Stones, relatively free from sermons

There are plenty of ruined buildings in the world but no ruined stones.

(Hugh MacDiarmid, “On a Raised Beach”, 1934).

Where I stay in the south side of Glasgow, red sandstone buildings are a common sight. It’s a lovely stone, which at least according to tradition originates in the region of Dumfries: this would presumably make it Permian in age (about 250–300 Ma old), having been laid down when Scotland was sitting in an arid zone just north of the equator. One of these days I hope to get round to tracking down more precisely the origin of the stone used in our own building…

Aside from its history, sandstone creates some beautiful textures as it weathers: I don’t know what these are like for the colour-sighted, but for us colourblind — who I suspect are more attuned to visual textures and patterns than most of you — they’re a real treat. Because of this, one of the advantages stone-faced buildings have over those built mainly of glass or concrete is that they age gracefully, becoming more interesting rather than merely shabby. A favourite building of mine is St Magnus’s Cathedral in Kirkwall (the final great achievement of Earl — later St — Rognvald, following some remarkably dirty politics and a less than successful Crusade). The fabric of the building is mostly sandstone: there’s a mixture of red and yellow stone, but to my eye it looks as if even within one colour of stone there are several types present — perhaps from different sources, or perhaps simply from different beds within the same source. (One of the things I’ve learned on my brief excursions into the field with grown-up sedimentologists is that sandstone is never simply sandstone.) And each type of stone — indeed, practically each block of stone — seems to weather slightly differently depending on its porosity, its orientation, its exposure and the fine details of grain sizes and cementation.

Here’s a photo of part of the surround of the main west door, taken on a holiday a few years ago:

West door at St Magnus's Cathedral

What fascinates me is that in those red sandstone blocks there seem to be three distinct modes of erosion. In the upper part of the doorframe, there’s what looks like fairly uniform erosion: I think the vertical ribs are the remnants of carved detail, though it’s hard to be certain and it does look as if some kind of gunk has ended up preferentially coating them — biologically or chemically? I’ve no idea.

Nice straightforward weathering

Over to the right, weathering of those slim pillars has given them an almost wood-like texture. It isn’t just uniform erosion exposing the grain, though: alternate layers have evidently weathered at different rates, leaving a distinct relief and massively accentuating the rather subtle laminations of the original stone. I’m puzzled by laminations: because they pick out the formation and development of bedforms they’re incredibly useful when interpreting sedimentary rocks; yet despite reading the accounts in a couple of sedimentology textbooks I don’t really understand how, in general, they occur. (As in the Wikipedia article they’re often attributed to variations in supply, but to my mind this replaces one mystery with a worse one. Personally I’d bet on a fine-scale segregation mechanism in the very thin bedload layer under a depositing flow, but I’ve yet to see this properly described.) At any rate, this differential weathering emphasises how different the properties of these very fine layers can be. I also get the impression there might be some loss of larger chunks going on towards the tops of the pillars: my prime suspect would be either some kind of freeze-thaw effect or perhaps a chemically-driven exfoliation process, but I could be way out.

"Wood-grain" effect on pillars

Finally, on the middle part of the doorframe we have the detail that provoked me to take these photos in the first place: these finger-sized pits like the cells in a wasps’ bike. Again I’m not certain, but I think these may be a miniature version of tafoni, described elsewhere as “honeycomb weathering” or alveoli. The jury seems to be out on the formation mechanism, but the most plausible culprit appears to be a kind of physico-chemical weathering involving salt which is drawn to the surface of the stone as pore water evaporates, and then flakes off taking chips of stone with it. Why there should be a feedback mechanism concentrating the erosion in these deep holes isn’t clear to me, since I’d have thought an evaporative effect would be more pronounced in less shadowed regions. For what it’s worth, all these examples are from the south side of the doorway (facing roughly west-north-west due to the recess); the north side is less weathered generally and shows no sign of these features.

Tafoni? No thanks, I've eaten already.

I’m not sure if there’s a moral to be drawn from this bit of geo-tourism, but if there is then I think it has something to do with what I find so fascinating about rocks: you rarely have to ask too many questions about them before you hit on a minor mystery; and if you get to the bottom of that there’s probably another one lurking below it. As Mark Twain put it with his customary reverence in the wonderful Life on the Mississippi: “There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.”

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