In the north-east quadrant of the broch interior, immediately above the ‘souterrain’ structure (detailed here), a spread of compact brown-orange clay over roughly-set flagstones formed a floor into which a large igneous boulder had been set. This boulder was a mortar or ‘knocking stone’ with a central V-shaped hole c. 0.25m deep, used for threshing (separating the grain from the husks): the ears of wheat or barley were placed into the hollow and pounded to remove the tough husks. The grains could then be processed further as desired, being ground into flour for bread, for example. The knocking stone at Clachtoll was found to be full of carbonised grain, which on initial assessment has been identified as hulled six-row barley. Full analysis in due course will examine the full range of cereal and plant evidence recovered during our excavations.
The floor surface into which the knocking stone was set had a distinct edge to the west, suggesting that the knocking stone may have been located within an internal compartment within the broch, though no superstructure relating to this survived. Given the carbonisation of the grain within the stone, it is highly likely that the mortar was in use when the broch was destroyed i.e. that it relates to the latest phase of flooring and hearth construction. This massive fire event will form the focus of next week’s post.