1. Introduction and Aims
A rock shelter is an opening of modest size and extension that is generally formed by weathering and erosion processes (e.g., induced by water run-off) of a rock that is less resistant than the surrounding rocks. Shelters are horizontally shallow, unlike caves generated by karstification, which are much deeper. Rock shelters can have an archaeological importance since they were often used by humans as refuge from the weather [
1,
2,
3,
4]. Prehistoric people frequented such shelters as a place to live, leaving behind tools and other artefacts that now assume a high archaeological significance [
5,
6,
7,
8].
La Calvera rock shelter is located in Picos de Europa National Park, 1180 m above sea level, close to Camaleño (Cantabria;
Figure 1), in the area of the Peña de Oviedo megalithic complex. From the geologic point of view, the Picos de Europa is an imposing mountain range within the Cordillera Cantabrica (
Figure 1), with a geological history linked to the Variscan and Alpine orogeneses. The Peña de Oviedo area, where the Calvera rock shelter site is located, is mainly characterized by Palaeozoic lithologies, such as limestone, quartzite, sandstone and shales, and subordinate Quaternary deposits (
Figure 1).
During the excavations in the La Calvera rock shelter, the archaeologists documented the presence of hearths and charcoal, historical pottery in the upper levels, and the presence of a lithic assemblage composed of different siliceous rocks (chert, rock crystal, quartzite) dating back to >8000 BP and linked to the first Holocene occupations of the Cantabrian Mountains [
9]. More than 500 lithic fragments were discovered with some retouched pieces (mainly cores, scrapers and small blades).
Chert is a very fine-grained siliceous rock composed almost exclusively of microcrystalline quartz and chalcedony. It is normally found in the form of layers or as nodules and lenses within carbonate rocks or alternated with clay-rich shale levels. Stratified cherts are essentially of biogenic origin and owe their formation to the accumulation of the siliceous shell or skeleton organisms (e.g., radiolarians, diatoms, silico-flagellates and sponges) [
10], even if recent studies suggest a significant role of diagenesis in silica redistribution in bedded cherts [
11]. Nodular cherts generally have a diagenetic origin and would form as a result of silicification processes inside the host rocks [
10]. The contribution of hydrothermal fluids interacting with seawater in the depositional environment has been reported locally by several authors [
12,
13]. Given their high hardness, chemical and physical resistance, conchoidal fracturing and its use as fire starter, this material had an important use in the ancient lithic industry, especially in prehistoric times. On the basis of the different chipping techniques of cherts, subperiods and, thus, the working material culture of civilizations can be identified.
Contrary to chert, the term quartzite is more ambiguous since historically it has been used with different meanings to describe a wide variety of rocks of both metamorphic and sedimentary origin [
14]. Moreover, distinguishing among different types of quartzite is often not easy due to the similar mineralogy and textures that different quartzites can share. As recently pointed out by Prieto et al. [
15] the full characterization of this material should be achieved through a detailed petrographic analysis integrated with digital imaging to recognize and quantify the textural features of quartz grains; however, this kind of approach is necessarily destructive and, thus, cannot be applied to highly valuable archaeological finds. In this paper, the term quartzite is used to refer to silicified quartzarenites (also known as orthoquartzite) almost completely composed of quartz. The discrimination between quartzite and cherts has been performed through macroscopic and microscopic observation (under reflected light), following the criteria proposed by [
14] and concerning the fracturing, the luster, the grain size of the samples.
The provenance and lithic supply in Cultural Heritage studies [
16,
17] are fundamental to define the dynamics involving human populations and their surrounding environment, mobility, and possible relations among different settlements; however, matching prehistoric artefacts and raw materials from potential quarries just on the basis of naked-eye examination is often controversial. Only a multidisciplinary approach, based on the use of different analytical techniques, allows for the full characterization of the archaeological geomaterials and their natural counterparts [
17,
18,
19,
20,
21,
22], leading to more objective and robust results [
23,
24,
25].
The study of the lithic assemblage from La Calvera rock shelter, has been performed through a two-stage, multi-analytical approach. Firstly, a large set of samples was analysed using non-destructive to micro-destructive techniques, based on smartphone imaging and molecular spectroscopy techniques [
26], to understand if it is possible to distinguish siliceous materials coming from different source areas and finally to identify the supply area of archaeological finds. Secondly, according to the first analyses, a subset of samples was selected to carry out a petrographic, mineralogical and petrophysical characterization (this study). This second stage is aimed at further characterizing the selected samples and to test the hypothesis postulated after the former analyses. To achieve this goal, the microstructural and textural features of both the amorphous and crystalline phases within the samples were observed through a scanning electron microscope (SEM-EDS) and analysed by powder X-ray diffraction (XRD) to detect minor phases and the degree of crystallinity. Finally, the physical and mechanical characteristics were determined to understand the technical properties of the artefacts in relation to their possible use as tools. A summary of the chemical composition of the different lithotypes is also reported.
2. Geological Setting and Natural Chert Occurrences
The Cordillera Cantabrica extends for about 480 km along the northern Spain coast, from Galicia (to the west) to Basque Country (to the east). The Picos de Europa area, where the La Calvera rock shelter is located (close to the Camaleño area,
Figure 1), is in the central part of the Cordillera. The current relief of the Cantabrian zone is the result of the superposition of the Variscan and Alpine orogeneses during the Palaeozoic and Cainozoic Eras, respectively [
27,
28]. The orogenic cycles led to the stacking of different tectonic units that mostly share the same sequence of lithostratigraphic formations, even if a local variability is observed. Indeed, from one unit to another, the same formation could have, or could even lack, different features and different thickness. Additionally, the name of the same formation can change depending on the unit or on the locality in which it occurs [
29].
The area of Peña de Oviedo, where the La Calvera shelter rests, is located on the south-eastern side of the Picos de Europa park, close to the Camaleño village (
Figure 1). In this area, only the Picos de Europa Unit (northwest) and the Pisuerga-Carrión Unit (south-east) were cropped out, separated by a regional thrust. The former is characterized by a predominance of upper Carboniferous limestone [
30] belonging to the formations of Picos de Europa and Barcaliente (previously known as Caliza de montaña). Small slices of limestones belonging to the Las Portillas and Alba (or Genicera) Fm., also occur in the southern side of the Barcaliente Fm. The Pisuerga-Carrión Unit consists of several formations (Barcena, Viorna, Narova, Remoña and Campollo Fms.), mainly siliciclastic (slates, shales, sandstone and conglomerates) with limestone intercalation, of the Carboniferous age [
31]. A sequence of allochthonous Devonian to Carboniferous rocks covers large areas of the Pisuerga-Carrión Unit; this sequence, referred to as Palentine Nappes or Palentian domain [
32] consists of alternating siliciclastic and calcareous deposits grouped in different formations. From the bottom to the top, they are the Gustalapiedra Cardaño Fm., Murcia Fm, Vidrieros Fm., Vegamian Fm., Alba Fm., and finally the Carboniferous sequence of the Potes group.
Quaternary covers consist of moraines, glacial/fluvio-glacial deposits, cemented landslides and boulder flows from the Pleistocene, and torrential cones, slope deposits and alluvium from the Holocene [
33,
34]. It was during this period, dominated by important glacial systems of the Cantabrian Mountains and interglacial stages [
35,
36,
37], that the Picos de Europa area assumed its current physiognomy and the typical rock shelters developed, mainly in the calcareous lithologies.
Several authors investigated the outcropping formations in the Cantabrian region to identify the potential sources of the lithic artefacts found in prehistoric human settlements [
29,
38,
39]. In particular, Herrero-Alonso et al. [
29] compiled an updated and complete inventory of the chert-bearing formations in a wide area of the Cantabrian region that includes the study area of this paper. According to these authors, in the surroundings of Peña de Oviedo, cherts, radiolarites and/or quartzite of knappable quality can be found in the Las Portillas, Barcaliente and Picos de Europa formations (all belonging to the Picos de Europa unit) and in the Vegamián formation (Palentine Nappe). The Alba (or Genicera) formation, also hosting knappable cherts, is found in both units forming decametre-thick slices. Quartzite occurrences in the surroundings of La Calvera have been reported in the Murcia Fm. [
40] and in the Vidrieros Fm. [
41], associated with chert nodules. None of the cited quartzites have a metamorphic origin, rather, they are the result of sedimentary/diagenetic processes. Chert-bearing formations are documented also in some areas of the Pisuerga-Carrión Unit, but they do not occur in the study area. The features of these cherts and the comparison with the archaeological finds of La Calvera will be outlined in the
Section 5.
3. Materials and Methods
Based on the spectroscopic analyses and colour features on a large sample set, ten representative samples were selected and analysed in this work (
Table 1,
Figure 2). One cobble of macrocrystal quartz or rock crystal (AR10), three samples of local grey chert (flakes: AR12.1 and AR18.2, chunk: RM03.4), one sample of ochre chert (AR21), two samples of quartzite (AR29 and AR34) and one sample of yellowish chert (AR37). Finally, for comparison with macroscopically similar chert, two samples of Domeño chert collected from the geological outcrop of Andilla (Valencia, Spain) were also analysed.
Each sample was cleaned with deionized water and a brush in order to remove superficial contaminations and incrustations prior to the analyses.
A small chip of each sample was powdered to carry out XRD analyses, which were performed at the Department of Chemical and Geological Sciences (University of Cagliari). XRD patterns were acquired by a PANalytical X’Pert Pro diffractometer (Malvern PANalytical, Almelo, The Netherlands) that works with theta-theta geometry using Ni-filtered Cu Kα1 radiation (λ = 1.540598 Å) and equipped with a X’Celerator detector. Operative conditions were acquisition range 5–70°, step size 0.008°, 0.19 s per step, voltage 40 kV and current 40 mA. Data were processed by X’Pert HighScore Plus (TM) 2.1.2 software using the PDF2 database (released in 2010 by ICDD, Newtown Square, PA, USA).
SEM analyses were performed on ten samples using a Quanta Fei 200 equipped with a ThermoFischer Ultradry EDS detector, at the CeSAR laboratories (University of Cagliari, Italy). Raw samples were put into the sample chamber without conductive coating to preserve them for further analyses, thus, low-vacuum conditions (0.3 to 0.5 torr) were used to dissipate electrons from incident beam. Variable spot sizes of 4–5 (in arbitrary units given by the Quanta Fei equipment) and an accelerating voltage of 15–25 kV were adopted during the analytical sessions.
Petrophysical and mechanical tests on the samples were performed on ten specimens of the most significant samples. Physical tests were carried out according to Buosi et al. [
42] and Columbu et al. [
43,
44] (see the
Supplementary SM1).
Multielement analysis was performed on a larger sample set, including ten chunks of grey chert used as raw materials, twenty-four fragments of the same grey chert, six cherts of different types, fifteen fragments of quartzite, six rock crystal fragments and eleven fragments of Domeño chert. Elemental concentrations of Al, K, Ca, Fe, Ti and Zr were detected using a S1 Titan portable energy dispersive X-ray fluorescence spectrometer (pXRF) equipped with a Rh X-ray tube (50 kV) and X-Flash® silicon drift detector (Bruker, Billerica, MA, USA). Internal calibration Geochem-trace was used. Each sample was analysed between two and up to five spots, and the results were then averaged.
5. Discussion
The investigations performed on the archaeological and geological samples yielded significant results regarding the sourcing, use and compositional characteristics of siliceous findings and raw materials coming from the area surrounding the La Calvera rock shelter archaeological site.
First, the petrographic and mineralogical investigations have allowed for the classification of the studied archaeological samples, mainly as cherts and subordinately as quartzites and “rock-crystals”. The mineralogical analyses by XRD on the lithic finds have revealed both higher crystallinity and grain size in the quartzite and crystal rock samples in comparison with cherts. The different crystallinity and grain size have also been confirmed by the petrophysical analysis. In detail: the real density to which it is positively correlated, while the porosity of rocks shows a negative correlation, confirms the presence of intraparticle planar pores among the particles (grains). The physical tests highlight the presence of two main different behaviours of the samples: the first population consisting of the archaeological cherts, quartzites and chunks of raw materials coming from the Calvera rock shelter, and a second subordinate sample population consisting of Domeño cherts from Andilla and the unclassified grey-ochre chert. Mechanical strength is high and comparable in the different samples analysed, due to the low overall porosity of the materials, which does not exceed 5%; however, the lower grain size and crystallinity of the chert microstructure, characterised by a more “brittle” physical and mechanical behaviour with pseudo-conchoidal microfracturing, probably facilitates the processing and production of sharper edges than quartzites and crystal rocks. The lithic assemblage of La Calvera shows that chert artefacts are commonly smaller than quartzite ones, likely due to different uses, which, in turn, are influenced by different petrophysical properties. The higher flakeability of cherts resulted in its suitability for arrowheads and cutting tools, whereas the coarser, less porous (<3%), and denser (>2.6 g/cm3) quartzites produced thicker and heavier tools, such as pestles or scrapers.
The measurement of the physical properties performed in this study was also aimed at testing the effectiveness of this approach in distinguishing among the different materials and, in future studies, using this information to identify potential sources. The data obtained seem to be inconclusive, but it must be considered that they are just preliminary results and that the method needs a wider sample set to be tested and statistically validated. From the available data, it can be observed that the foreign samples of Domeño, introduced to test the method, show a distinct behaviour in all diagrams of
Figure 7 and can be grouped together with ochre cherts. Yellowish cherts have physical features between those of the Domeño and La Calvera materials. Considering that the analysed samples display detectable differences in their petrophysical properties, even if similar from a mineralogical point of view, we believe this approach is worth being tested further.
SEM-EDS allowed for the definition of the microstructural and textural characteristics of samples, highlighting specific discriminant compositional information on single samples by the identification of minor or accessory phases, e.g., the relatively high content of calcite, distinctive of Andilla Domeño cherts and also confirmed by XRD results. However, a clear signature of each specific source (or material) cannot be found by SEM-EDS and/or XRD analyses since it would require detailed partially destructive investigation on every lithic fragment.
Based on the above-described analyses, some provenance hypothesis can be made.
The excavation campaigns at La Calvera rock shelter discovered abundant chunks and cores of grey chert (samples AN01.2, AN05.1, A12.1, AR18.2) from different archaeological levels, making this lithic material the most representative of the site. Considering these abundant findings and the proximity of La Calvera to chert-bearing rocks [
29,
41], a local supply of this grey chert can be reasonably hypothesized.
Grey and black cherts are also commonly found within the Cantabrian Range but only the Alba Fm. (Lower Carboniferous), Barcaliente Fm. (previously known as Caliza de Montaña, Mountain Limestone, Upper Carboniferous) and Picos de Europa Fm. (Upper Carboniferous) are locally present [
29]. In addition, a report of greyish chert associated with dark quartzites is described by Castillo-Diez [
41] within the Vidrieros Fm., in close proximity to the shelter of La Calvera, and in contact with the quartzites of the Murcia Fm. Although it is difficult to distinguish between the aforementioned cherts based on of the obtained results, the presence of some mineralogical peculiarities detected by XRD and SEM-EDS (i.e., presence of phyllosilicates, massive texture, absence of fossils, negligible amounts of Ca and Mg), would lean toward the Barcaliente Fm. cherts [
29] or to the Vidrieros/Murcia Fms. The latter can be identified just by its proximity and by the macroscopic description since no analytical data can be found in the literature. On the other hand, we cannot exclude that further outcrops of the same chert, nor that cherts and quartzite boulders of secondary origin (i.e., from colluvial or alluvial deposits), were exploited [
40].
As regards the yellow chert (AR37), petrographic and mineralogical data corroborate the first identification with a Piloña chert outcropping, based on chemical and macroscopic features, in eastern Asturias about 50 km NW of the studied site [
38,
45]. This chert circulated in the area and is present in other Mesolithic sites of the area [
46]. The performed analyses pointed out the difference between the grey-ochre chert artefact (AR21) and local chert, suggesting a different and possibly non-local provenance. Concerning quartzite samples, little information is available in the literature ([
40] and references therein), but a local source is the most plausible assumption since dark quartzite occurs in the Vidrieros Fm. [
41] and in the Murcia Fm [
40]. Rock crystal, as well, is possibly local [
46], but a precise identification of the source area cannot be established.
6. Conclusions
The multidisciplinary research gave interesting results for the characterisation of siliceous rock artefacts and the raw materials provenance study, confirming the support of the non-destructive SEM analyses.
The petrographic and mineralogical characterisation carried out on a subset of samples previously analysed by colour analysis and other spectroscopic techniques allowed for the definition of the provenance of the different siliceous rocks, especially for cherts. According to the analytical results, most of the archaeological chert samples have chemical characteristics compatible with natural ones outcropping in the same area, confirming the close supply, which could have possibly favoured the occupation of the site. As regards the origin of the quartzite from the territory, the previous studies do not provide enough information, and samples from geological outcrops should be added to obtain more robust data. Rock crystal samples probably have a local raw material supply, because quartz crystals are commonly found in Mesolithic contexts in the southern areas of the Picos de Europa Fm. and in eastern Asturias.
The study of the physical and mechanical properties has been proved to be a new and very fruitful approach in the characterization of cherts because it can provide useful information on the different mechanical behaviours of siliceous samples, which certainly conditioned the workability and, thus, the uses, technical functions and production of tools of antiquity. The lower crystallinity of cherts affects its flakeability and the typical conchoidal fracturing, resulting in sharp tools suitable for arrowheads and small cutting tools. Quartzite, which is coarser, less porous and denser, was used to produce larger tools such as beating masses and scrapers.