressive thinning of the ice core will give a reduction in velocity and, eventually, halt flow. In that context, only a sufficient amount of matrix debris could maintain the rock glacier movement. Further investigation of horizontal and subsidence displacements on the Besiberris rock glacier will help to address these questions properly.