Luminescence Dating of Rock Surface. the Case of Monoliths from the Megalithic Sanctuary of Ossimo-Pat (Valle Camonica, Italy)
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applied sciences Article Luminescence Dating of Rock Surface. The Case of Monoliths from the Megalithic Sanctuary of Ossimo-Pat (Valle Camonica, Italy) Anna Galli 1,2,* , Laura Panzeri 2,* , Paolo Rondini 3 , Raffaella Poggiani Keller 4 and Marco Martini 2 1 Istituto CNR-IBFM, via F.lli Cervi, 93, 20090 Segrate, Italy 2 Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy; [email protected] 3 Dipartimento di Studi Umanistici, Università degli Studi di Pavia, Corso Strada Nuova 65, 27100 Pavia, Italy; [email protected] 4 Già Soprintendente per i Beni Archeologici Della Lombardia- Ministero dei Beni e Delle Attività Culturali e del Turismo, 20100 Milano, Italy; [email protected] * Correspondence: [email protected] (A.G.); [email protected] (L.P.) Received: 30 September 2020; Accepted: 20 October 2020; Published: 22 October 2020 Featured Application: Dates obtained by optically stimulated luminescence (OSL) of rock artefacts and the underneath soils were shown to be in very good agreement with the archeological evidence. Highly satisfying results contribute to the understanding of a megalithic sanctuary. Abstract: Ossimo-Pat megalithic sanctuary (Valle Camonica, BS, Italy) is one of the most relevant archaeological findings of the southern alpine region, for the variety of its structures and the quality of its engraved monoliths. Its unique state of preservation gives the opportunity to apply the luminescence dating of the rock surface method. Here, we investigate the use of optically stimulated luminescence (OSL) for dating five cobbles from the site and compare cobble-surface derived ages to quartz OSL ages from sediments and to archaeological evidences. The obtained ages confirm the archaeological studies and open the way to a new hypothesis. Keywords: optically stimulated luminescence; surface dating; megalithic sanctuary 1. Introduction There are many examples of rock surfaces, rock art and stone structures of unknown age. In particular, in prehistoric archaeology, long-lasting building traditions can sometimes be difficult to date, if the context does not offer useful chronological tools, such as artifacts or organic remains associated to the structures. Moreover, this particular issue is not a marginal one, if we consider that the study of megaliths, drystone walls and other structures, such as cairns or mounds, is often crucial to our understanding of the society and traditions of very ancient cultures, devoid of other forms of communication. In this scenario, the Ossimo-Pat (Valle Camonica, BS, Italy, see Figure1) megalithic sanctuary is an exemplary case. Appl. Sci. 2020, 10, 7403; doi:10.3390/app10217403 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7403 2 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 9 FigureFigure 1. 1. LocationLocation of the Ossimo-PatOssimo-Pat site site in in Middle Middle Valle Valle Camonica Camonica (GIS (GIS elaboration elaboration on LIDAR on LIDAR DTM base). DTM base). Its construction started in the first half of the fourth millennium BCE and was structured and used during the Copper Age (late 4th–3rd millennium BCE), with ceremonial structures and engraved monolithsIts construction articulated started in ain north-south the first half alignment. of the fourth The southernmostmillennium BCE structure and iswas a sequencestructured of and usedceremonial during the mounds, Copper built Age upon (late a concentric 4th–3rd seriesmillennium of stone BCE), circles with enclosing ceremo an innernial structures rectangular and engravedspace, monoliths which usually articulated contained in a objects, north-south such as alig flintnment. weapons The and southernmost tools or ornaments. structure The is centrala sequence of ceremonialpart of the mounds, sanctuary built was upon an alignment a concentric of monoliths series of of stone various circles lithologies, enclosing mainly an inner sandstones rectangular of space,diverse which granulometry, usually contained carved withobjects, figures such and as symbols flint weapons and probably and tools depicting or ornaments. mythical ancestors The central part orof gods.the sanctuary The alignment was ledan toalignment a monumental of monoliths tomb and of then various to a series lithologies, of at least mainly five votive sandstones stone of diversecircles, granulometry, which contained, carved similarly with tofigures the southern and symbols mounds, and a number probably of flint, depicting stone, metal mythical and ceramic ancestors or gods.artefacts, The probablyalignment the led result to a of monumental unknown cult tomb activity and [1,2 then]. The to site a series was frequented of at least until five its votive sudden, stone partial, dismantling and further abandonment, at the very beginning of Bronze Age, at the end of circles, which contained, similarly to the southern mounds, a number of flint, stone, metal and the 3rd millennium BCE. Later frequentation during the final Bronze Age and the whole Iron Age ceramic artefacts, probably the result of unknown cult activity [1,2]. The site was frequented until its is attested by a series of fireplaces that were lit in front and at the backside of the monoliths still sudden,vertically partial, standing. dismantling Ossimo-Pat and further is one of abandonment, the most relevant at the archaeological very beginning findings of Bronze of the southern Age, at the end ofalpine the 3rd region, millennium for the variety BCE. ofLater its structures frequentation and the during quality the of final its engraved Bronze monoliths,Age and the as wellwhole as Iron Age foris attested its unique by statea series of preservation, of fireplaces which that were gives lit the in opportunity front and at for the a documentationbackside of the of monoliths the exact still verticallymoment standing. of its disuse. Ossimo-Pat As part is of one the of UNESCO the most World relevant Heritage archaeological Site n.94 “Rock findings Drawings of the in Vallesouthern alpineCamonica,” region, for the the sanctuary variety occupiesof its structures an area ofand more the than quality 4000 of square its engraved meters, and monoliths, it is located as well on the as for its uniqueright hydrographic state of preservation, flank of the medium which Valle gives Camonica, the opportunity at an altitude for of 810a documentation m asl. The site is arrangedof the exact momenton the of edgeits disuse. of a terrace As part overlooking of the UNESCO a steep drop Wo intorld theHeritage Valle dell’Inferno Site n.94 “Rock (the strikingly Drawings named in Valle Camonica,”“Valley ofthe Hell”), sanctuary placed o onccupies a wide an elevated area of plateaumore than comprised 4000 squa by twore meters, dominant and peaks: it is located the Pizzo on the Camino (2491 m asl) to the southwest and the Concarena (2549 m asl) to the north-east. The rocky right hydrographic flank of the medium Valle Camonica, at an altitude of 810 m asl. The site is substrate of the area is mostly made of Triassic (from the Scitic to the Carnic) limestones, while the arranged on the edge of a terrace overlooking a steep drop into the Valle dell’Inferno (the strikingly surfacing lithology is often covered by morainic deposits, left by the glaciers, which can be encountered namedup “Valley at an altitude of Hell”), of 1650 placed m asl. on (source: a wide F.elevated Franzoni, plateau Piano dicomprised Governo delby two Territorio, dominant Comune peaks: di the PizzoOssimo, Camino 2012 (2491 (http: m// www.comune.ossimo.bs.itasl) to the southwest and/Allegati the Concarena/all_50012_GEO_Relazione%20Geologica.pdf (2549 m asl) to the north-east.)). The rockyThe substrate monoliths of the chosen area tois bemostly engraved made are of veryTriassic likely (from coming the Scitic come to from the theseCarnic) deposits, limestones, but the while the surfacinggeological surveylithology and is study often is stillcovered ongoing. by Excavationsmorainic deposits, and analyses left are by being the carriedglaciers, on which since 1994 can be encounteredand are still up ongoing, at an altitude under theof direction1650 m asl. of Ra (source:ffaella Poggiani F. Franzoni, Keller Piano (Figure di2). Governo del Territorio, Comune di Ossimo, 2012 (http://www.comune.ossimo.bs.it/Allegati/all_50012_GEO_Relazione%20Geologica.pdf)). The monoliths chosen to be engraved are very likely coming come from these deposits, but the geological survey and study is still ongoing. Excavations and analyses are being carried on since 1994 and are still ongoing, under the direction of Raffaella Poggiani Keller (Figure 2). Appl. Sci. 2020, 10, 7403 3 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9 Figure 2. Ortophotograph of the Ossimo-Pat site. There are twotwo potentialpotential methodsmethods forfor datingdating thethe surfacessurfaces ofof suchsuch artefacts:artefacts: the measure of cosmogenic radionuclidesradionuclides concentrations concentrations [3] and[3] theand applicationthe application of luminescence of luminescence dating (in dating particular (in Opticallyparticular StimulatedOptically Stimulated Luminescence, Luminescence, OSL) [4,5]. OSL) The [4,5]. first approach The first approach depends ondepends cosmogenic on cosmogenic nuclides 10 26 36 (e.g.,nuclidesBe, (e.g.,Al, 10Be,Cl) 26 build-upAl, 36Cl) build-up with time with in minerals time in exposedminerals to exposed cosmic rays.to cosmic Therefore, rays. measuringTherefore, theirmeasuring concentrations their concentrations allows determination allows determination of how long of rocks how havelong beenrocks exposed have been at orexposed near the at surfaceor near ofthe the surface Earth. of In the this Earth. paper, In wethis focus paper, on we the focus alternative on the approach alternative based approach on OSL. based This on method OSL. This was originallymethod was developed originally to developed date the depositionto date the deposition of sediments of sediments [6] and it depends[6] and it ondepends the ability on the of ability some minerals,of some minerals, such as feldspar such as and feldspar quartz, toand absorb quartz, and storeto absorb energy and from store environmental energy from ionizing environmental radiation.