Cobbles - 2

The Diary's identification of rock types is very much a hit-and-miss affair, so it was good to have Rob Gill, who runs GEOSEC Slides, a small business based in Achnaha, write in with a couple of photographs to show how rock identification should be done.

Rob says, "I found this cobble at Ormsaigbeg a couple of years ago. As it was obviously different from most others I took it to Achnaha and made a section from a bit of it. It is a garnet amphibolite (or possibly granulite) most probably Lewisian. I cannot prove that, though it is difficult to think where else it could have come from. The spots are garnets surrounded by a rim of feldspar, a depletion zone as the garnets grew at the expense of the amphibole.

"I do not know its source, but is quite distinctive, so it might be possible to trace it."

This picture shows Rob's thin section, with the distinctive pink of the garnets surrounded by paler felspar, set in a matrix of finer crystals.

Metamorphic rocks such as this are fascinating to study. It may have started as something like a mudstone, which was deeply buried in the core of a mountain chain, and heated and squeezed so a totally new set of minerals grew. During this process, however, some minerals may have grown and been absorbed again, changing into others.

It's also interesting to grapple with the idea that, if it is Lewisian in age, it's over a billion years old.

Rob's website, which is here, is well worth a visit to see the beautiful rock thin sections he produces.