4.0 Differentiation Within Categories: visibility of earthen long mounds

4.1. Introduction

Although less studied than the other monuments of the region until comparatively recently, the earthen long mounds are understood to be among the earliest Neolithic monuments in the Carnac area (Boujot and Cassen 1993). Such a view has been supported by recent excavations at Petit Mont (Lecornec 1994) and Lannec er Gadouer (Cassen 2000). In contrast to the passage graves (which are the main other type of earlier Neolithic monuments in the area), the landscape context of the earthen long mounds have been more difficult to characterise by topographic measures (Roughley 2001).

4.2. Visual characteristics

Visibility was clearly important in establishing Neolithic monuments in the Carnac area (Roughley 2001; Roughley forthcoming). However, while passage graves have much larger long-range viewsheds (the mean is twice that of a random sample of 500 points), this is not such a significant feature of earthen long mound locations. The locations in which passage graves were constructed are also significantly less visible over shorter distances (up to 0.5km) than would be expected – they are situated on small hills which create false horizons (Roughley 2001; Roughley forthcoming). In contrast, the earthen long mounds were situated in locations with greater visibility at a local scale, slightly greater than would be expected. Although the local viewsheds of earthen long mounds, unlike the passage graves, do not differ so distinctly from the expected, the absence of any sites with small local viewsheds suggests that local visibility may have been a factor in the choice of location.

The viewsheds of the earthen long mounds were considered individually to examine further the visual characteristics of the locations in which they were constructed. Many of the viewsheds of the earthen long mounds are very similar in extent and shape (Fig. 7). The similarity of the viewsheds is in part related to the spatial distribution of the earthen long mounds, which are not randomly distributed spatially, but significantly clustered. Although in many cases members of a group are nearest neighbours, this is not always the case. The viewsheds were therefore grouped by similarity: viewsheds which largely overlap are considered together, viewsheds with few common areas are assumed to belong to a different group.

Figure 7: Viewsheds of earthen long mounds by group

Viewsheds by group

Within each of the groups defined by viewshed similarity, there is a high degree of inter-visibility between the monuments, and a low amount of inter-visibility with members of other groups. Figure 8 shows the proportion of sites of each group inter-visible with members of a group. There is a contrast between the high degree of inter-visibility within each group and lack of inter-visibility between groups. The clear dominance of inter-visibilities within groups shows the discrete nature of those groups. However, as inter-visibility is highly spatially autocorrelated, the clustered distribution of the earthen long mounds does make this more likely than for a spatially randomly distributed group of points. Assessment as to the statistical significance of these results is therefore difficult.

Figure 8: Inter-visibility between groups of earthen long mounds

Chart showing inter-visibility

Some of the groups identified are less separate. The distinction between Groups 7 and 8 is slight (Fig. 8), even though they are spatially distant from one another (Fig. 7), with Group 3 sitting between them. This is because they share a coastal location and perspective.

Group 9 can be seen not to fit the pattern. There is a high degree of inter-visibility between this group and other groups, and between other groups and Group 9. The group has a central distribution (see Fig. 9) but this is not the reason for its high inter-group inter-visibility, as other groups are also surrounded by earthen long mounds. Group 9 consists of the Manio group and Kerlescan (which are all in close proximity to where the Carnac alignments were later constructed), and earthen long mounds close to the Carnac Mounds at Moustoir and St Michel. The Manio group is the only cluster of earthen long mounds to be situated on a locally high area of visibility. The other clusters are not situated on such local maxima in the total visibility surface, unlike neighbouring passage graves. (Roughley 2002a).

Figure 9: Viewshed for Group 9

Viewshed map

The viewsheds of Group 9 (shown in Fig. 9) cover much of the basin to their north, and extend towards the coast in other directions. When the viewshed areas over a range of distances were plotted for each tertre individually, the sites of Group 9 were found to be different in character. Group 9 sites have long-distance viewsheds which are much larger than those for most other tertres, but have small local viewsheds (Fig. 10). This helps to confirm the distinct nature of the locations of these monuments from the other earthen long mounds. Further analysis will help to clarify the extent to which the viewshed characteristics identified are statistically significant.

There is some similarity in the boundary of the viewsheds with the main watersheds (Fig. 10). The watersheds are the local horizons for many of the sites. The topography, slight as it is, can be seen to have a considerable impact on the locales in which the earthen long mounds were constructed. The viewsheds of Group 9 can be described further: Group 9 sites all sit on or near watershed boundaries. This concurs with Boujot et al's observation on the importance of the location of the Manio earthen long mounds as the boundary between the coastal area and inland (Boujot et al. 1998).

Figure 10. Distribution of earthen long mounds by group with respect to watersheds (black) and rivers (cyan)

Map showing watersheds


The distribution of earthen long mounds in the Carnac area is spatially non-random, being strongly and significantly clustered, but does not correlate with broad topographic trends (Roughley 2001). Initially, visibility analysis did not suggest a clear relationship between locations chosen and visibility. However, further exploration of the data has suggested some interesting possibilities, and has allowed the spatial distribution of the earthen long mounds to be considered further within the context of this landscape. Monuments appear to be in groups which are inter-visible, and have views of the same areas of the landscape. The horizons are often local watersheds. Groups are not inter-visible. There are a small number of sites which do not fit this pattern, and further research is needed to explore this more fully.

4.2.1 Clustering

Clustering of sites was investigated using Ghat. Ghat measures the distance from each point to its nearest neighbour (Diggle 1983). This can be compared with a Monte Carlo simulation of random distributions which are based on the Poisson distribution. The simulated distributions must have the same boundary and number of points as the distribution in question. The number of iterations can be increased to enhance the confidence of the test; 100 iterations were used here. The highly non-random distribution of the tertres tumulaires is shown clearly in Figure 11. It is outside the expected envelope for a random distribution. Tertres are strongly, and statistically significantly, clustered, with most sites very close to another site.

Figure 11: Clustering of earthen long mounds

Cluster graph

4.3. Interpreting earthen long mound locations

Although not a complete description of the landscape context of the earthen long mounds of the Carnac area, the visibility analysis above provides some insight into the ways in which the landscape was constructed and engaged with. The groupings of the earthen long mounds may be understood more fully in the context of further excavation. In the meantime, some interpretations are offered.

It may be that sites within each group were constructed by the same group of people and their descendants, but at different times. For example, three of the monuments at Bovelann (Erdeven) share an alignment, whereas the fourth is orientated at 90 degrees to the others (Boujot and Cassen 1995), suggesting that it may have been constructed separately.

Earthen long mounds have a variety of construction methods (Boujot and Cassen 1993). The earthen long mounds of Mané Ty Ec, Mané er Layeu and Mané Pochat er Uieu, which are all in the same cluster, have very similar structures, with a distinct curb and circular cist structure at one end (Miln 1883). The morphological similarities between these sites suggest that they were built over a relatively short time period.

Chronological differences may be present between groups. Group 9 earthen long mounds might be later and part of a move towards greater social cohesion. Alternatively, the spatially central monuments with large viewsheds of Group 9 may have had a different role. The design of Group 9 sites is varied. Le Manio quadrilateral is architecturally similar to Le Jardin aux Moines (Néant-sur-Yvel, Broceliande) which has been dated to the later middle Neolithic (Briard 1989). Their viewsheds have similar characteristics to those of passage graves, and their landscape context may be part of a complex interaction in southern Morbihan between earthen long mound and passage grave traditions.

4.4. Conclusions

Through investigating the visual characteristics of the locations in which earthen long mounds were constructed, new insight can be gained into the monuments. Although visibility analysis of the locations of earthen long mounds has been less straightforward, and potentially less conclusive, than for the passage graves, interesting possibilities have emerged. Various areas of the landscape appear to have been marked with the construction of distinct groups of earthen long mounds. Whether the monuments were constructed over a few years or by succeeding generations may be elucidated in the future through further excavation. The clustered distribution of monuments suggests that each monument does not relate to a separate territory as has been proposed by Renfrew for Wessex (Renfrew 1973). However, it is possible that each group of monuments was constructed by a group of people living within that part of the landscape. Alternatively, a large group of people may have moved round the landscape, and constructed monuments in different places according to particular circumstances.


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Last updated: Thur Nov 11 2004