the ad 1717 rock avalanche deposits in the upper ferret valley (italy): a dating approach with...

The AD 1717 rock avalanche deposits in theupper Ferret Valley (Italy): a dating approachwith cosmogenic 10Be

NAKI AKCAR,1* PHILIP DELINE,2 SUSAN IVY-OCHS,3 VASILY ALFIMOV,3 IRKA HAJDAS,3 PETER W. KUBIK,3

MARCUS CHRISTL3 and CHRISTIAN SCHLUCHTER11Institute of Geological Sciences, University of Bern, 3012 Bern, Switzerland2EDYTEM Lab, Universite de Savoie, CNRS, Le Bourget-du-Lac, France3Institute of Particle Physics, ETH Honggerberg, Zurich, Switzerland

Received 4 February 2011; Revised 20 July 2011; Accepted 9 October 2011

ABSTRACT: On 12 September AD 1717, a rock volume larger than 10 million m3 collapsed onto the Triolet Glacier,mobilized a mass composed of ice and sediment and travelled more than 7 km downvalley in the upper Ferret Valley,Mont Blanc Massif (Italy). This rock avalanche destroyed two small settlements, causing seven casualties and loss oflivestock. No detailed maps were made at the time. Later investigators attributed accumulations of granitic bouldersand irregular ridges on the upper valley floor to either glacial deposition, or the AD 1717 rock avalanche, or a complexmixture of glacial deposition, earlier rock avalanche and AD 1717 rock avalanche origin. In this study, we presentcosmogenic 10Be exposure ages from nine boulders in the extensive chaotic boulder deposit with irregular ridges, twofrom Holocene glacier-free areas, and one from a Little Ice Age moraine. Exposure ages between 330� 23 and483� 123 a from eight of nine boulders from the chaotic deposit indicate that at least seven were deposited by the AD1717 rock avalanche. The other three boulders yielded 10Be exposure ages of 10 900� 400, 9700� 400 and244� 97 a, respectively. Our results are in good agreement with the existing chronology from dendrochronology andlichenometry, and radiocarbon analysis of wood samples, but not with older 14C ages from a peat bog in the upper partof the valley. Based on the new age control, the rock avalanche deposits cover the whole bottom of the upper Ferretvalley. Copyright # 2012 John Wiley & Sons, Ltd.

KEYWORDS: Alps; inheritance; Mont Blanc Massif; natural hazard; surface exposure dating.

Introduction

Mass movements are one of the main erosional processes in theevolution of mountain landscapes. Their effect on landscapechange for a given locality can be determined when themagnitude, timing and the frequency of the events areconstructed (Cockburn and Summerfield, 2004). For this,determination of the timing of recurrence intervals is essentialand this can shed light on the understanding of displacementmechanics, causes, triggers (tectonic structures, debuttressingfollowing retreat of glaciers, climate and/or seismic events), andtheir interaction (Ivy-Ochs and Schaller, 2010, and referencestherein). The timing can be determined by surface exposuredating, which is a widely accepted dating tool in Quaternarygeology and geomorphology (Gosse and Phillips, 2001, andreferences therein). In order to exposure date a landslide, headscarps, sliding planes and large boulders are potential samplingsites. Despite the fact that these sites contain new rock surfacescreated during mass movements, surface exposure dating canbe complicated and difficult (Ivy-Ochs and Schaller, 2010). Forinstance, low angle sliding planes may yield ’too young’ agesdue to snow, sediment and/or vegetation cover (Ivy-Ochs et al.,2009a). In contrast, sliding planes may yield ’too old’ ages dueto inheritance, depending on the exposure time beforelandsliding (e.g. long exposure, of the order of millions ofyears, would require deep-seated sliding planes in order toexclude inheritance). As well as the sliding planes, exposuredating of boulders in a landslide can result in ’too old’ ages withrespect to independent age controls (Nichols et al., 2006;Sewell et al., 2006; Ivy-Ochs et al., 2009a), since boulders ontop of the landslide deposit often originate from the outersurface of the pre-slide bedrock due to mechanisms of massmovement (Voight, 1978). Despite these potential compli-cations, several landslides have been exposure dated with

cosmogenic nuclides in various areas: Europe (e.g. Ballantyneet al., 1998; Prager et al., 2009), North America (e.g. Nicholset al., 2006), South America (e.g. Hermanns et al., 2004) andAsia (e.g. Barnard et al., 2001; Ivy-Ochs and Schaller, 2010,and references therein).In this study, we focus primarily on the cosmogenic

10Be exposure dating of boulders from one of the largestcrystalline rock avalanches in the Alps, which occurred on thenight of 12 September AD 1717 in the Ferret Valley (MontBlanc Massif, Italy; Fig. 1). A rock volume larger than 10million m3 suddenly collapsed onto the upper part of the TrioletGlacier in the upper reaches of the Ferret Valley. Thevoluminous rock mass, mixed with ice, was mobilized andavalanched violently downvalley (Fig. 1). It destroyed twosmall settlements and instantly killed seven herdsmen and 120cows (Porter and Orombelli, 1980, and references therein). Itreached the lower part of the valley (7.2 km from its source)with a descent of around 1860m (Porter and Orombelli, 1980;Deline and Kirkbride, 2009).For a long time, the extent of the deposits of this rock

avalanche has been under debate. In the upper part of thevalley, accumulations of granitic boulders and irregular ridgesover a distance of 2 km terminate downvalley with a 5m higharcuate front (Deline and Kirkbride, 2009). This deposit wasfirst attributed in large part or even totally to glacial deposition(e.g. Sacco, 1918). In their detailed study, Porter andOrombelli(1980) interpreted this deposit as the rock avalanche of AD1717 (Fig. 1). Based on a single 14C date, Aeschlimann (1983)debated this interpretation and claimed that the main part ofthis deposit is a Lateglacial moraine. Recently, Deline andKirkbride (2009) concluded that this deposit is a complex ofglacial, an earlier rock avalanche and of the AD 1717 rockavalanche deposits. They also point out the difficulties indifferentiation of rock avalanche and glacial deposits in theFerret Valley. Therefore, our secondary focus in this study wasto provide more information about the extent of these deposits

JOURNAL OF QUATERNARY SCIENCE (2012) 27(4) 383–392 ISSN 0267-8179. DOI: 10.1002/jqs.1558

Copyright � 2012 John Wiley & Sons, Ltd.

*Correspondence: N. Akcar, as above.E-mail: [email protected]

and contribute to their differentiation from glacial deposits byconstructing their chronology with in situ cosmogenic10Be surface exposure dating. In this paper, we briefly describethe deposits of a 291-year-old rock avalanche (before 2008)and a give a short summary of the debate on their extent.Following the information on the samples, we present sub-millennial-scale 10Be exposure ages from the upper FerretValley and discuss their implications.

Study area

Situated in the southern flank of the Mont Blanc Massif, theFerret Valley is a major headward tributary of the Aosta Valleydrained by the Doire River (Fig. 1). This part of the Mont BlancMassif consists mainly of coarse-grained porphyritic granite(Mont Blanc Granite) and it forms the northern side of thevalley, which rises up to 2000m above the present valley floor(Fig. 1). Bedrock lithology of the southern flank consists ofJurassic limestone, schist and flysch, which are shown as’sedimentary and metasedimentary rocks’ in Fig. 1. In thisvalley system, the Triolet and the Pre de Bar glaciers are thelargest, since the others are restricted to the steep lateraltributaries that are separated from the main valley by a bedrockbarrier. The floor of the upstream part of the valley is overlain bythe recent moraine complex of these two glaciers (Deline andKirkbride, 2009). Downstream, the valley floor is covered by anassemblage of morphologically distinct units comprisinggranitic boulders for a distance of 1.9 km from the recent

moraine complex of the Triolet Glacier (Fig. 2). In its distalsector, this deposit is a chaotic accumulation of blocks of alldimensions �1000m3, with an openwork structure partlyfilled by deposition of alluvium, which resulted in the formationof smooth plains and small meadows (Porter and Orombelli,1980; Deline and Kirkbride, 2009). In Bois de Greuvettaz(Figs 2 and 3), where this deposit terminates on a flat alluvialplain, its morphology, where unmodified, is hummockywithout any regular pattern. Despite this chaotic morphology,linear ridges standing as much as 4m above the average surfaceare found (Porter and Orombelli, 1980; Deline and Kirkbride,2009). The more obvious are longitudinal and transverse ridgesin the Plan de Greuvettaz, which are 10–30m wide, 1–10mhigh and 100–800m long (A2 in Fig. 2). Arranged in concentricsegments, these ridges mainly comprise a matrix-supporteddiamicton, and local openwork structures occur. Along thecrest line of the ridges, large boulders are found (Deline andKirkbride, 2009). Further to the northeast, upstream, the depositis overlain by two polygenetic debris cones originating from thenorthern (Greuvettaz adret cone) and southern sides (Greu-vettaz ubac cone) of the main valley (Deline and Kirkbride,2009) (Figs 2 and 3). The part of the deposit close to the recentTriolet moraine complex displays a diverse morphology: (i) alarge alluvial terrace in the Plan d’Arp-Nouvaz, where largegranitic boulders are randomly distributed; (ii) chaotic boulderaccumulation in the Bois d’Arp-Nouvaz, where openworkstructures are filled by alluvium; and (iii) a 40m wide and300m long coarse ridge at Bois de la Biche, with openworkblock accumulations and a crudely terraced morphology(Deline and Kirkbride, 2009) (Figs 2 and 3). On the Tsa-de-Jeanrock spur overlooking the valley floor, at the bottom of thenorthwest-facing side, a small deposit of granite boulders isperched (Fig. 2). Unlike in several rock avalanche deposits(Prager et al., 2009; McColl and Davies, 2011), toma hills arenot present in the upper Ferret Valley.

Previous studies

The chaotic granitic deposits that cover the upper Ferret Valleyhave been studied for almost 300 years. The earliest historicaldocumentation concerning the AD 1717 rock avalanche waswritten shortly after the disaster by Michel-Joseph Pennard, alocal inhabitant (Porter andOrombelli, 1980). A few years later,de Tillier (1966) provided a more complete record on the rockavalanche. When de Saussure (1796) travelled through theFerret Valley in 1781, he observed granitic rock debris coveringthe Triolet Glacier and ascribed this cover to a massive rockfallin the 1720 s. In the 19th century, the deposits of the upperFerret Valley were attributed to glacial deposits (e.g. Agassiz,1845) except for Virgilio (1883), who interpreted the deposits ofArp-Nouva and Greuvettaz as a glacial outburst flood. Agassiz(1845) related the advance of the Triolet Glacier during theLittle Ice Age (LIA) to the reduced ablation beneath the rockdebris covering the glacier (Deline and Kirkbride, 2009, andreferences therein). In the 20th century, studies attributed thisdeposit to either a glacial origin or to the AD 1717 rockavalanche. Sacco (1918) interpreted this deposit as a glacialdrift deposited between the 16th and 19th centuries. Zienert(1965) also attributed it to a glacial origin of Lateglacialadvance. Mayr (1969) argued for both glacial and rockfallorigin. He related the deposits in Arp-Nouva to the 1717 event,whereas he interpreted the terminal part of the deposit inGreuvettaz as a part of an earlier (mid Holocene) morainesystem (Porter and Orombelli, 1980; Deline and Kirkbride,2009, and references therein). Porter and Orombelli (1980)attributed the whole deposit of the upper Ferret Valley to therock avalanche of AD 1717. They argued that the avalanche

Figure 1. Sketch map of the upper Ferret Valley showing the path andextent of deposits of the AD 1717 rock avalanche proposed by Porterand Orombelli (1980), bedrock geology and maximum extent ofglaciers in the valley system during the LIA (redrawn from Orombelliand Porter, 1988; modified after Deline and Kirkbride, 2009). Thisfigure is available in colour online at wileyonlinelibrary.com.

Copyright � 2012 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 27(4) 383–392 (2012)

384 JOURNAL OF QUATERNARY SCIENCE

originated from the Aiguille de l’Eboulement (3599m a.s.l.),which means ’rockfall peak’ in French (Fig. 1). Here toponymyis the unique argument to locate the scarp area, since norockwall area can particularly be distinguished as a scar in this

highly fractured part of the Mont Blanc Massif. They concludedthat the AD 1717 event had a volume of (16–20)� 106m3 andcovered the valley floor over an area of 2.7 km2 with aminimum sediment thickness of 5m, after having descendedfrom 3600m a.s.l. down to 1740m, travelling a runout distanceof 7.2 km. Its apparent coefficient of friction (height over length,H/L ratio) of 0.23 and travel angle (Fahrboschung (F)¼ tan�1

H/L) of 12.988 are typical for large rock avalanches on toglaciers:H/L and F are 0.19, 0.18 and 0.15, and 10.948, 10.088and 8.538, for 10–20� 106m3 rock avalanches on the AlaskanSherman (1964), Black Rapids (2002) and McGinnis Peak(2002) glaciers, respectively (Jibson et al., 2006). Porter andOrombelli (1980) discussed multiple field criteria basedon several characteristics (e.g. morphology, texture, surfacegradient, basal contact), which they used to recognize the rock-fall origin of the deposit. They applied dendrochronology andlichenometry to determine the age of the rock avalanchedeposit. They found that the oldest tree on the deposit datedback to AD 1742, which was considered a close minimum agefor the deposit. According to the lichen data, they concludedthat the oldest lichen on the boulders commenced growingbetween AD 1717 and 1727.Based on the uncalibrated radiocarbon date of 885� 60

14C a BP (UZ-293) from a peat some hundredmeters upvalley ofArp-Nouvaz, Aeschlimann’s (1983) interpretation was that this

Figure 2. Map of the upper Ferret Valleydeposits and the location of the sampledboulders for surface exposure dating: 1, limitof the deposits of the Plan d’Arp-Nouva/Greu-vettaz (a, recognized; b, inferred); 2, diamictridges (moraine crests as interpreted by Delineand Kirkbride, 2009; a, recognized; b,inferred); 3, till as proposed by Deline andKirkbride (2009); 4, chaotic blocks; 5, alluvialdeposits; 6, megablocks; 7, local rockfalldeposit; 8, recent moraine complex of TrioletGlacier; 9, diamicton consisting of graniteblocks within a schistose matrix; 10, graniticveneer (a), with megablocks (b); 11, deposit ofgranite blocks; 12, till comprising weatheredgranite blocks; 13, snow avalanche couloir;14, limit and number of Arp-Nouva/Greuvet-taz and Tsa-de-Jean sectors; 15, peat bog; 16,location of radiocarbon dates (topographicmap: 1:10 000 with 10m contour interval.Archivi topocartografici della Regione Auton-oma Valle d’Aosta – permit no. 52–18 August1999) (modified from Deline and Kirkbride,2009).

Figure 3. The upper Ferret Valley and geographic locations men-tioned in the text. View towards the southwest. This figure is availablein colour online at wileyonlinelibrary.com.


10BE DATING OF THE 1717 ROCK AVALANCHE DEPOSITS IN THE FERRET VALLEY 385

area was not reached by the LIA advance of the Triolet Glacier,whereas it was overridden by the main Holocene advance.Therefore, he refuted Porter and Orombelli’s (1980) interpret-ation and argued that the boulder deposit of the upper FerretValley is mainly the product of two Lateglacial advances,namely the last two minor advances of Egesen (Figs 2–4 andTable 1). Orombelli and Porter (1988) responded to Aes-chlimann’s (1983) interpretation with a supplementary study inwhich they sampled a basal peat from a peat bog near Arp-Nouva (site no. 2) for radiocarbon dating and applied a Schmidthammer to determine the degree of rock weathering on thegranite boulders in the upper valley. Three radiocarbonmeasurements in three different laboratories (Paris, Zurichand Rome) from two samples from the same basal peat in Arp-Nouva (site no. 2) gave three different uncalibrated ages:105� 70 (Gif-?), 1020� 65 (UZ-?) and 2320� 150 14C a BP (R-1890) (Figs 2–4 and Table 1). Based on these results, Orombelliand Porter (1988) concluded that the entire accumulation ofboulders in the upper Ferret Valley is a single and uniform bodyof sediment of late Holocene age.Deline and Kirkbride (2009) supplied a new interpretation of

the deposits of the upper Ferret valley. Following a detailedsummary of the previous studies, interpretations and existingdata on these deposits, they provided evidence for thedeposition age based on the new 14C ages, Schmidt hammerrebound values and boulder edge-roundness measurements. Inthis study, six 14C ages were gathered from three trenchesopened around the margin of the AD 1717 rock avalanchedeposit defined by Porter and Orombelli (1980). Uncalibratedradiocarbon ages of two peat and one wood sample from atrench close to the sampling site of Orombelli and Porter (1988)in the Arp-Nouva peat bog are 1030� 30 (LY-9382), 279� 42(LY-9943) and 896� 36 14C a BP (LY-9506) (Figs 2–4 andTable 1). A wood sample from the Greuvettaz ubac debris cone(Figs 1–3 and Table 1) on the southern side of the valley yieldedan uncalibrated radiocarbon age of 405� 30 14C a BP (LY-9942). Two wood samples from the third trench opened at themargin of the deposit front, embedded in the layers underlyingthe granite deposit, yielded uncalibrated radiocarbon ages of370� 40 14C a BP (GX-27762) and (100.10� 0.53)% of themodern (AD 1950) 14C activity (GX-27765) (Figs 2–4 andTable 1). Based on these new ages and geomorphologicalcharacteristics of the deposit, Deline and Kirkbride (2009)argued that the deposit of the AD 1717 catastrophe was lessextensive than interpreted by Porter and Orombelli (1980). Therock avalanche would have been deflected to one side ofthe valley, so that older deposits along the other side werepreserved. They also concluded that there was an earlier rockavalanche that collapsed onto the Triolet Glacier before AD1000. Glacial deposits andmoraines from the older advances ofthe glacier were partly overlain by the deposits of thesetwo avalanches.

Samples

Twelve granitic boulders in the upper Ferret Valley werecarefully chosen with respect to geomorphic setting and size.We sampled with a hammer and chisel following the strategiesdefined in previous studies (e.g. Akcar, 2006). The height of theboulders varies from 1.3 to 6.0m. A description of the samplesis given in Table 2. Samples Trio-1, Trio-2 and Trio-3 (Fig. 5)were taken from three megablocks in the southwest of theGreuvettaz ubac cone (Figs 2 and 3). These three large bouldersare situated on the left margin of the valley with a distance ofless than 50m from each other. The space between theboulders was partially filled by talus deposits. Trio-4, Trio-5 and

Trio-7 are three boulders on the crestline of the transverseridges in the Plan deGreuvettaz (Figs 2 and 3). Trio-4 and Trio-5are located on two successive ridges in the middle of the valley,whereas Trio-7 is on a ridge close to the southern valley side.Trio-6 (Fig. 5) is the largest boulder on a longitudinal ridge bythe Greuvettaz ubac debris cone, where Deline and Kirkbride(2009) obtained an uncalibrated radiocarbon age of 405� 3014C a BP (LY-9942) from the trench in the deposits of the debriscone (Figs 2–4). Trio-8 (Fig. 5) is one of the boulders in the Boisd’Arp-Nouvaz close to the peat bog in the Plan d’Arp-Nouvazwhere Orombelli and Porter (1988) and Deline and Kirkbride(2009) recorded the radiocarbon ages from peat and woodsamples (Figs 2–4 and Table 1). Trio-9 was collected from aboulder embedded in the northeastern part of a Triolet LIAmoraine close to the Doire (Fig. 6), which comprises morainesformed in early 18th- and 19th-century advances at this part ofthe valley (Aeschlimann, 1983). This moraine ridge is over-

Figure 4. (A) Plot of calibrated 14C ages obtained in previous studiesagainst AD 1717 (1Aeschlimann, 1983; 2Orombelli and Porter, 1988;3,4,5Deline and Kirkbride, 2009). Laboratory number, depth (whenavailable) and source of each sample are indicated by numbers, andthe type of organic matter is also stated; see Discussion section aboutthe discrepancy between radiocarbon dates and stratigraphy ofsamples regarding the rock avalanche deposit. (B) Plot of calibratedage ranges for wood sample GX-27765 from trench no. 5, which in theoriginal publication was given as 100.1�0.53% of modern carbon(1950). Here F14C (fraction modern carbon 1.001� 0.0053) is usedwith OxCal 4.1.7 (Bronk Ramsey, 2010; https://c14.arch.ox.ac.uk/oxcal.html); r:0.2; post-bomb atmospheric NH1 curve (Hua andBarbetti, 2004).



topped by the 1850 advance of the Triolet Glacier. BouldersTrio-1 to Trio-9 were sampled in June 2008. Boulder Trio-10(Fig. 5) is located at the margin of the deposit close to the trenchopened by Deline and Kirkbride (2009), where they obtainedan uncalibrated radiocarbon age of 370� 40 14C a BP (GX-27762) and (100.10� 0.53)% of the modern (AD 1950)14C activity (GX-27765) (Figs 2–4). Trio-11 (Fig. 6) is thelargest boulder of a granitic deposit lying on the southern side ofthe metasedimentary Tsa-de-Jean rock spur; Trio-12 (Fig. 6) issituated higher than Trio-11 on the Tsa-de-Jean spur (Figs 2and 3). Boulders Trio-11 and Trio-12 are definitely outside ofthe rock avalanche deposits in the upper Ferret Valley: Tsa-de-Jean spur, which rises 120m above the eastern part of the

Triolet moraine complex, was only covered on its northernside, by a small lobe of the AD 1717 rock avalanche (Fig. 2,D3). These boulders, resulting from rockfall on to the past FerretValley Glacier, were likely deposited by this glacier during itsretreat. Samples Trio-10, Trio-11 and Trio-12 were sampled inMay 2010.

Surface exposure dating

The samples were prepared at the Surface Exposure Laboratoryof the University of Bern. The quartz was extracted from thesamples and purified using a modified version of the techniqueintroduced by Kohl and Nishiizumi (1992; see also Ivy-Ochs,

Table 1. Radiocarbon ages from the upper Ferret Valley deposits (from Deline and Kirkbride, 2009).

Site No.(Fig. 1) Sample location Depth (m) Material Laboratory No.

Uncalibratedage (1s BP)

Calibrated age (2s)(AD, except as stated)

From To

Stratigraphic position: above the interpreted rock avalanche deposits1a Surface Peat UZ-293 885� 60 1030 12542b Surface Peat Gif-?d 105� 70 1660 19602b Surface Peat UZ-?d 1020� 65 884 11702b Surface Peat R-1890 2320� 150 792 BC 54 BC3c Trench 3 0.13 Peat LY-9506 896� 36 1038 12153c Trench 3 0.31 Wood LY-9943 279� 42 1478 19523c Trench 3 0.33 Peat LY-9382 1030� 30 898 11174c Trench 4 0.70 Wood LY-9942 405� 30 1433 1624

Stratigraphic position: below the interpreted rock avalanche deposits5c Trench 5 1.5 Wood GX-27765 (100.10� 0.53)% of

modern (1950) 14C activity1694e 1956e (17.1%)

1812e 1839e (12.4%)1841e 1854e (3.1%)1858e 1862e (0.7%)1866e 1918e (44.7%)1951e 1956e (17.3%)

5c Trench 5 2.2 Wood GX-27762 370� 40 1446 1635

Radiocarbon ages are calibrated with OxCal 4.1 (https://c14.arch.ox.ac.uk/oxcal.html; Bronk Ramsey, 2009) in combination with the INTCAL09calibration dataset (Heaton et al., 2009; Reimer et al., 2009). a Aeschlimann (1983); bOrombelli and Porter (1988); cDeline and Kirkbride (2009).dUnknown laboratory numbers. e Calibrated with OxCal 4.1.7 (https://c14.arch.ox.ac.uk/oxcal.html); r:0.2; post-bomb atmospheric NH1 curve(Hua and Barbetti, 2004).

Table 2. Description of samples from the upper Ferret Valley, Mont Blanc Massif.

Samplename

Altitude(m)

Latitude, 8N(DD.DD)WGS84

Longitude,8E (DD.DD)WGS84

Boulderheight(m)

Samplethickness

(cm)

Thicknesscorrectionfactora

Thicknesscorrection factor

(with snow cover)b

Shieldingcorrectionfactorc

Trio-1 1775 45.8663 7.0476 6.0 3.0 0.9756 0.9580 0.9197Trio-2 1772 45.8661 7.0470 3.5 3.0 0.9756 0.9580 0.9197Trio-3 1774 45.8659 7.0468 4.0 3.0 0.9756 0.9580 0.9197Trio-4 1755 45.8636 7.0414 1.3 3.0 0.9756 0.9580 0.9285Trio-5 1755 45.8641 7.0409 1.7 5.0 0.9597 0.9424 0.9501Trio-6 1766 45.8697 7.0499 1.6 4.0 0.9676 0.9501 0.9472Trio-7 1760 45.8648 7.0446 1.5 4.0 0.9676 0.9501 0.9232Trio-8 1775 45.8728 7.0523 1.4 3.0 0.9756 0.9580 0.9537Trio-8Ad 1775 45.8728 7.0523 1.4 3.0 0.9756 0.9580 0.9537Trio-9 1863 45.8808 7.0594 2.3 5.0 0.9597 0.9424 0.9505Trio-10 1746 45.8617 7.0389 2.0 5.0 0.9597 0.9424 0.9514Trio-11 1858 45.8742 7.0575 6.0 3.0 0.9756 0.9580 0.9691Trio-12 1940 45.8767 7.0597 4.0 5.0 0.9597 0.9424 0.9427

aCorrection for sample thickness was done after Gosse and Phillips (2001), with mean attenuation length of 160 g cm�2 and rock density of2.65 g cm�3. b Calculated applying a snow cover thickness of 30 cm with an average density of 0.3 g cm�3 during 4 months of the year for all exposuretime. c Calculated for topographic shielding and dip of the surface, after Dunne et al. (1999). d TRIO-8 sample reprocessed and re-measured (see Fig. 7).



1996; Akcar, 2006). Purified quartz from eight samples (Trio-1to Trio-8) were processed following the lab protocol defined inAkcar (2006). Samples Trio-10, Trio-11, Trio-12 and Trio-8A(duplicate sample of Trio-8) were processed according to theupdated lab protocol applied in this study (Table 2). Pure quartzsamples were spiked with 0.15–0.20mg 9Be and dissolvedwith concentrated HF. After evaporation and fuming of HFwith HNO3, aqua regia and HCl, the samples were passedthrough the anion column to remove Fe and precipitated ashydroxides. We separated aluminium and beryllium on ion-exchange columns in 0.4 M oxalic acid (AG 50WX8 resin,100–200mesh, hydrogen form) (Blanckenburg et al., 1996).We co-precipitated beryllium with iron (150–200mL100mL�1 9Be spike) as hydroxides at a pH value of �9; thenthe Be/Fe precipitate was mixed with the AgNO3 solution(35mL 100mL�1 9Be spike; M. Christl, P.W. Kubik, and J.Lachner, unpublished results). The suspension was dried on ahotplate at around 90–1008C. Beryllium and iron were baked,

and silver was reduced to metallic form in a furnace at 6758Cfor the accelerator mass spectrometric measurements at theETH tandem facility in Zurich (Kubik and Christl, 2010). Theweighted average 10Be/9Be full process blank ratio was(3.13� 0.36)� 10�15.

10Be exposure ages were calculated with the CRONUS-Earthonline calculator of Balco et al. (2008; http://hess.ess.washington.edu/math/) using wrapper script 2.2, main calcu-lator 2.1, constants 2.2.1 and muons 1.1. Altitude/latitudescaling of the production rate was made according to the time-dependent Lal (1991)/Stone (2000) scheme using a productionrate due to spallation (at sea level, high latitude), of 4.39� 0.37atoms g�1 SiO2.a

�1 (CRONUS calculator update from v. 2.1 tov. 2.2 published by Balco in October 2009). Rock density wastaken as 2.65 g cm�3. The shielding of the surroundingtopography (based on Dunne et al., 1999), and thickness ofthe sample (using an exponential attenuation length of160 g cm�2) are considered in the calculation of apparent

Figure 5. Sampled boulders in the upperFerret Valley. (A) Trio-1, Trio-2 and Trio-3,located to the west of the Greuvettaz ubaccone. (B) Trio-6, located to the east of theGreuvettaz ubac cone. (C) Trio-10, locatedon the boundary between the Plan d’ArpNouvaz and Bois d’Arp Nouvaz. (D) Trio-10, located at the margin of the boulderdeposit in the Bois de Greuvettaz. Thisfigure is available in colour online atwileyonlinelibrary.com.

Figure 6. (A) View of the upper Ferret Valleytowards the northeast. (B) Boulder Trio-9,embedded in the LIA moraine complex ofthe Triolet Glacier; dashed line delineatesthe limit of the LIA moraine complex. (C)Boulder Trio-11 on the slope of the Tsa-de-Jean rock spur. (D) Boulder Trio-12, on theTsa-de-Jean spur. This figure is available incolour online at wileyonlinelibrary.com.



exposure ages. An erosion rate of 3.0� 0.5mmka�1 andsnow cover of 30 cm (0.3 g cm�3 density, 4 months per year,full exposure time) is estimated, whereas the correction forvegetation is excluded since some samples are above the tree-line and the biomass in the valley was exploited until the mid20th century (Deline and Kirkbride, 2009), i.e. their effect on thecosmogenic isotope production is negligible. The cosmogenicnuclide data for the samples are given in Table 3.

Results

In Table 3, the amount of quartz dissolved and 9Be spike,10Be concentration, apparent exposure ages and erosion andsnow-corrected exposure ages are given for each sample. Thecorrections to account for thickness, dip of rock surface andshielding of the surrounding topography were included in thecalculation of apparent ages, but erosion and snow correctionwere excluded.

10Be concentrations from the glacially transported Trio-11and Trio-12 are 181 200� 7600 and 206 200� 6400 at g�1,respectively. Those for the other samples are less than 10 000at g�1, including the LIA boulder Trio-9, except for Trio-1which has a 10Be concentration of 14 167� 4352 at g�1. Theuncertainties for the low-concentration samples are of the orderof 25% for samples prepared with the old lab protocol (Akcar,2006), it is around 12% for Trio-10 and 8% for Trio-8A, whichwere prepared with the new one. The 10Be concentrations ofTrio-8 (347� 88 at g�1) and Trio-8A (324� 22 at g�1) areconsistent within 1s; accordingly, the uncertainty was reducedby almost a factor of four.Apparent exposure ages from boulders Trio-1 to Trio-8 and

Trio-10 vary from 324� 22 to 474� 121 a, except for Trio-1(854� 249 a) (Table 3). These results indicate that theseboulders belong to the deposits of the AD 1717 rock avalanche,although boulders Trio-1 and Trio-6 likely contain inheritednuclide concentrations. These results with 1s externaluncertainties are plotted against AD 1717 in Fig. 7. The LIA

boulder Trio-9 has an apparent exposure age of 240� 96 a(Table 3). Considering the fact that the LIA moraines of theTriolet Glacier likely span the 18th to 19th centuries, theexposure age of Trio-9 is younger. Despite this fact, it is stillconsistent with the time window of the LIA. Apparent, anderosion and snow (3� 0.5mm ka�1) corrected 10Be exposureages for Trio-11 and Trio-12 are 9.3� 0.4, 9.7� 0.4, 10.4� 0.3and 10.9� 0.4 ka, respectively (Table 3). These last twoexposure ages indicate that these boulders are not related tothe AD 1717 rock avalanche.

Table 3. Cosmogenic nuclide data and 10Be exposure ages.

Samplename

Quartzdissolved (g)

9Be spike(mg)

AMS ratio(�10�14)

Error in AMSratio (%)

10BeConcentration

(at g�1)

Apparentexposureage (a)

Exposure age (a)Erosion and snow corrected

(e¼3�0.5mm ka�1)

Lab protocol: Akcar (2006)Trio-1 74.5694 0.3063 5.97 29.1 14 167�4352 854�249 (266) 872�255 (272)Trio-2 101.8808 0.3061 3.45 48.6 5 747�3072 335�176 (184) 342�180 (188)Trio-3 101.1471 0.3062 4.62 43.1 7 950�3676 465�211 (221) 474�215 (225)Trio-4 100.8236 0.3063 4.10 31.1 7 015�2363 412�136 (145) 420�139 (148)Trio-5 101.5170 0.3065 3.42 29.2 5 720�1839 333�105 (112) 339�107 (114)Trio-6 102.1582 0.3065 4.81 24.3 8 226�2139 474�121 (131) 483�123 (134)Trio-7 101.2566 0.3033 3.33 22.2 5 510�1352 326�72 (86) 332�80 (87)Trio-8 74.9508 0.3070 2.80 22.7 6 211�1590 347�88 (95) 354�90 (97)

Lab protocol: this studyTrio-8Aa 103.2082 0.1496 6.23 6.7 5 731�406 324�22 (36) 330�23 (37)Trio-9 99.8678 0.3050 2.72 35.7 4 482�1809 240�96 (101) 244�97 (103)Trio-10 101.2402 0.2028 5.03 12.1 5 761�745 338�43 (58) 345�44 (54)Trio-11 100.9303 0.2035 147.75 4.2 181 200�7600 9 300�400 (900) 9 700�400 (1000)Trio-12 107.1801 0.2049 177.21 3.1 206 200�6400 10 400�300 (1000) 10 900�400 (1000)

Reported blank ratios and concentrations are referenced to 07KNSTD (Kubik and Christl, 2010). Accelerator mass spectrometry (AMS) measurementerrors are at 1s level, including statistical (counting) error and error due to normalization of standards and blanks. The error weighted average10Be/9Be full-process blank ratio is (3.13� 0.36)� 10�15. Exposure ages are calculated with the CRONUS-Earth exposure age calculator (http://hess.ess.washington.edu/math/ (v. 2.2); Balco et al., 2008 and update from v. 2.1 to v. 2.2 published by Balco inOctober 2009) and time-dependent Lal(1991)/Stone (2000) scaling model. Production rate uncertainties are reported in parentheses. A half-life of 1.39Ma for 10Be (Korschinek et al., 2010;Chmeleff et al., 2010) is used for the age calculations.a TRIO-8 sample reprocessed and remeasured (see Fig. 7).

Figure 7. Plot of the snow- and erosion-corrected exposure ages (1sexternal uncertainties) for boulders from the AD 1717 rock avalanchedeposit in the upper Ferret Valley. Note that Trio-8 and Trio-8A areduplicate samples (Table 3).



Discussion

Scientists have debated the origin of the upper Ferret Valleydeposits for a long time. This debate deals mainly with theextent of the rock avalanche deposits. Some scientists favouredthe view that the whole extent of the deposits of the upper FerretValley were of rock avalanche origin, whereas some othersfavoured a glacial origin. Of these, Deline and Kirkbride (2009)developed a complex interpretation with two rock avalanchesand Lateglacial deposits, based on geomorphology andradiocarbon ages. Our 10Be exposure ages from eight boulders(Trio-1 to Trio-8 and Trio-10) indicate that the chaotic graniticdeposit with irregular, subconcentric ridges is a deposit of theAD 1717 rock avalanche (Fig. 7). The boulder exposure agesare slightly older than the known date of AD 1717, althoughthey are consistent within the errors. This likely reflectsinherited cosmogenic nuclide concentrations. This may havebeen acquired in the pre-slide bedrock or in the glacial setting.Inheritance is highest in sample Trio-1 (872� 255 a), andpossibly in Trio-6 (483� 123 a); perhaps boulders weretransported on the passive carapace during the event (Daviesand McSaveney, 2009). For instance, the cosmogenic 10Beconcentration measured in boulder Trio-1 (14 167� 4352at g�1 SiO2) is equivalent to around 500 years of surfaceexposure. This subrounded megablock, similar in size (a-axis>15m) and roundness to the very close-by Trio-2 (342� 180 a)and Trio-3 (474� 215 a), located on top of the same ridge,was most probably deposited by the AD 1717 rock avalanche.Trio-1 (872� 255 a) could have been mobilized by theAD 1717 rock avalanche from the older moraine complex ofthe Triolet Glacier, i.e. inheritance due to reworking, or itcould have originated from the surface of the collapsed rockmass during the rock avalanche, i.e. pre-exposure before theevent. In addition, the size of Trio-1, Trio-2 and Trio-3 (Table 1)could explain their preferential deposition on the margin ofthe rock avalanche path, possibly originally embedded in thedeposition ridge. Therefore, it seems unlikely that Trio-1belongs to an older event, for which no field evidence isavailable.Our results confirm the interpretation of Porter and

Orombelli (1980) that the accumulations of granitic bouldersand irregular ridges on the upper Ferret Valley floor weredeposited by the AD 1717 rock avalanche. 10Be exposure agescorrelate well with their results from dendrochronology andlichenometry; and they are also in agreement with the existingradiocarbon dates of the wood samples (Table 1 and Fig. 4; seeDeline and Kirkbride, 2009). In contrast to the radiocarbon agesfrom wood samples in different trenches (Figs 2 and 4),exposure ages do not correlate with the existing radiocarbondates from the peat, except for the ’Gif-?’ (AD 1660–1960;Table 1 and Fig. 4; see Aeschlimann, 1983; Orombelli andPorter, 1988; Deline and Kirkbride, 2009). Orombelli andPorter (1988) related inconsistently older radiocarbon agesfrom the peat bog in the Plan d’Arp-Nouvaz to the ’hard-water’effect originating from the carbonate bedrock at the source ofthe spring which feeds the bog, on the metasedimentarybedrock (Fig. 1). Orombelli and Porter (1988) did not exclude ageological explanation for the radiocarbon age published byAeschlimann (1983). They proposed that the sampling site(sampling site no. 2 in Fig. 2) is located in a sheltered position sothat the peat bog could be preserved without any cover afterbeing overridden by the rock avalanche.Our interpretation of the existing recalibrated radiocarbon

ages (Table 1 and Fig. 4) is as follows. (i) Four wood samplesfrom trenches (Deline and Kirkbride, 2009) date the AD 1717rock avalanche, as mentioned above. The wood piece withcalibrated 14C age of AD 1478–1952 (Table 1) sandwiched

between the two peat layers from trench 3 is interpreted as thepenetrative root of a younger tree by Deline and Kirkbride(2009); however, the time span gathered from this samplecorrelates well with 330� 23 a 10Be exposure age from sampleTrio-8A. The wood sample in trench 4 yielded a calibrated14C age of AD 1433–1624 (Table 1; Deline and Kirkbride,2009), and is similar to the 10Be exposure of 483� 123 a ofboulder Trio-6, but neither date correlates with the timing of therock avalanche. At the margin of the 1717 rock avalanchedeposit in Bois de Greuvettaz, Deline and Kirkbride (2009)reached the layers below the rock avalanche deposit in trench5. The calibrated 14C age of AD 1446–1635 from a woodsample embedded in the underlying layer is older than the rockavalanche, which fits the stratigraphy. Radiocarbon analysis ofthe wood sample (GX-27765) in the underlying layer close tothe base of the rock avalanche deposit yielded a concentrationof (100.10� 0.53)% of the modern (AD 1950) 14C activity. Thecalibration results in six probability intervals between AD 1694and 1956 and the oldest interval yield a calibrated age of AD1694–1727 (17.1% probability). The age of the AD 1717 rockavalanche is well within the calibrated age range and is in goodagreement with 345� 44 a exposure from boulder Trio-10(Fig. 4). (ii) Peat samples are considerably older and notablyinconsistent. They also show age inversions; i.e. stratigraphi-cally younger samples yield older radiocarbon ages thanstratigraphically older samples (e.g. samples from trench 3;Table 1 and Fig. 2). In particular, results from the subsamples ofOrombelli and Porter (1988) show inhomogeneity of the peatsamples. Five millimetre- to centimetre-scale thick peat layers,T-1 to T-5 in trench 3 (Deline and Kirkbride, 2009) areembedded within the schistose sand and silt layers. Thedominance of the schistose clast lithology in these layersindicates that the source for these sediments is mainly theTorrent de Bellcombe catchment basin. Already depositedorganic matter in the catchment area of this high discharge riveris probably eroded, transported and redeposited in the peat bogin the Plan d’Arp-Nouvaz. If rock deposit overlain by the peatbog is part of the AD 1717 rock avalanche, the old radiocarbonages and inhomogeneity of the peat layers could be explainedby the reworked organic matter. Thus granitic hummockyaccumulations located downstream in the Plan d’Arp Nouvazand on the foot of the south valley side would also be a part ofthe AD 1717 rock avalanche. Methodological testing of thisinterpretation is, however, still missing. By contrast, correctpeat 14C ages would mean that the peat bog was overridden bythe rock avalanche, but completely preserved without beingoverlain by the rock avalanche deposits as suggested byOrombelli and Porter (1988) for the small peat bog whereAeschlimann (1983) sampled (sampling site no. 1 in Fig. 2) orrock avalanche was stopped there without destroying the peatbog. This may be supported by an electrical tomography profilethat shows a low resistivity (100–600 V m) up to 10m deep inthe peat bog, suggesting the absence of a rock debris layer(Deparis, 2003). In both cases, 10Be exposure age of Trio-8A(330� 23 a) shows that the rock avalanche deposit of Boisd’Arp Nouvaz does not pre-date AD 1000 (as proposed byDeline and Kirkbride, 2009), but is part of the AD 1717 rockavalanche.

In this study, we determined the extent of the AD 1717 rockavalanche deposit using mapping and 10Be exposure ages,which confirms the runout distance of the avalanche and thearea of the deposit on the valley floor (0.9 km2) calculated byPorter andOrombelli (1980). This contributes to the assessmentof natural hazards and risks in the Mont Blanc Massif area,characterized by many resident and visiting people andconsiderable infrastructure such as international highwaysand busy cable cars. Deline and Kirkbride (2009) calculated a



volume of 2.5–5.0� 106m3 for the deposit on the valley floor(Arp-Nouva/Greuvettaz plain), based on their interpretation ofthe extent of this deposit area from radiocarbon ages (0.5 km2).Based on the new age control, the revision of the volumedeposited in this area is 4.5–9.0� 106m3, the upper valuecorresponding to the calculation by Porter and Orombelli(1980). It should be noted that, while the transverse electricaltomography profile in the Plan and Bois de Greuvettaz areasuggests an 8m mean thickness for the rock avalanche deposit(Deline and Kirkbride, 2009), its thickness as proposed by asecond tomography profile in the Plan d’Arp-Nouvaz/Bois de laBiche area could be 5m (as assumed here) or 20m (Deparis,2003). Taking into account the deposit upon the glacier thatsupplied the morainic complex in the following two centuries,the total deposited rock volume was probably larger than10� 106m3. The total volume of the mixed rock–ice depositwould be expected to have been larger than 20� 106m3, basedon analogy to two rock avalanches that occurred in thecatchment area of the Brenva Glacier in 1920 and 1997, wherea large volume of glacier ice that was incorporated in the rockavalanche (70–75% ice; Deline, 2009). This large volume ofmixed rock–ice deposit should have dammed the Doire Riverfor a short period and formed a lake, as suggested by thedeposition of glacio-lacustrine sediments (Deline and Kirk-bride, 2009).As in the case of Ferret Valley, a notable number of

catastrophic landslides in several repeatedly glaciated moun-tainous areas in the world have been misinterpreted as ofglacial origin and such misinterpretations have had directconsequences in determining, in particular, Lateglacial glacierfluctuations and in modelling related palaeoclimate change(e.g. Hewitt, 1999, and references therein). As mentionedabove, (mis)attribution of landslide deposits to glacial origindirectly affects risk assessment of natural hazards in mountai-nous areas. However, distinguishing between the two types ofdeposits – glacial and landslide – is not always simple andrequires careful field observations and multi-analytical tech-niques, such as at least more than one dating tool. Asexemplified in the recent rock avalanches in the Brenva area(Deline, 2009), where nearly two-thirds of the avalanchedeposit were glacier ice, we may not expect a thick sedimentsequence from the AD 1717 rock avalanche. In addition, mostof the finer sediments should have been washed away by theRiver Doire, leaving an array of boulders. Therefore, theincorporation of glacier ice may explain the absence of goodoutcrops related to the rock avalanche. As noted previously inseveral studies (e.g. Porter and Orombelli, 1980; Davies, 1982;Hewitt, 1999), low arcuate ridges and furrows transverse to thedirection of movement are often the result of rock avalanches.This may explain the origin of the longitudinal and transverseridges in the Plan de Greuvettaz. As the provenance of the AD1717 rock avalanche and the Triolet Glacier are mono-lithologic (porphyritic granite), differentiation of thesedeposits in the field is impossible; even petrographicalanalysis of the boulder would not guarantee this separation.The same is also valid for the angularity of the boulders, sinceboulders deposited by the rock avalanche are basically amixture of those from collapsed bedrock and those that werealready being transported by the glacier (i.e. reworkedboulders of glacial origin). In such a difficult field context, inaddition to the contradictory and inverse radiocarbon resultsfrom peat and wood samples, our exposure dating of bouldersin the Ferret Valley brought the extent of the rock avalanchedeposits to light and ended the debate on the origin of thesedeposits. Our application of cosmogenic nuclides exposedthe importance of the combination of field evidence withchronological data.

The boulder Trio-9 embedded in the LIA moraine yielded asnow- and erosion-corrected exposure age of 244� 97 a(Table 3). This younger exposure age of Trio-9 could resultfrom: (i) an englacial transport of the boulder after AD 1717;this would mean that the outer moraine of the Triolet morainiccomplex was not built before the 18th century; or (ii) its recentexhumation from an older-than-1717 outer moraine by fluvialerosion from the Pre-de-Bard Glacier outlet river (Figs 1 and 2).Exposure ages of 9.7� 0.4 ka from Trio-11 and 10.9� 0.4 ka

from Trio-12 indicate that: (i) deposition of these boulders isrelated to the early Holocene stages of the Ferret Glacier; (ii) norock avalanche detached from the Mont Blanc Massif reachedaltitudes of 1850–1950m on the southern side of the upperFerret Valley during the main part of the Holocene, except forthe AD 1717 one with its secondary lobe (Fig. 2, D3). Trio-12(10.9� 0.4 ka) seems to be related to ice margin build-upduring the Egesen stadial (Ivy-Ochs et al., 2009b). It cannot beexcluded that Trio-12 may also be originating from a rockfalland likely glacially transported in the supraglacial level duringKartell stadial (Ivy-Ochs et al., 2009b). Similarly, we cannotexclude that Trio-11 (9.7� 0.4 ka) belongs to a small part of arock avalanche that travelled across the glacier when it wasreadvancing. In order to clarify these, a Lateglacial chronologyof the glacial deposits in the Ferret Valley needs to beconstructed.

Conclusions

In order to solve the debate on the extent of the AD 1717 rockavalanche deposit in the upper Ferret Valley in the Mont BlancMassif (Italy), we exposure dated 12 granitic boulders usingcosmogenic 10Be. Nine boulders stem from the extensivegranitic boulder deposition with irregular ridges, which iscurrently under debate; two from Egesen to early Holocenestages of the Ferret glacier; and one from a LIA moraine. Snowand erosion (0.3� 0.5 cm ka�1) 10Be exposure ages fromboulders related to the Lateglacial and from the LIAmoraine are10 900� 400 and 9700� 400 a, and 244� 97 a, respectively.These exposure ages are in good agreement with field evidence.Our results from the extensive chaotic deposit vary between330� 23 and 483� 123 a except for one boulder, which weinterpret to show clear evidence of inheritance with anexposure age of 872� 255 a. Consequently, these exposureages reveal that these boulders belong to the AD 1717 rockavalanche, which confirm the main part of Porter andOrombelli’s (1980) interpretations. Our results are consistentwith the results from dendrochronology, lichenometry andradiocarbon analysis of the majority of wood samples, whereasthe radiocarbon ages from peat samples are often inconsistent,older and stratigraphically inverted. Results from this studydirectly contribute to improving our skill in distinguishingbetween rock avalanche deposits and moraine sets in valleys ofglaciated mountains – two assemblages of landforms that oftenlook alike – and to the assessment of natural risks in the MontBlanc Massif area.

Acknowledgements. Wewould like to thankDr Aristide Franchino forhis help in the field. We would also like to thank the Zurich acceleratormass spectrometry facility operated by the Swiss Federal Institute ofTechnology, Zurich, Switzerland. We are grateful to Christoph Pragerand Derek Fabel for their constructive comments and suggestions. Thisstudy was funded by the Swiss National Science Foundation (ProjectNo. 200001-111878), the Surface Exposure Dating Laboratory at theUniversity of Bern (Switzerland) and the EDYTEM Laboratory at CNRSand Universite de Savoie (France).

Abbreviations.AMS, accelerator mass spectrometry; LIA, Little IceAge.



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