Internet Archaeol. 26. Risch et al. The petrographic composition of the Hiregudda Sanganakallu dolerite dyke

One of the main questions that arises in view of the scale of quarrying and axe production at Hiregudda concerns the material properties of the dolerite dyke targeted by prehistoric peoples. As already mentioned, this type of basic dyke is a common geological feature of southern India, but at the Hiregudda locality, evidence of large-scale prehistoric exploitation is especially notable. It appears to have been a particular focus of axe production activity. While petrographic characterisation is necessary in order to define the distribution network of the Sanganakallu artefacts, it is possible that they were exchanged or traded over large parts of southern and central India.

Field survey enabled us to observe that marked differences exist between the quarried dolerite outcrop at Hiregudda and another dyke just 180-200m further north that also traverses the hill. This latter much wider and larger outcrop, which runs parallel to the quarried dyke, and extends along the top of Hiregudda, was a local focus of petroglyph creation (Boivin 2004b; Fawcett 1892). One can imagine that the dark colour of this chain, formed by large boulders running through a light granite hill, was perceived to be of particular symbolic importance. The rock can best be defined as a coarse-grained, extremely hard gabbro. It was used as raw material for percussion and abrasive tools on all three hill settlements, as well as in the quarry.

Figure 3: Thin-sections of the gabbro from the northern dyke and dolerite from the southern dyke (axe quarry) of Hiregudda (Photographs by David Gómez-Gras). A) general view of the intergranular texture of the gabbro rock containing crystals of augite (green) and plagioclase (light) under plane polarized light (PPL), and B) under crossed polars (PPX). C (PPL) and D (PPX) images of the microcrystalline intergranular texture in dolerite rock. E (PPL) and F (PPX) enhanced images showing augite crystals (red, blue and orange colours) occupying the spaces between plagioclase laths (grey)

Rock samples were taken from both dykes as well as from archaeological materials and submitted for petrographic analysis through thin-sectioning (Fig. 3). The two identified rock types can be characterised as follows:

The textural arrangement of the crystals forms a 3-D holocrystalline isotropic framework in both rocks and confers high mechanical strength. Moreover, the microcrystalline grain size of the dolerite allows a conchoidal fracture, one of the necessary conditions for flaked axe production.

Experimental tests were carried out in order to gain a better understanding of the material properties of the rock and its behaviour during the knapping, pecking and polishing stages, as these represent the documented techniques used in Hiregudda axe production. Flaking of natural blocks and slabs confirmed the conchoidal fracture habit of the material when submitted to heavy impacts. Pecking and polishing tests proved the extreme hardness of the dolerite. It is worth mentioning here that part of the experiments were carried out by local male villagers, whose usual income derives from manual quarrying of the granite and who are familiar with the mechanical properties and working of stone. Axe blanks collected from areas disturbed by modern quarrying were ground on granite bedrock with the addition of water and sand, which had previously been prepared by crushing the same granite. Without the addition of this abrasive material, the grinding process proved practically impossible and required considerably more strength. Every 30 minutes, the axe blank was cleaned, documented and weighed. Soon it became evident that most grinding efforts had to be carried out on the thickest part below the edge, confirming the importance of a previous competent flaking strategy. The mean material loss, or abrasion wear index, was hardly 3g of dolerite an hour. The grinding of afanitic rocks from the axe factory of Plancher-les-Mines (Vosges) on sandstone slabs resulted in wear indices between 5-13g/hour (Petrequin and Jeunesse 1995, 41). Given the hardness of the Sanganakallu dolerite, the grinding of a reasonably well-prepared blank into a usable edged tool of 90-100mm length would probably have required around 6 hours of work. Much more effort would be necessary in order to obtain a completely polished axe.

2. The Petrographic Composition of the Hiregudda Sanganakallu Dolerite Dyke