Title image: Pentre Ifan Dolmen, Pembrokeshire
Could lichen growth on portal tombs give us a clue to the original height of the cairns which once covered these tombs?
An interesting article in the Autumn 2019 issue of ‘Archaeology Ireland’ leads us to think differently about the cairns that covered portal tombs. Evidence from the lichen suggests that the cairn covering only went so far up the tomb and that more of the impressive dolmen-like structure would have been visible than if it was completely covered.
Portal tombs are the most striking of the Neolithic tombs and are characterized by their enormous capstones, balanced precariously on a few upright stones. Typically, the chamber has two portal stones which form the entrance and these can be impressively high, as in Legananny tomb below. They have a single large capstone which slopes down onto a backstone at the rear. More stones would have formed the walls of the chamber but these didn’t support the capstone.
There are about 180 known portal tombs in Ireland with the largest number of them being in the northern part of the country. They are also found in Wales , mainly in the extreme south west, and in Cornwall. In Wales and Cornwall they are usually called cromlechs or dolmens – but not all cromlechs or dolmens are of the portal tombs design!
Many portal tombs have evidence of a long cairn which would have stretched out behind the chamber, making a long trapezoidal-shaped mound. It has often been thought that this cairn also covered the chamber and only the entrance would have been visible. However, it now seems that this may not have been the case.
Eoin Halpin, in his article “Growing evidence for portal tomb cairns?” has examined lichen growth on many portal tombs in Ireland and found that on the external upright stones there is often a marked difference in lichen growth between the upper and lower parts of the stones.
A ‘tideline’ is usually visible, where the lichen above it is much more profuse and of a greater variety than the lichen below the line. Lichen needs sunlight to grow and the density of lichen above a certain height could indicate that that part of the stone had been exposed to daylight for much longer, while the stone below had been hidden from daylight by the cairn material.
The conclusion is that cairn material was only built up to a certain height, so that people would probably have been able to see the capstone and part of the structure.
The lichen evidence supports the views of other archaeologists who have come to the same conclusion that part of the tomb structure was left open to view and that the cairns covering portal tombs were not as high as originally thought.
It makes sense that these fine structures, which were far from easy to build, were built to be seen, and that the tombs were as impressive looking in their day as the monuments we see left in the landscape today.
Reference: Growing Evidence for Portal Tomb Cairns, Eoin Halpin ponders lichen growth as an indicator of cairn height. Archaeology Ireland, Issue No.129, Autumn 2019