I must admit, when I first thought of using xenolith for my X post, I was thinking of those strange boulders that are found miles away from where any rock like them forms part of the landscape. Many of those have been moved by ice, as in glaciers, but they are properly termed erratics. They are great natural phenomena (that being my A to Z theme this year) and sometimes lead to artificial phenomena like stone circles and piles. I’m particularly fond of this one, in Cornwall, which I stopped and drew a picture of many years ago, now. I think my friend Helen bought it.
Xenoliths are fragments of rock that become embedded in other rocks – usually through rock formation under the Earth’s crust (igneous rocks). They can also be embedded in sedimentary rocks, chunks covered by decomposing animal skeletons and laid down over the millennia to become limestones and the like. The most common xenoliths are in granites. There is a lovely grey granite – Shap granite (in the header picture) – with pink rock captured in it that is quarried in the Lake District. I think the pink bits are actually autoliths, in that they are made from the same material as the surrounding rock, but crystallised faster.
This picture is of xenoliths; the embedded rock is older than the surrounding rock.
Take a look at the monuments around you. There might be interesting things in the stone, apart from graffiti. The photo above is of xenoliths in the stone of monument buildings in Hull (courtesy of East Yorkshire RIGS Group). And if you ever visit the Albert Hall in London, cross the road to the Albert Memorial and look at the paving around the base of the memorial – it is stuffed with fossils and xenoliths!