Dombate and its archaeological context

The adoption of the Neolithic way of life in the northwest of the Iberian peninsula has traditionally been considered to be relatively recent when compared to other Iberian regions (for a recent review, see Prieto Martínez et al. Reference Prieto Martínez, Mañana Borrazás, Costa Casais, Criado Boado, López Sáez, Carrión Marco, Martínez Cortizas, Rojo Guerra, Garrido Pena and García-Martínez de Lagrán2012). The data from areas such as the Morrazo Peninsula (Pontevedra) may nonetheless indicate the existence of an initial Neolithic regional horizon—with the presence of impressed pottery—that precedes the development of the monumental architectures in the area. Sites such as A Cunchosa (Cangas, Pontevedra: Suárez Otero Reference Suárez Otero and Rodríguez Casal1997), O Regueiriño (Moaña, Pontevedra: Prieto Martínez Reference Prieto Martínez2010) or Monte dos Remedios (Moaña, Pontevedra, with a pit dated to 5780±40 bp, 4722–4534 cal. bc,Footnote ¹ Ua-32670: Bonilla Rodríguez et al. Reference Bonilla Rodríguez, César Vila and Fábregas Valcarce2006) seem to correspond to an early moment of the neolithization process.

Galician and northern Portugal megalithic monuments have been the object of scientific study since the last third of the nineteenth century and major advances were made in the characterization of the phenomenon during the 1980s and 1990s. Mounds are found throughout the region, with a preference for locations in elevated land. Location in low topographic areas is less common (but see e.g. López-Romero et al. Reference López-Romero, Güimil-Fariña, Mañana-Borrazás, Otero Vilariño, Prieto Martínez, Rey García and Vilaseco Vázquez2015, 355–7).

Regarding chronological issues, however, accurate analysis of the moments of construction, use and abandonment of the monuments is largely biased in the region by the poor preservation of organic materials suitable for radiocarbon dating (e.g. bone). Most available radiocarbon dates for the Neolithic and the megalithic phenomenon thus come from charcoal remains or from sediments found in different archaeological contexts. Owing to the exceptional preservation of painted decoration in some of the monuments and to advances in AMS radiocarbon dating, a very significant milestone was achieved in the late 1990s and early 2000s with the direct dating of the decorated orthostats in monuments such as Coto dos Mouros (Rodeiro, Pontevedra, CAMS-83631: 5540±70 bp, 4523–4259 cal. bc) or Pedra Cuberta (Vimianzo, A Coruña, CAMS-77923: 5010±60 bp, 3953–3666 cal. bc). These dates confirmed the hypothesis that the paintings were part of the symbolic programme of the monuments from the moment of their construction and that they were not a much later addition (Steelman et al. Reference Steelman, Carrera Ramírez, Fábregas Valcárce, Guilderson and Rowe2005).

Considering all this, the oldest dates available for tumular contexts in Galicia and northern Portugal place the emergence of the phenomenon in the second half of the fifth millennium bc. This is the case for instance of Forno dos Mouros 5 (Ortigueira, A Coruña, UA-20009: 5635±50 bp, 4580–4353 cal. bc), Coto dos Mouros (Rodeiro, Pontevedra, CAMS-83631: 5540±70 bp, 4523–4259 cal. bc) or Areita (Paredes da Beira, Vila Real, GrA-18518: 5170±60 bp, 4226–3797 cal. bc: Alonso Mathías & Bello Diéguez Reference Alonso Mathías, Bello Diéguez and Rodríguez Casal1997). From these initial moments it seems that different types of architectures were built and coexisted; the architectural solutions for the funerary space were thus varied, with orthostatic and non-orthostatic chambers, with pits or with the presence of wooden implements. During the first half of the fourth millennium this architectural diversity continues, with polygonal chambers without passage (e.g. Cha de Parada, Porto; Portela do Pau, Viana do Castelo; Carapito 1, Guarda; all in northern Portugal), mounds without inner orthostatic structures (e.g. Ponte da Pedra, A Coruña; Cha de Santinhos 2, Porto: see e.g. Jorge Reference Jorge1987), large earthen mounds (e.g. Madorra da Granxa, Lugo), small tumuli (e.g. Monte de Dorna, A Coruña) and passage graves. It is during the second half of the fourth millennium that passage graves became probably the most common megalithic architecture in the region; among them, the passage grave of Dombate stands out from the rest for its characteristics and biography.

First described by M. Murguía (Reference Murguía1865), Dombate soon drew the attention of local historians and archaeologists. It eventually became internationally known during the 1930s, when some of the most renowned European archaeologists of the age (Breuil Reference Breuil1933; Leisner Reference Leisner1938) discussed different aspects of its architecture and symbolism, contributing to the scientific renewal that was slowly taking place in Galicia and in the north of Portugal (López-Romero Reference López-Romero2013, 83).

The first scientific excavations were undertaken in the late 1980s (Bello Diéguez Reference Bello Diéguez1992/1993). It was then discovered that to an original small megalithic chamber (c. 2.4×1.9 m) covered by a relatively small mound, a second, larger, seven-stone chamber (3.4×2 m) provided with a passage was added nearby during the first half of the fourth millennium cal. bc (Alonso Mathías & Bello Diéguez Reference Alonso Mathías, Bello Diéguez and Rodríguez Casal1997). A large mound (c. 24 m in diameter) was built and partially covered with a crust of medium-sized stones (coraza). According to Bello Diéguez (Reference Bello Diéguez1992/1993) and Lestón Gómez (Reference Lestón Gómez, Cebrián del Moral and Yáñez Rodríguez2011), the whole mound would be probably covered by this coraza; however, it is often the case in the region that this arrangement was more prominent in the area surrounding the entrance (Fig. 2; see e.g. the case of the excavated mound in Forno dos Mouros in Toques, A Coruña: Aboal & Porto Reference Aboal and Porto2012). The later mound in Dombate covered not only the recent monument, but also the earlier one, making it invisible. Later research has confirmed that this phenomenon of integration was a quite recurrent trend of the funerary monument-building tradition in this (e.g. Mañana-Borrazás Reference Mañana-Borrazás2005) and other European regions, one that also concerned other forms of megalithic monuments—i.e. standing stones and stelæ—and that was repeated over time, as indicated by the many punctuated stages of construction, reuse and re-signification (Bradley & Williams Reference Bradley and Williams1998).

Figure 2. Plan of the dolmen of Dombate. The megalithic structures of the old and new chambers can be appreciated. (Bottom) general plan of the structures found during the last interventions with special attention to the ditches and hearths. (Modified by Anxo Rodríguez Paz after Bello Diéguez Reference Bello Diéguez1992/1993, 141, fig. 1.)

All orthostats in the chamber and corridor of the passage grave are decorated with paintings—black and red linear motifs and dots on a white background—and engravings (these appear only in the chamber). These decorations seem to be contemporary to the date of construction of the monument (first half of the fourth millennium bc), as the preparation layers for the paintings and some ¹⁴C contexts suggest (Carrera Ramírez Reference Carrera Ramírez, Cebrian del Moral and Yáñez Rodríguez2011, 243). There is evidence that the stone surface was regularized with a white mortar made of kaolin and water with the probable addition of an organic adhesive. The motifs painted over this preparation layer are dominantly red lines—organized in geometric patterns—delimited by alternating red and black dotted lines. The paintings seem to be organized into two levels, vertically separated by a horizontal red band. The upper register has almost completely disappeared from the chamber. In the corridor, the painting of this upper level consists of zigzags made in black. The lower level is better preserved throughout the monument, and the motifs depicted there seem much more complex. Big red bands delimit irregular spaces, which are smaller and more varied in the chamber (Fig. 3, bottom and right).

Figure 3. The dolmen of Dombate. (Top, left) The mound surrounds the megalithic structure. The stone cover is more prominent close to the entrance. A number of idols were found set on their sockets at the opening of the passage. A closing stone blocks the entrance to the corridor. (Bottom and right) The interior of the megalithic structure presents a very elaborate art programme, including paintings and carvings.

One of the most interesting results of the 1980s excavations was the identification of a series of ‘idols’ placed in sockets especially made to locate them standing up at the entrance of the dromos, an open-air passage way leading into the megalithic corridor, indicating the special nature of this area within the whole monument. A total of 20 ‘idols’ were identified, 17 of them standing in situ and forming a line in front of a series of flat slabs that were placed in the manner of a threshold (see Fig. 3, top). All of them have a more or less anthropomorphic shape, in some cases emphasized by means of incision or carving (i.e. abstract representation of the head and shoulders). They are not exclusive to Dombate; similar items are known in the Neolithic of northwest Iberia, having mostly been found in other megalithic passage graves (e.g. Cova da Moura in Noia; Mina da Parxubeira in Mazaricos: see Fábregas Valcárce Reference Fábregas Valcárce1993). Further excavations in the corridor enabled the discovery of a vertical stone c. 1 m in height blocking its access (see Fig. 3, top). This stone, still in situ at the time of the excavation, was the main element evidencing the closure of the corridor after the primary use of the passage grave. The setting of this element was dated 3011–2586 cal. bc (CSIC-892: 4230±70 bp: Alonso Mathías & Bello Diéguez Reference Alonso Mathías and Bello Diéguez1995) as a terminus post quem thanks to the presence of a small hearth underlying a series of stones placed immediately behind the vertical stone.

Recent excavations have completed our view of the monument's history and meaning. A most striking result of this recent work has been the identification of a number of hearths and ditches around the monument (Lestón Gómez Reference Lestón Gómez, Cebrián del Moral and Yáñez Rodríguez2011; Fig. 2). Three hearths were found in close proximity to it. They seem to be related to the monument, as they are set in the layer that corresponds to the occupation floor contemporary to it (Lestón Gómez Reference Lestón Gómez, Cebrián del Moral and Yáñez Rodríguez2011, 202–5). A fourth hearth was found c. 90 m to the east of the monument, most probably related to a settlement area (see description of ditch 1 below) that was in use after the access to the monument had been closed (Lestón Gómez Reference Lestón Gómez, Cebrián del Moral and Yáñez Rodríguez2011, 221).

A total of three ditches were identified. While ditch 2 and ditch 3 might be contemporary to the Neolithic occupations of Dombate, ditch 1 (c. 1 km of estimated perimeter) seems to be later in date. The northeastern limit of this ditch is located just 4 m southeast of the monument, and probably delimits a Bronze Age settlement area. The exact nature of the link between the monument and the area enclosed by the ditch remains unknown, but it inevitably recalls the debate about the correlation between the domestic and the symbolic spheres in past societies (Criado Boado et al. Reference Criado Boado, Gianotti García, Villoch Vázquez and Jorge2000). Increasing evidence of such linear features beneath or around monuments is being found elsewhere in Galicia (e.g. Chousa Nova: Domínguez-Bella & Bóveda Fernández Reference Domínguez-Bella and Bóveda Fernández2011; As Gándaras: Méndez Fernández Reference Méndez Fernández2007), drawing our attention to the structural, chronological, cultural and symbolic complexity of the sites.

Illumination event and paintings

A different but necessarily complementary level of analysis concerns the setting of Dombate and its relationship to the multiple dimensions of the landscape, including the sky (García Quintela & González García Reference García Quintela and González-García2009; Knapp & Ashmore Reference Knapp, Ashmore, Ashmore and Knapp1999). The relation of the megalithic monuments with the sky is one of the most debated problems in archaeoastronomy (see e.g. González-García & Belmonte Reference González-García and Belmonte2010). For several decades archaeoastronomy has tried to devise a methodology to discover if the orientation of megaliths was related to the heavenly bodies. Despite numerous efforts, it was not until the last decades of the past century that a number of works showed that if taken together, several groups of possibly related megaliths in relatively close areas appeared to present patterns of orientation that arguably were related to the sun or the moon when close to the horizon (Hoskin Reference Hoskin2001; Ruggles Reference Ruggles1999).

Those results proved intentionality in the orientations, but they did not tackle the cumbersome problem of the intent behind such orientations. Recent developments focus on a more cultural context in what has been termed cultural astronomy (Iwaniszewski Reference Iwaniszewski and Aveni1989). A complementary approach recently advocated is that of skyscape archaeology (Silva & Henty Reference Silva and Henty2015). However, in general most of these results ignored the relationship of the monuments with their surrounding landscape.

Landscape archaeology has no a priori reason to stop its gaze at the horizon line, especially considering that the sky presents a number of well-understood regularities (Ruggles & Saunders Reference Ruggles, Saunders, Ruggles and Saunders1993). This is an advantage, because those regularities allow the researcher to replicate with a high degree of confidence how it was at any moment in the past. According to Sprajc (Reference Sprajc2018), ‘the sky can be considered an integral part of any archeological landscape’, and as such may have specific meaning (Ashmore Reference Ashmore, David and Thomas2008) and provides hints to understand the landscape organization (Knapp & Ashmore Reference Knapp, Ashmore, Ashmore and Knapp1999). The built landscape, including megaliths, and the meaningful landscape are thus mutually related (García Quintela & González García Reference García Quintela and González-García2009; Knapp & Ashmore Reference Knapp, Ashmore, Ashmore and Knapp1999). However, we do not have clear references as to the cultural and social understanding, connection and character that the sky had for the peoples who built the megaliths, if it had any at all. In short, we lack their social meaning of the sky. The interaction of the material remains with the landscape, and in particular the sky, may provide a hint towards that social meaning (Criado Boado Reference Criado Boado2012).

The case of Dombate

In this respect, the discussion of the astronomical significance of Dombate has been limited, until now, to the analysis of the orientation of its chamber and passage towards winter solstice sunrise (WSSR here after) for the epoch (Hoskin Reference Hoskin2001, 235; see Table 1), but little research has been conducted to comprehend the possible implications of this orientation, one of the most common in Galicia (Fig. 4; González-García et al. Reference González-García, Criado-Boado, Vilas, Henty, Brady, Ginzburg, Prendergast and Silva2017).

Figure 4. Orientation diagram for 62 passage graves in Galicia. Short lines indicate the orientation of each dolmen. The outside strokes indicate the cardinal directions and the positions of sunrise and sunset for winter solstice (WS) and summer solstice (SS).

Table 1. Data for the orientation of the main axis of the two monuments of Dombate (old and new; Hoskin Reference Hoskin2001 data are provided in the last row for reference; latitude 43°11'28.5”N; longitude 8°58'4.23”W; 198 m.a.s.l.). The columns identify the the azimuth (A), the altitude of the horizon in that particular direction (h) and the astronomical declination (δ). For details, see text.

Table 1 presents the orientation data for both the old and new monuments of Dombate. These measurements were obtained with two professional tandems (Suunto 360), including a compass plus a clinometer. The data provide the mean of several measurements of the orientation of the axis of symmetry for the corridor and chamber of the two monuments. This axis was defined by finding the centre of the corridor and the chamber back stone and marking these points with a surveyor's rod. The nominal precision of the compass is ¼° while that of the clinometer is ½°. However, given the procedure of determination of the measured line, a conservative possible error of 10 cm should be estimated in placing the centres in a line of c. 10 m. This means that a conservative estimate of the error in a single measurement for the azimuth would be closer to ½°. The standard deviation of all measurements is below this level. The magnetic readings were checked for the possibility of local magnetic alterations by taking readings from different sides of the line defined above, finding no significant deviations that may indicate local magnetic alterations. Also, the readings were corrected by comparing with astronomical observations at the moment of sunrise. The measurements presented in Table 1 are our own for both the old and new monuments and we present those by Hoskin (Reference Hoskin2001, 235) for the new monument for comparison. Finally, the last column provides the astronomical declination, where atmospheric refraction has been considered (Schaeffer Reference Schaeffer1993), which is the appropriate quantity to compare local directions in a given landscape (as provided by the azimuth and the altitude of the horizon) with global ones such as the rising or setting of heavenly bodies. In this respect the error in our measurements translates into an uncertainty of ¾° in declination. For the epoch of construction of both monuments of Dombate the southernmost rise of the sun at WSSR happened at a declination of c. −24°.

It should be stressed that the data in Table 1 are for the symmetry axis, while the width of the window of visibility as defined by Silva (Reference Silva2014) is of 43½°, (from 119½° to 163°) and the symmetry axis is slightly off this window of visibility. When located at the back stone along this symmetry axis, the one we have used to define the orientation of the passage grave, the window defined by the entrance is 7¾°.

One implication of this orientation and the physical configuration of the chamber and corridor is that, despite facing the area where the rise of several heavenly bodies would happen, only during a small number of days centred on WSSR would the inner parts of the chamber and its paintings be lit up by direct sunlight (Fig. 5).

Figure 5. Horizon reconstruction indicating the sun's path as seen today (full circle) and in 3000 bc (empty circle).

Because the entrance to the corridor has a width as seen from the back stone of 7¾°, illumination would occur for one month before and after WSSR.

The illumination event is now precluded by the location of the closing stone mentioned above at the entrance of the corridor. Figure 6 indicates the illuminated parts today due to the small breaks at the corners of that stone. It is worth noting that inspection on site of the illumination on 21 December 2014 indicated that the illuminated part never reaches further up than the painted red geometrical pattern of the back stone.

Figure 6. Illumination of the paintings as seen on 20 December 2013. (Left) View of the corridor from the chamber. The small openings above the closing stone allow small beams to illuminate the back stone (right).

In order to mimic the effects of the illumination without removing the closing stone and replicate the illumination at the time of use of the chamber, we have built a 3D model of the mound and megalithic structure and incorporated it into a Digital Terrain Model (DTM) of the surrounding area in order to provide the landscape context. The Digital Terrain Model was obtained from the Spanish Geographic Institute (Instituto Geográfico Nacional).Footnote ² The model was built by interpolation from LIDAR flights of the Spanish Plan of Aerial Orto-Photography (PNOA). The model has a passband of 5 m and employs the ETRS89 geodetic reference system and UTM projection. A horizon was produced incorporating this model to the Horizon software, kindly provided by A.K. Smith. The horizon thus has a resolution of 7½ arcmin for the distance of the horizon of interest for the simulation: this is nearly a quarter of the solar disc. The model was oriented according to the measurements taken by the authors and presented above, and incorporated into the DTM by taking six GPS measurements around the monument.

The 3D model of the megalithic structure was built using Blender from an archaeological plan of the monument (Bello Diéguez Reference Bello Diéguez1992/1993) and by comparing it with the laser-scan of the passage grave (Cebrián del Moral & Yáñez Rodríguez Reference Cebrián del Moral, Yáñez Rodríguez, Cebrian del Moral and Yáñez Rodríguez2011). We did not use directly the laser-scan-based model as this was a really heavy model to be incorporated within the astronomical software. Thus our model has a simplified structure with fewer points and is easier to handle. The mean deviation of our model compared to the laser scan is 2.5 cm, which translates into an angular deviation of less than 10 arcmin.

These features were duly treated into the Stellarium softwareFootnote ³ employing the Scenery 3D feature (Zotti Reference Zotti2016) that allows setting the correct time, permitting us to observe the process of illumination of the back stone paintings during WSSR (see Video in the Supplementary Material). Figure 7 gives a virtual reconstruction of how the illumination would proceed on WSSR for different times as the sun rises and climbs up in the sky a few minutes after sunrise at the time of use of the tomb. It can be observed that, after a first flash of light projecting the entrance frame into the red and white paintings of the back stone, such a trapezoidal frame shifts downwards to the right of the stone as the sun gains altitude above the horizon, until the back stone returns to shadows after a while (nearly 45 minutes). It is again interesting to note that the illuminated part never reaches further up than the red horizontal band, as first hinted at by the on-site observation.

Figure 7. Reconstruction of the illuminated part of the back stone if the closing stone were removed after the simulations described in the text. As the sun climbs up in the sky and moves towards the south, the illuminated area becomes progressively smaller, and shifts towards the northern edge of the stone. The horizontal dotted line indicates the level of the upper part of the red paintings.