This study analyzed glacial and periglacial landforms in the Southern French Alps to reconstruct the landscape evolution since the last glacial maximum and better understand rockglacier development patterns. Cosmogenic dating and weathering rind measurements identified three post-LGM glacial stages and three generations of rockglaciers. Most rockglaciers likely developed in the second half of the Holocene, and their distribution depended on both climate and geomorphic conditions following glacier retreat. The results provide new constraints on late glacial retreat in the region, suggesting valley glaciers persisted lower than previously thought until the early Holocene.

1. Rockglaciers genesis and growth in a degrading mountain
cryosphere (Southern French Alps)
E. Cossart1, M. Fort1, D.L. Bourlès2, R. Braucher2,
J. Carcaillet3
(1) PRODIG, UMR 8586 – CNRS, Universités Paris 1 & Paris-Diderot (Paris 7), 2 rue
Valette, F-75005 France
(2) CEREGE, UMR 6635 – CNRS, Université Aix-Marseille 3, Europôle Méditerranéen de
l'Arbois, BP 80, 13545 Aix en Provence Cedex 04, France
(3) LGCA, Laboratoire de Géodynamique des Chaînes Alpines, Grenoble.
Global Change and the World’s Mountains
Perth, Scotland, September 26-30, 2010

2. Rockglaciers genesis and growth in a degrading mountain
cryosphere (Southern French Alps)
E. Cossart1, M. Fort1, D.L. Bourlès2, R. Braucher2,
J. Carcaillet3
(1) PRODIG, UMR 8586 – CNRS, Universités Paris 1 & Paris-Diderot (Paris 7), 2 rue
Valette, F-75005 France
(2) CEREGE, UMR 6635 – CNRS, Université Aix-Marseille 3, Europôle Méditerranéen de
l'Arbois, BP 80, 13545 Aix en Provence Cedex 04, France
(3) LGCA, Laboratoire de Géodynamique des Chaînes Alpines, Grenoble.
OUTLINE
• Issue: deglaciation and rockglacier
development vs climate change
• Studied area
• Methods
• Results: chronology of deglaciation vs
rock glaciers development
• Discussion: control factors and significance
at the regional scale
Global Change and the World’s Mountains
Perth, Scotland, September 26-30, 2010

3. Introduction
- Glacial and periglacial features widespread in European Alps
 potential record of climatic and environmental change
But…
 small number of valuable ages of both glacial and periglacial landforms
 complex significance of some landforms (i.e. rock-glaciers)
 What alpine landscape evolution since the Last Glacial Maximum?
 Patterns of rock-glacier genesis?
- Study area : Briançonnais area, Southern French Alps
 very significant deglaciation pattern of the French Alps since the LGM
 many rock-glaciers identified, pristine or fresh (Evin 1987, Francou 1988)
But debates on…
 The Late-Glacial glaciation pattern: valley or cirque glaciation?
 Age of rock-glaciers? mostly assumed to be Late-Glacial features
 Application of numerical (CRE ages) and relative dating methods
(weathering rind thickness)

4. Introduction
- Glacial and periglacial features widespread in European Alps
 potential record of climatic and environmental change
But…
 small number of valuable ages of both glacial and periglacial landforms
 complex significance of some landforms (i.e. rock-glaciers)
 What alpine landscape evolution since the Last Glacial Maximum?
 Patterns of rock-glacier genesis?
- Southern French Alps:
 Very significant deglaciation pattern since the LGM
 Many rock-glaciers identified, pristine or fresh (Evin 1987, Francou 1988)
But debates on…
 The glaciation pattern during the Late-Glacial: valley or cirque glaciation ?
 Age of rock-glaciers? mostly assumed to be Late-Glacial features
 Application of numerical (CRE ages) and relative dating methods
(weathering rind thickness)

5. Study area: Briançonnais
- Former Durance glacier during the Last
Glacial Maximum (LGM):
 one of the major valley glacier in the
French Alps (more than 100 km long)
 trunk glacier receiving most of the glaciers
of the Southern French Alps
 Ice thickness reaching at least 1000 metres Mt Blanc
in the upper part of the watershed (Briançon)
After Campy &
Buoncristiani; in
Ehlers and
Gibbard (2004)

6. Northern part: Clarée valley
Clarée glacier during
the LGM
 Right-bank tributary of
former Durance glacier
 Ice at least 850 metres
thick at the confluence
north of Briançon
Durance
glacier

7. Current permafrost and
glacier extent in the
Clarée valley
- Lower Limit of Permafrost
(LLP):
 Possible permafrost: 2510m
 Probable permafrost: 2700m
(in Cossart et al., 2008)
- Glacier extent?
 no more glacier
 Regional Equilibrium Line
Altitude (Vallouise): 3200m
E 3150 m W
 Large extent of « periglacial
belt »

8. Methods (1): Field mapping
Identification of former glacier remnants  relative chronology of
deglaciation
 Morainic ridges: position of former glacial fronts
 Roches-moutonnées: reconstruction of minimal thickness of valley glacier
Identification of
erratics, cf. lithological Quartzite Rock-bar
contrasts
Erratics (dolomite)
Moraine made of
dolomites

9. Methods (1): Field mapping
Identification of former glacier remnants  relative chronology of
deglaciation
 Morainic ridges: position of former glacial fronts
 Roches-moutonnées: reconstruction of minimal thickness of valley glacier
Identification of
erratics, cf. lithological Quartzite Rock-bar
contrasts
Inventory and
description of rock-
glaciers (RG)
 Geometric extent
 Subdued vs. fresh
landforms Moraine made of
dolomites
Assumption: Rock Glacier
development after glacier
decay

10. Methods (2): Relative chronology
Position of morainic landforms
 External ones  Older
 Internal ones  Younger
Characterization of the freshness/
activity of rock-glaciers
 Vegetation extent
 Steepness of the front
Measurements of weathering rinds (on
sandstone boulders)
 Yellow to red oxydation cortex (5YR to 10R)
 Samples on top of ridges (avoid late snow
influence; maximize stability of boulders)
 15 to 20 samples for each site
 5 to 10 thickness measurements for each sample
(accuracy 0.5 mm)

11. Methods (3): Cosmogenic Ray Exposure (CRE) dating, 10Be
Sampling strategy  Sampling on stable, roches-moutonnées surfaces
Assessment of:
- retreat of glacier front
 Sampling at various sites from down- to
upvalley
- ice-thickness lowering
 Sampling along cross-sections of valley slopes
CRE clock is set when the
rock-bar is free of either
ice or till-cover
Sampling:
• On the edge (toss side) of
rock-bars
• Next to the steep lee side
of rock-bars
• On surfaces affected by
striae (avoid rejuvenated
surfaces)

12. Results (1): 3 post-LGM glacial stages
Stage 1: Most external moraines identified ≈150 m below the trimline (yet post-
LGM) = lateral moraines = valley glaciation
Stage 2: frontal moraines, located at the outlet of cirques  Incipient cirque
glaciation
Stage 3: small frontal moraines at the foot of cirque faces  last stage before
complete deglaciation

13. Younger 3 generations of rock-glaciers
Class 3
 15 small samples: 103 to 103 m3
 Front elevation: above 2600 m.a.s.l
 no vegetation, water at 0.2-1°C
 Active landforms (creeping screes)
Class 2
 14 samples
 Volume: 103-104 m3
 Front altitude: 2500m
 Symptoms of
degradation, yet water
seepages at 0.2-1.5°C
Class 1
 2 samples: 104 m3
 Front altitude: 2380 m
 Completely vegetated
Older  Relict landforms

14. Relative scenario
Most recent
features
Younger features Oldest features
Thinner weathering rinds Weathering rinds reach their maximal value

15. CRE dating: 1 Late Glacial stage, 2 Holocene stages
Disappearance Cosmic ray exposure :
of valley glacier  2 main stages identified
LGM  LGM between 25 and 30 ka BP
(cf. trimline)
Cirque Glaciation  Preservation of a small glacier
in Vallouise tongue during the Late Glacial
(between 9 - 11 ka BP)
 Cirque glaciation after 8 ka BP
Vallouise
Valley :
 same
pattern as in
Clarée valley
 Cirque
glaciation at
the beginning
of the
Holocene

16. Probable scenario
LIA features
Age younger than 5.0 ka CRE Age = 11.0 ka
Late Glacial features
 Second half of the Holocene CRE Age = 7.9 ka

17. Synthesis and interpretation
Glacier variation:
 A re-assessment of the Late-Glacial
period:
 A low ELA (~ 2200 m.a.s.l)
 Persistance of a valley glacier YD
 2 stages of cirque glaciation during the
Holocene:
 Stage 2 = probably Subboreal
 Stage 3 = LIA
Rock-glacier chronology:
 Class 1: only a few Late-Glacial
features
 Class 2: main stage of rock-glacier
development during the second half of
the Holocene
 A general, altitudinal shift of RG:
indicator of a rise of the Lower Limit of
Permafrost ?

18. Climatic vs. geomorphic
significance of Rock-Glaciers
Class 1 rock-glacier
Deglaciated plateau
Class 1 rock-glaciers:
 only 2 samples
 Rock-glacier development possible
on an early deglaciated plateau (ice-
free during the Late Glacial)
 Disconnectivity between the
deglaciated plateau and the Late-
Glacial tongue Limited debris source
 no evacuation of RG debris
Class 3 rock-glaciers:
 15 small active samples
 permafrost conditions
 Rock-glacier development hampered
by limited debris sources and supply
 The altitude of RG front reflects -a
Small creeping features
minima- the extent of permafrost belt

19. Conclusions
 Climatic vs. Geomorphic significance of rock-glaciers
- Three generations of rock-glaciers but:
- Late Glacial: development of rock-glaciers limited by lack of deglaciated areas
- Present: growth of rock-glacier limited by lack of debris supply
- Most rock-glaciers probably developped during the second half of the
Holocene (Subboreal?)
 Location and age of RG depend not only on climatic but also on
glaciological and geomorphic conditions
 CRE results provide new constrains on glacial retreat in Southwestern
French Alps:
- Useful method for glacial erosional landforms dating
- Our data not in agreement with the hypothesis of an early retreat of
glaciers within the whole Southern French Alps
- valley Clarée glacier front at altitude lower than 2000 m.a.s.l. during the Late-
Glacial
- major stage of recession: only at the beginning of the Holocene (as in the Western
valleys)

20. Conclusions
 Climatic vs. Geomorphic significance of rock-glaciers
- Three generations of rock-glaciers but:
- Late Glacial: development of rock-glaciers limited by lack of deglaciated areas
- Present: growth of rock-glacier limited by lack of debris supply
- Most rock-glaciers probably developped during the second half of the
Holocene (Subboreal?), i.e. earlier than previously thought
 Location and age of RG depend not only on climatic but also on
glaciological and geomorphic conditions
 CRE data provided new constrains on glacial retreat in Southwestern
French Alps:
- Useful method for glacial, erosional landforms dating
- Our results: not in agreement with the former hypothesis of an early
retreat of glaciers within the whole Southern French Alps
- valley Clarée glacier front: still at elevation lower than 2000 m.a.s.l. during Late-
Glacial
- major stage of recession: only at the onset of Holocene (as in the Western Alpine
valleys)

21. Thank you for your attention
First author Etienne COSSART For further details, see COSSART, FORT & al., Catena 80 (2010) 204-21