Curious Rock Formations at Inverkirkaig
On 13th March 2023 expert wall-builder Iain Roberts of Nedd found at Inverkirkaig (NC0719) some Lewisian gneiss bedrock with curious markings. The rock had been unearthed from a depth of a metre or more in excavations for house foundations.
The markings consist of attractive concentric rings, some tinged red (photos 1-2). They are about 6 cm across and seem to be an integral part of a thin band of hard insoluble minerals following a fracture plane in the rock. Iain kindly gave me a sample, and I sent photos off to a couple of geologists with an interest in Assynt rocks, but neither was able to suggest a cause.
Recently, geologist Grant Nicoll, Iain’s brother-in-law, came across a similar phenomenon in gneiss rocks that had collapsed across the cemetery track at Nedd (NC1331), and looked for an explanation on the Internet. They are Leisegang Rings, deposits of iron oxide, which usually occur in iron-rich sedimentary rocks, such as sandstones.
However, the account of them in Wikipedia says that they ‘can also occur in igneous and metamorphic rocks that have been chemically weathered’, when there is ‘diffusion of oxygen into pore space containing soluble ferrous iron’, oxidising it to red ferric iron (which gives the colour to rust). We know that parts of the local gneiss are rich in iron and other minerals, but the account continues ‘few scientists have studied their mineralogy’, so we may never know precisely how and when they were formed.
Ian M. Evans
Photos by I.M.Evans