Geomorphology of the Aran Valley (Upper Garonne Basin, Central Pyrenees)

Figures & data

Figure 1. Location of the Aran Valley within the Pyrenean range, including the main lithological units of the study area.

The map shows the location of the Upper Garonne Basin, with the distribution of the different lithologies (including detrital, igneous, carbonated, and metamorphic rocks, as well as the main faults).

Figure 2. Steps followed during the elaboration of the geomorphological map; (A) combination of hillshade (40% transparent) with orthophotomaps; (B) first draft of the geomorphological map; (C) field validation of the first geomorphological draft with pictures, such as till cover (C₁), superficial deformation of till cover at the plateau (C₂), peat area (C₃), and alluvial fan (C₄). (D) final geomorphological map respecting the Main map legend.

This combined picture shows four pictures illustrating the workflow adopted to create the main map; (A) combination of the hillshade with orthophotomaps where several terrain features can be identified, such as a rock glacier and alluvial fans; (B) first sketch of the geomorphological map; (C) pictures from the field illustrating the till cover (C1), superficial deformation of till cover at the plateau (C2), peat area (C3), and alluvial fan (C4); (D) final geomorphological map respecting the Main map legend.

Table 1. Attributes of the remote sensing products used for the geomorphological map.

Product	Source	Characteristics and spatial resolution	Information
Orthophotomaps	Institut Cartogràfic i Geològic de Catalunya (ICGC) (http://www.icc.cat/appdownloads/index.html?c=fmeof25c)	RGB, 25 cm/pixel	Terrain characteristics (e.g. vegetation cover, outcrops, deposits, etc)
Lidar data	ICGC (http://www.icc.cat/appdownloads/?c=dlfxlidar)	Collected in 2011. Point cloud density of 0.5-2.7 m^2	Ground surface characteristics (e.g. flat surfaces, furrows, crests, etc) and delimitation of landforms
Stereoscopic viewer	Instituto Geográfico Nacional (IGN; http://www.ign.es/3d-stereo/)	Photograms of the Plan Nacional de Ortofotografia Aérea. Variable resolution (22-45 cm/pixel)	Characteristics of the ground surface morphology
3D models satellite images	Browser of Google Earth (https://earth.google.com/web/)	IGN imagery (large scale) and Landsat imagery (small scale). NASA SRTM Digital Elevation dataset (30 m) with the IGN imagery (25x25 cm) and Landsat imagery (30x30 m)	Validation of the ground and terrain characteristics and a broader framework of geomorphological features

Table 2. Location and morphological characteristics of the landforms and processes drew in the Main map.

Categories	Landforms and processes	Geomorphological setting	Morphological criteria	Mapping identification criteria	Reference
Glacial	Cirque	Headwaters of the valleys	Amphitheatre-like basin surrounded by steep walls	Steep walls displaying a concave area with a (semi)elliptical morphology	Joly (1997); Lambiel et al. (2013)
	Ridge (or arêtes)	Water divide between glacial cirques/valleys	Truncated spur dividing adjacent glacial cirques or valleys	Sharp and narrow rock alignments	Giles et al. (2017)
	U-shaped valley	Trunk and tributary valleys shaped by glaciers	U-shaped transverse valley section with concave slopes	Parabolic cross-profile (steep-sided and flat bottom)	Evans (2013)
	Rock step (>50°)	Polished surface in glacial cirques/valleys	Slope rupture with a rounded shape and asymmetric up/down valley faces	Higher slope degrees in lee’s bare rocks	Joly (1997); Giles et al. (2017)
	Watershed breach	Headwaters of the valleys (cols)	Concave water divide eroded by past glacial diffluence	Lower areas within an elevated alignment, able to connect two opposite valleys containing bare rocks and/or boulders	Joly (1997); Giles et al. (2017)
	Overdeepened basin	Depressed area in glacial cirques/valleys	(Semi)circular depression surrounded by polished surfaces or glacial deposits	Current flat areas frequently include lakes or peatlands	Joly (1997); Lambiel et al. (2013)
	Hanging valley	Lateral glacial valley hanging above the main glacial valley floor	Glacially eroded valley floor standing above the adjacent valley	Different base levels between lateral valleys and the tributary/main valley	Joly (1997); Giles et al. (2017)
	Riegel	Bottom of glacial cirques/valleys	Upstanding transverse ridge with polished surfaces	Rock barrier across the bottom of the glaciated valley/cirque with or without a narrow exit	Lambiel et al. (2013); Giles et al. (2017)
	Roche moutonnée	Bottom of glacial cirques/valleys	Convex surfaces showing polished surfaces upstream and quarrying downstream	Bare surfaces with a smooth up-facing slope and steep down-facing slope	Lambiel et al. (2013); Giles et al. (2017)
	Polished surface	Slope or bottom of glacial cirques/valleys	Relatively horizontal glacially abraded bedrock	Bare rocks with a smooth surface	Giles et al. (2017)
	Striation	Polished rock surface in glacial cirques/valleys	Millimetric to metric long scratches or grooves commonly on stoss side of the polished rock surfaces	Only identifiable in field observations	Lambiel et al. (2013); Giles et al. (2017)
	Debris-free glacier moraine	Bottom and slopes of glacial cirques/plateaus/valleys	One or several crests with subangular to subrounded boulders and with a depression upvalley	Elongated ridges with smooth surfaces	Joly (1997); Lambiel et al. (2013)
	Debris-covered glacier moraine	Bottom and slopes of glacial cirques	Supraglacial till with angular boulders showing frequent longitudinal furrows and crests	Parallel ridges with supraglacial boulders on top or surfaces with collapses (minor-closed depressions) and debris thickness increasing towards the glacier front	Evans (2005)
	Till	Wide range of settings affected by past glaciations (summit surfaces, slopes, plateaus, valley bottoms)	Unsorted glacial sediment	Soft surfaces covered with unsorted sediments and embedded boulders where commonly vegetation colonized the fine fraction of the deposit	Lambiel et al. (2013); Benn and Evans (2010)
	Till collapse	Till deposits	Depression on till cover	Closed depressions	–
	Kame terrace	Sides of glacial valleys	Relatively horizontal deposit generated by meltwater streams along the side of a glacial valley	Smooth and flat (table-like) surfaces	Joly (1997); Lambiel et al. (2013)
	Erratic boulder	Widespread deposit across formerly glaciated areas	Isolated boulder lying on the surface	Large boulders needing of field validation	Joly (1997); Lambiel et al. (2013)
Periglacial	Rock glacier	At the foot slope and bottom of glacial cirques	Large accumulation of boulders displaying perpendicular ridges and furrows and a steep front	Large boulders with different vegetation matureness and morphological changes ridges-like	Joly (1997); Kääb (2013)
	Protalus lobe	At the foot of talus slopes in glacial cirques/valleys	Accumulation of boulders with one lobate ridge at the front	Large boulders with different vegetation matureness and morphological changes ridge-like	French (2007); Kääb (2013)
	Block stream	Slopes of glacial cirque/valleys	Linear accumulation of coarse angular boulders	Angular boulders with scarce vegetation flowing downslope	French (2007); Knight and Harrison (2009)
	Patterned ground	Relatively flat surfaces or gentle slopes, generally at high altitudes	Geometric landforms composed of sorted/non-sorted circles (flat surfaces) and stripes (downslope).	Only identifiable in field observations	Joly (1997); French (2007)
	Block field	Relatively flat surfaces or gentle slopes at the summit level	Accumulation of in-situ boulders on the mountaintops	Only identifiable in field observations	Giles et al. (2017)
	Solifluction lobe	Slopes of glacial cirques/valleys	Tongue-shaped landforms or terrassettes occurring in vegetation and sediments	Only the biggest lobated structures are identified on vegetation and sediments	Joly (1997); Giles et al. (2017)
	Ploughing boulder	Gentle slope of glacial valleys	Boulder showing downslope displacement with a linear depression upslope	Boulder’s pathway is recorded in vegetation and topography	Joly (1997)
Nival	Late-lying snow patch	Hollows of glacial cirques	Semi-permanent snow field	Snow patches	Joly (1997); Lambiel et al. (2013)
	Avalanche corridor	Steep slope of glacial cirques/valleys	Corridor of frequent avalanche occurrence	Linear areas with no arboreous strata along the slope	Joly (1997)
	Protalus rampart	At the foot of slopes in glacial cirques/valleys	Accumulation of well-sorted sediments transversal to slope and at the foot of late-lying snow patches	Difficult to identify with remote sensing products	Joly (1997); Lambiel et al. (2013)
Karst	Doline	Outcrop of limestones	Isolated closed depression	Dots identified as topographic depressions	Joly (1997)
	Uvala	Outcrop of limestones	Set of coalescent dolines next to each other	Set of dots connected with each other identified as one topographic depression	Joly (1997)
	Karst window	Contact between limestone and silt-grained bedrock	Depression where a stream falls into a karst aquifer	Only identifiable in field observations	–
Slope	External scar	Slopes or interfluves of glacial cirques/valleys	Delineation of the slope failure origin	Steep areas of downslope ruptures	Lambiel et al. (2013)
	Internal scar/antiscar	Surface of rock slope failures in glacial cirques/valleys	Downslope or upslope scar	Steep areas of perpendicular and linear ruptures on the mass movement. The position of the highest degrees on the rupture defines if is downslope or upslope	–
	Talus slope	At the foot of rock walls in glacial cirques/valleys	Irregular and angular rock debris covering the valley sides	Large boulders forming a talus or cone morphology	Joly (1997); Giles et al. (2017)
	Rock slope failure	Slopes of glacial cirques/valleys	Break of slope caused by mass movements	Steep downslope scars with irregular displaced body surface and steep front	Joly (1997); Giles et al. (2017)
	Rock fall	At the foot of the rock wall of glacial cirques/valleys	Non-sorted deposit of large angular boulders	Large boulders showing a chaotic distribution on foot slopes	Lambiel et al. (2013)
	Gully/Ravine	Slopes of glacial cirques/valleys	Narrow runoff channels eroding the bedrock/soil.	Abrupt channels directly affecting the bedrock	Rapp (1960); Lambiel et al. (2013)
	Debris flow	Slopes of glacial cirques/valleys	Channel, levée margin and sometimes lobate feature on the terminal areas of talus slopes	Narrow channels on sediments	Joly (1997); Lambiel et al. (2013)
	Lobate front	Foot slopes of glacial cirques/valleys	Prominent toe of the rock slope failures	Arched morphologies facing downslope	–
Alluvial	Alluvial fan	Foot slopes of glacial valleys	Triangular-shaped deposits of gravels and fine-grained sediments	Triangular-shape morphologies	Joly (1997)
	Alluvium	Flat area on the slopes of glacial valley	Alluvium defined by a relatively horizontal surface	Soft and flat surfaces covered with herbs	Parry (2011)
	Mudflow	Slope of glacial cirques/valleys	Silty-rich deposit with lobate morphology in the distal section	Poorly defined morphologies with fan-shaped and lobate terminus	Matsuoka et al. (2005)
	Peatland/wetland	Depressed area of glacial cirques/valleys	Dense vegetation cover with poor drainage within overdeepened basins	Relatively horizontal surfaces covered with peat sediments	Joly (1997)
Fluvial	Gorge	Bottom of glacial valleys	Deep fluvial incision on bedrock	Longitudinal deeply incised bedrock	Joly (1997); Lambiel et al. (2013)
	Plain	Bottom of glacial valleys	Relatively horizontal surface formed by fluvial sediments	Relatively soft and flat surface parallel to the riverbed	Joly (1997)
	Terrace	Bottom of glacial valleys	Lateral staggered level of fluvial sediments	Staggered flat levels constrained by steep slopes, parallel to the main axis of the river	Joly (1997); Lambiel et al. (2013)
Hydrology	River/stream	Bottom of glacial cirques/valleys	Linear water body that is permanent (river) or ephemeral (stream)	Linear water bodies	Joly (1997); Lambiel et al. (2013)
	Lake	Bottom of glacial cirques/valleys	Permanent water body with irregular shape	Water bodies from natural origin, coinciding with overdeepened basins or landslide obstructions/scars	Joly (1997); Lambiel et al. (2013)

Figure 3. Examples of the different glacial landforms identified in the area: (A) lateral moraine around 1600 m at the Nère Valley; (B) E-W perspective of the main Garonne Valley with an erratic boulder of granite distributed on a slate outcrop above Salardú village; (C) glacial lake distributed in an overdeepened basin at the Unhòla Valley; (D) glacial cirque of the Montardo Peak at the Valarties Valley.

Evidence from the Aran Valley indicative of ancient glacial processes. (A) alpine meadows and a moraine ridge; (B) U-shaped valley of the Aran and an erratic boulder; (C) the Montoliu Lake that accumulates its waters at the bottom of an overdeepened basin in the Unhòla Valley; (D) overview of the Montardo glacial cirque.

Figure 4. Examples of the different periglacial landforms identified in the area: (A) relict rock glacier distributed across the glacial cirque floor of Locampo, Ruda Valley; (B) active talus slope with protalus lobes at the Nère Valley; (C) relict block stream at 2200 m in the Ruda Valley; (D) inactive sorted cirques at 2480 m in the Mauberme cirque, Unhòla Valley.

This combined figure presents 4 pictures from the Aran Valley indicative of the periglacial processes: (A) shows a relict rock glacier; (B) shows a protalus lobe at the foot of a talus slope; (C) shows a block stream; (D) shows a patterned ground feature composed of angular boulders forming a circle with no signs of present-day activity.

Figure 5. Examples of other geomorphological landforms identified in the area: (A) long-lying snow patch at 2350 m, in the north face of the Armèros Peak; Unhòla Valley (B) dolines located in the Sascorjada glacial cirque, Toran Valley; (C) Rock slope deformation at the Varradòs Valley; (D) alluvial fan connected with the fluvial plain at the Joèu Valley.

This combined figure presents 4 pictures from the Aran Valley indicative of other geomorphological processes: (A) shows a long-lying snow patch; (B) shows three dolines within alpine meadows; (C) shows a complex slope with scars in the upper red line and deposit in the arrow showing evidence of the magnitude of this large mass movement; (D) shows an alluvial fan in contact with the fluvial plain at the valley bottom.

Figure 6. Sequence of environmental phases shaping the landscape of the Aran Valley: (A) maximum ice extent of the last glacial cycle; (B) glacial tongues flowing downvalleys during a glacial advance/standstill (Oldest Dryas) within the long-term glacial retreat of the Garonne paleoglacier that favored also intense slope readjustment; (C) alpine and cirque glaciers (Younger Dryas) with very intense postglacial slopes dynamics, and the occurrence of permafrost that promoted the formation of rock glaciers; (D) deglaciated cirques with periglacial processes prevailing at higher elevations (inactive rock glaciers), and slope, alluvial and fluvial processes at lower elevations.

This sketch figure summarizes the environmental evolution in the Aran Valley during four stages: (A) the first one shows the valley entirely occupied by the ice during the maximum ice extent of the last glacial cycle; (B) the second shows how glaciers started retreating during the Oldest Dryas but they still extended over the main valleys; (C) the third sketch presents glaciers already confined within the headwaters of the valleys, mainly in cirques, during the Younger Dryas; (D) the last stage corresponds to postglacial conditions, such as those of those, with prevailing periglacial processes.

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Data availability statement

The data described in this article are openly available at the ZENODO data repository (https://doi.org/10.5281/zenodo.4644265).