This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. At first sight, urban geology sounds like an oxymoron. How can you do geology with no rocky outcrops anywhere in sight within the built-up environment of cities? Well, it turns out you can do a great deal of geology, and Ruth Siddall has been doing just that for the past 10 years. Along the way, she has developed nearly 50 urban-geology-themed walks, mostly in London, as well as built up a database of over 4,300 urban localities of geological interest throughout the UK. Both the walks and the database are freely available to all. She is a postdoc at Trinity College Dublin, studying the social history and geological provenance of stone in 18th-century buildings in Britain and Ireland. Ruth Siddall, welcome to Geology Bites.

Ruth Siddall: Thank you very much, Oliver.

Oliver Strimpel: What exactly is urban geology?

Ruth Siddall: Well, I think it’s something that can be defined in different ways. Some people would define urban geology in the context of geotechnical engineering: what is the geology of a city, and how does that impact building a bridge or building an underground railway line? But for me, it’s very much learning about the geology of building stones and what stories those stones tell in terms of their social history, how they came from quarry to building, but also what you can learn as a geologist, or a would-be geologist, from looking at building stones.

Oliver Strimpel: Why should one do urban geology instead of going into the field to see the rock in situ, not just in naturally occurring outcrops, but also in quarries and road cuts?

Ruth Siddall: Well, not everybody has the mobility or the funds to be able to do that. If you’re going to go to a quarry, or if you’re going to walk up a mountain and look at rocks, then you need a certain level of fitness, and that’s not necessarily there for everybody. And if you have mobility issues, or if you’re blind, there are other factors that can stop you observing the geology. And the thing about urban geology is that even though the urban environment has its own hazards, it’s somewhere that people are used to being in. And anybody can access city streets, in theory, and look at the stone on the side of buildings. And you can learn a surprising amount about geology from looking at building stones.

Oliver Strimpel: So obviously it’s going to be hard to do structural geology or geomorphology when the rocks are not in situ. So what kinds of geology can you learn about in the city?

Ruth Siddall: Yeah, you’re absolutely right. The big problem is that you cannot look at the three-dimensional setting of a rock as in the field. So you can’t do structural geology measurements and record orientations of outcrops or anything like that. But if you’re interested in petrography, petrology, paleontology, sedimentary structures, all of these kinds of things you can see. You can really learn about rock description, tools and terminology. You can see a huge range of rocks as well, and I think that’s really important. If I wanted to go and see a high-grade gneiss, sitting here in northeast London, I would have an expensive train trip or road trip or flight to northwest Scotland or to Canada or to Brazil, and then probably have to walk out into a rural environment. Or I could just go down to the City of London and see one of these rocks on the side of a building. I’m a mineralogist and petrologist, and a lot of that information you can see on a building stone. And I think it’s a really good way of teaching petrology, petrography, paleontology, ecosystems, environments of deposition, all of these sorts of things, in an accessible manner.

Oliver Strimpel: Can you give some examples of how we can glean each of these sorts of knowledge by looking at building stone?

Ruth Siddall: Yeah. Well, I think in terms of petrology, especially for hard rocks, igneous and some metamorphic rocks, polished surfaces are just great, because you have a single plane. It’s like looking at a thin section, only it’s right there in front of your eyes. Especially if you’ve got a hand lens, you can see a huge amount about intergrowth relationships, processes of crystal growth, order of crystal growth, deformation structures, and you’re not just limited to a rough, weathered surface in a quarry.

Oliver Strimpel: Can you see examples of structures or features that you’d be hard pressed to see in the field?

Ruth Siddall: Yeah. I’ve seen, in a variety of limestone called, slightly confusingly, Petit Granit (it’s not a granite, it’s obviously a limestone; it’s Carboniferous limestone from Belgium), one of the things that I’ve seen quite a lot in building stone examples: fossil rostroconchs. They’re rather bizarre bivalves, and I’ve seen a number of these in the building stone, and yet these are hardly recorded in field observations. They’re rare beasts, but we know they’re there because of the building stones. So that’s a paleontological example.

I’m very interested in hard rocks, and I love looking at the metamorphic rocks, especially in polished surfaces. And you can see kinematic indicators. Obviously the measurements of them don’t tell you anything, because it’s not in situ, but at least you can show students and other interested people what these things look like, what they should be looking for when they’re out in the field looking at the direction of thrust movement, for example. So yeah, you can learn about textures, mineralogy, so much. You can do everything that you can do in the field except structural orientation.

Oliver Strimpel: One of the stones that’s been used really extensively in London is Portland stone, which comes from Dorset. That’s a sedimentary rock. Are you able to see things in that in building stone that you’d find it difficult to find in the field?

Ruth Siddall: Yes, certainly. So Portland stone is really London’s classic building stone. It’s been used here since the early 17th century, and it’s late Jurassic, Tithonian, although we call that Portlandian here in the UK. These are marine oolitic limestones that were deposited in a very clean Portland sea, so very white or pale gray limestones, oolitic, variably fossiliferous. And there are three main facies of Portland stone that are used as building stone. One is an almost pure oolite, more or less fossil free. A second is a mixture of oolite and mainly oyster fossils, and that’s called the Whitbed, which is the main building stone that’s used. And then there’s a variety called Roach, where the fossils have leached out. It turned from a sediment into a sedimentary rock rather more quickly than the original aragonite of the shells could transform into more stable calcite. And that’s a really weird-looking spongy rock, full of fossil-shaped holes, casts and molds of fossils.

But the Whitbed is the one that is used mostly. And if you go out in the field, go to the Isle of Portland, go to the quarries, you really only have access to the abandoned quarries, which are weathered, overgrown, difficult to access, sheer cliff faces. You don’t really see the detail of the kind of things that you want to look at. What indicators have we got of the environment of deposition? You’re seeing faces cut orthogonal to the bedding. So you don’t really see bed surfaces in the Portland stone quarries, but we do see bed surfaces on paving stones of Portland stone. And so we see trace fossils like Thalassinoides, which are hard to see in the quarries.

We also have patch reefs, which are quite endemic to the Portland seas, and they’re like reef mounds, about four meters in diameter and four meters high. And they’re not coral reefs. They’re composed of coralline algae, a species called Solenopora portlandica, which is, as the name suggests, endemic to the Portland seas, and also oysters encrusted together. And this is a really unusual ecosystem, very unique to Portland. But we see it there in buildings, because these reefs are encountered occasionally within the quarries, and the quarrymen keep them and slab them and store that stone until they’ve got enough reefs to be able to sell to a building contractor to clad a whole building. And so you can go and look at them in the buildings. You don’t see them in the quarries. You can see all this life going on, not only the coralline algae and the bivalves, but trace fossils, borings, all kinds of things going on. So you get a real feeling for the ecosystem of the Portland seas that you just do not get in the field.

Oliver Strimpel: With metamorphic rocks, obviously marble is extensively used, but can you actually see metamorphic facies, different metamorphic grades, in some of the building stone?

Ruth Siddall: You’re more likely to see high-grade rocks like amphibolites and granulites, and that’s because they’re hard, and they’re also very attractive. They tend to be coarse grained, beautiful. So lots of gneisses, for example, gneisses and migmatites, very popular in 1980s and 1990s architecture. And most of these, wherever you are in the world, tend to come from places like India and Brazil, Archean high-grade gneiss terrains. We don’t see so many schistose rocks, and that is because of the nature of the stone. It’s too friable; it breaks on the schistosity. There are a few examples where the plane of the schistosity is used as a cladding, and there’s an example from the Norwegian Caledonides called Otta schist, which is a graphite–actinolite schist, a greenschist. But I must admit, those kinds of greenschist facies, and indeed blueschist facies, are pretty rare. If you want something built from blueschists, you need to go to somewhere like the island of Sifnos in the Aegean, where this is the local stone and it’s used for building stone walls. But yeah, it’s not the kind of stone that you see on a city block.

Oliver Strimpel: But what about slate? Slate is used for roofing, and that has cleavage.

Ruth Siddall: I was just going to mention that. That’s probably the most common metamorphic rock that most people encounter: relatively low-grade metamorphic rock, mudstones. I’m originally from North Wales, so Welsh slate is to me the best slate ever. And that stone, which was quarried throughout the 19th and 20th centuries, and indeed still today, was shipped all over the world as a roofing slate. So yeah, slate is the most common building material. The problem is it’s on the top of the building, so it’s very difficult to see from street level.

Oliver Strimpel: Apart from avoiding crumbly rock, if you like, or schistose rock, what properties make a rock desirable as a building stone material?

Ruth Siddall: I think it’s two things. It’s partly aesthetics, what it looks like, but also, possibly more importantly, the strength of the rock. We tend to measure the compressive strength of rock in megapascals. You want something that’s going to have good load-bearing strength, and you also want something that has got a decent block size. So let’s go back to Portland stone. By block size, I mean what is the maximum size of rock that you can extract from the quarries? And for Portland stone, it’s probably blocks with dimensions of about two meters by three meters by two meters, large, large blocks that you can then cut into building blocks, into ashlars.

And then compressive strength: how strong is that stone? How much load can it bear? And again, for Portland stone, that’s quite a lot for a limestone. It has a compressive strength of about 45 megapascals, which is quite high for a limestone. But just to put that in context, your average granite has a compressive strength of about 250 megapascals. So there’s a big difference. So granites are often used for foundations of buildings. If you look at, say, a Victorian building, you’ll often find granite foundations. But again, block size is important, and block size is governed by bedding in sedimentary rocks, but also by joints in the quarry. Some granites can have quite small joint spacing, and they’re pretty much useless as a building stone, however strong they are. But you could use them, say, for cobblestones or curbstones or some kind of aggregate. But with things like the Cornish granite, you can get a similar kind of block size to what you can get from Portland stone. And that’s the kind of granite you want if you want to build strong foundations for a building, or indeed if you want to go and build a lighthouse. You need big blocks of solid, flawless stone. So those kinds of physical parameters, which are things that we don’t really look at too much as field geologists, are really important for stones.

Oliver Strimpel: In your urban research, have you always managed to identify the rocks and discover where they came from?

Ruth Siddall: Yes and no. There are some stones that I’m very familiar with, and I know the quarries and I can recognize them. They feel a bit like old friends: Portland stone, Bath stone, for example, Cornish granite. But there are rather more, I suppose, exotic stones that I don’t know that you’ll see on a building, and I really have to go and do a bit of research to try and find out what they are. Sometimes you’ll get a friendly architect or engineer or stone contractor who will be able to tell you exactly what it is and what quarry it comes from. But in some cases, architects and stone contractors just don’t know where their stone comes from, and it’s been sold to them under a trade name. And yes, it will come with a certificate showing it has such-and-such a compressive strength or whatever. But those kinds of stones can be really, really difficult to track down. I spend a lot of time looking through commercial stone catalogs trying to make things match. But yeah, it’s not always possible to identify the more exotic stones.

Oliver Strimpel: What about historical monuments like Saint Paul’s Cathedral in London, or some of the cathedrals of Italy?

Ruth Siddall: Yeah, certainly we do have records going back a long way for the use of stone. St. Paul’s particularly is a good example. Christopher Wren was the architect there, and there’s a huge archive of his documents relating to the building which has been preserved. And these include orders of stone from Portland, for example, which is the main building stone of Saint Paul’s Cathedral. We see similar things in Italy. You mentioned Florence: Brunelleschi, who built the cathedral, the Duomo, Santa Maria del Fiore, again left very, very clear descriptions of how much stone he wanted and which quarry it came from. But sometimes those records never existed with that level of detail, or they’ve been lost or destroyed. So you’re guessing a lot.

Oliver Strimpel: The ancients were great builders of stone: the Egyptians, the Greeks, the Romans. Do we know where they got their stone from and how they managed to transport it all those distances across their empires?

Ruth Siddall: We do. There’s quite a lot of very, very solid archaeological work done on the archaeology of quarries. Most quarries used in antiquity have now been discovered or rediscovered. Some of them, like Carrara, have been in continuous use since the Roman period. In ancient Egypt, stone was pretty much used locally, and the Nile was the main route of transport for carrying granite, for example, from Aswan down in the south of Egypt to cities on the Nile in the north. And there were limestone quarries and sandstone quarries as well. The pyramids are built from stone quarried close by on the Giza plateau. But there wasn’t very much export of stone out of Egypt until the Roman period.

I think it probably was the Romans who were the first society to really take stone seriously. They very much valued stone, and they exported it throughout the empire. We see stone used by the Romans which could have come from Egypt, and we now see it in the Museum of the Roman Amphitheatre in London. They had no problems at all about shipping stone across the empire. Their structural building stones were often locally sourced. The kind of stone that they shipped was their decorative stone, used for sculpture, mosaics and interior revetment and cladding.

Oliver Strimpel: So apart from Portland stone, which you’ve already mentioned, what kind of stone was used to build some of the large institutional buildings of London?

Ruth Siddall: Mainly Portland stone, and you get this very classic combination of a Portland stone superstructure on foundations of Cornish granite. So the granite’s there in the basement, with the Portland stone superstructure above it. Cornish granite was really, really important in London building, more so than Aberdeen granite. I think everybody tends to think of the Aberdeenshire, the Scottish, granites as being the most famous ones. But certainly when the first railways were constructed, it was no coincidence that they went to Exeter and they went to Aberdeen. And that was partly to bring back stone, because shipping is always a risk for heavy loads of stone. If you have a shipwreck, there’s no way that a cargo of stone can be salvaged, so more reliable ways of transporting stone were important. Even today in the City of London, you’re asked to build with Portland stone unless there’s a good reason not to do so. But that varies from city to city. You go to Manchester, and it’s all sandstones from the Pennines, Carboniferous.

Oliver Strimpel: What about stones from abroad? Did Britain exploit its colonies for stone like it did for other natural resources?

Ruth Siddall: Much more the other way. We exported Portland stone to the colonies. So you do see Portland stone buildings in India, in Australia, in New Zealand, and I think that was part of the control, part of really implanting that belief that England was best. And there are some interesting legacies to unpick about the export of stone. But no, we didn’t get very much stone coming the other way, although you do see, particularly in the 19th century, when transport was available to do this, war memorials being made out of local stone, and embassies often incorporate local stone. So at the Australian High Commission in London (Australia is part of the Commonwealth), stone from Australia was brought to build the embassy in London in the early part of the 20th century, actually during the First World War. So we see granite foundations there which come from New South Wales rather than from Cornwall, but the superstructure is Portland stone once again.

Oliver Strimpel: As I mentioned in the introduction, you’ve compiled many urban geology walks in London. Could you walk us through one of your favorites? And we’ll post pictures taken along this walk on the episode web page for listeners who’d like to follow along.

Ruth Siddall: Yes, of course. I think doing an urban geology walking tour is a really good way to get your eye in for building stones. I think many of us just hurry past buildings on the way to work or whatever and don’t really look at them. But once you start looking, you see all these amazing things. The walk I’m going to talk you through is Urban Geology in London No. 39, which takes us from the Monument to the Great Fire of London to the Tower of London, along what was once a medieval thoroughfare called Eastcheap. But it’s now, apart from the end point and the start point, largely modern buildings.

So we’re going to start at the Monument, which is a really interesting structure. It was built between 1671 and 1677 by Sir Christopher Wren and also Robert Hooke, who’s probably more famous these days as a physicist and chemist rather than an architect. He was also quite an accomplished geologist, and both Wren and Hooke were interested in astronomy. And the Monument serves two purposes. It’s a single Doric column on a square plinth, and it’s about 61 meters high, and you can go up a spiral staircase inside the column to a viewing platform on the top. But this was also built as a zenith telescope. It had two uses: firstly as a monument and tourist attraction, but also as a telescope. And so you could go and stand at the bottom of the Monument and look directly up at the zenith. And there was a little laboratory there, which is now the ticket office if you go and visit the Monument.

The base and the column are both built out of Portland stone. This stone was really coming into prominence then. It had only been used in London from the beginning of the 17th century. But after the Great Fire in 1666, King Charles II and his architects, like Christopher Wren, saw this as a huge opportunity to build London as a modern city, as a stone-built city, rather than a city built of wattle and daub and timber and thatch. And that’s what they did. And they chose Portland stone as the stone that they were going to build their new city in. This really started the commercial-scale quarrying of Portland stone. And as I mentioned with Saint Paul’s, we do have good documentary evidence that Wren was corresponding with the quarries and ordering stone from them. And the Monument is, as I say, about 60 meters high. And if you turned it on its side, the top of the Monument lying on the ground would be at the spot where the Fire of London is thought to have started, on Pudding Lane.

Around the Monument, in the square in which it sits, there are lots of modern office buildings which are also built out of Portland stone, and they are of the more unusual varieties of Portland stone that I mentioned earlier, reef Portland stone, for example, that would not have been used by Wren and his fellow architects. It was seen as an inferior stone because it was too rubbly and too rich in fossils. They were specifically asking for clean, white Portland stone. They didn’t want stone that was rubbly and full of shells, as they put it. We now know all Portland stone is as strong as the other.

So we follow that route, and then on Eastcheap we find ourselves surrounded by 20th-century and indeed 21st-century office buildings. A lot of these are clad in granite. So at 16 Eastcheap, we see a 2.7-billion-year-old granite from South Dakota, which is called Dakota Mahogany. And this is a really beautiful orthogneiss. It’s not particularly foliated, it has a weak foliation, but it’s a beautiful chocolate-brown stone. But when you get up close, you can see it’s got a granitic mineralogy. It was a hugely popular building stone in the 1980s and 1990s.

And then from there we can walk onto Plantation Place, and this is a passage which runs parallel to Eastcheap. And the buildings there are all clad with a stone called Jura marble. Now, this is not a true marble in the geological sense. Masons use the word marble to mean any soft stone, I mean not a granite, any soft stone that will take a good polish. So this includes hard limestones as well as true marbles in the geological sense. So this is a limestone, but what I really love about it is that it’s full of really well preserved ammonites, nautiloids and belemnite fossils. And they’re really clear to see. You can see nice, clear spirals, and people who don’t know anything about geology can really see these fossils clearly and see what kind of animals they were. So you can talk about the functional morphology of ammonites versus nautiloids, for example. And it comes from Bavaria in Germany.

And then as we move along the Plantation Place passage, we find ourselves standing outside a most incredible-looking building called Minster Court. It was built in the 1980s, 1990s, in a style that architecturally is known as postmodern Gothic. It looks like something from Gotham City. In fact, it has been used in a number of films. And it’s completely clad with this really beautiful granite migmatite called Rosa Tuscoda, from Minas Gerais state in Brazil. It’s a really beautiful banded gneiss. You can see really good examples of mineral segregation between feldspars and more biotite-rich layers, and it’s folded and deformed. So it’s a really good place for talking about the processes that are going on in high-grade metamorphic rocks, and it’s coarse grained as well, so you can really see the properties of individual minerals in it: the quartz, the micas, mainly biotite, and two feldspars.

And then we come to a Victorian building that was once a pub on Great Tower Street. And this has a frontage that is clad in various granites, two of which are really classic examples of the kind of granites that were being used in late Victorian and Edwardian building in London. These were being shipped from northern England, from Scotland, and also from Scandinavia. So we see an example of Shap granite, which any geologist who has been educated in the UK will be familiar with. It’s a really good porphyritic granite, with very well defined phenocrysts of potassic feldspar in a finer-grained matrix, but nevertheless a coarse-grained rock overall. And we also see this really brick-red, very coarse-grained granite, which is from Scandinavia, in fact from the Kalmar coast of Sweden. And this is one of a group of Swedish granitoids that we call the coastal reds. There are various outcrops and intrusions that have been quarried. Shap granite, I should have mentioned, is Caledonian, so about 430 million years old. The Swedish granites are 1.4 billion years old, so much, much older rocks, and they’re in fact related to the Wiborg-style rapakivi granites that you may have come across, with their ovoid textures. This kind of granite doesn’t contain ovoids, but it has these very large brick-red feldspar crystals in it. It’s a quite stunning rock.

And then finally we’ve got a Canadian rock, an anorthosite called Blue Eyes in the trade catalogs, and it’s called Blue Eyes because it has schillerescent feldspars in it. Schillerescence is an effect related to the refraction of light in thin films, but here we’re getting refraction of light within the feldspar crystal, between twin planes and cleavage. And as a result of that, we get this kind of iridescent color, which makes these feldspars shimmer with peacock-blue and green shades as you move your point of view around it. And it’s a really, really stunning rock because of this phenomenon of schillerescence. And again, it’s a very ancient, early Proterozoic rock.

And then we rapidly move up the geological time scale as we come to the memorial to the Siege of Malta. This is cut from a single block of Maltese limestone. And I’m really interested in war memorials, because they are often made of stone from the country of the troops or the event that they’re commemorating. And in this case, it’s the Lower Coralline Limestone of Malta and Gozo, which is Oligocene in age. This piece was quarried on the island of Gozo, which is the smaller island of the Maltese archipelago. And I think what’s really interesting about it is that it’s absolutely stuffed with fossil sand dollars, which are very flat sea urchins. The species is Scutella subrotunda, for those that are interested. And you can see these creatures in cross section very nicely in this big block of limestone.

And then finally, we find ourselves outside the Tower of London, which is one of the few medieval buildings still existing in this area. And much of medieval stone building was built out of a stone called Kentish ragstone. I mentioned earlier that when Christopher Wren and his architects were clearing and rebuilding the City of London, they didn’t want to repeat the mistakes of the medieval world, when most buildings were built out of timber and plaster and thatch. They wanted to build from stone. So the buildings that survive from the medieval period are those that are built out of stone, and these are churches and the Tower of London, and they were built out of Kentish ragstone. Kentish ragstone has a small block size. It forms in beds maybe 30 centimeters thick at the most, and it’s quite well jointed. And it doesn’t cut into nice squared ashlar either, so the masonry looks a bit rubbly. Kentish ragstone is part of the Greensand, below the Chalk, so an early Cretaceous sediment, rich in glauconite, which gives it its name of greensand. It’s the classic building stone, really, of Roman and medieval London, and indeed I wouldn’t be surprised if many medieval structures built from Kentish ragstone used stone that was actually quarried from Roman archaeological remains, rather than quarried from the site where it outcrops, 20 to 30 miles away from London. It would have had to be brought up the Thames on boats. London’s built on clay. We’re much better at producing bricks than stone. So I really like this walk, because you see the history of London in stone, from ragstone to these modern granites from Brazil, but you also see a really good history of different styles of buildings and the use of stone varying through that time.

Oliver Strimpel: I know you’ve concentrated your urban geology on the UK, but are there cities in other countries that you think would have a particularly rich urban geology?

Ruth Siddall: Any city or town anywhere in the world that is built of stone will have a rich urban geology. I think this is something that you can do anywhere. There are some places where the use of stone is quite incredible. Italy, for example, has such a huge history of quarrying and a large number of very high-quality stones, like Carrara marble and pietra serena. Paris has got great stone. The Lutetian limestone was literally quarried from the catacombs underneath Paris and used to build the city above. But there’s also a large number of decorative marbles that were sourced in French territories, in the French Pyrenees, in what is now Belgium. These are used to great decorative effect in places like the Palace of Versailles and the Opéra Garnier and the Louvre: just a huge wealth of stone, something you wouldn’t necessarily think of about Paris.

There are places anywhere in the world. Washington, DC, has got great stone, imported from all over the USA for various monuments and buildings. I mentioned Athens, a city built of marble to this very day. I was also recently in India, visiting Jaipur and Agra. And India is also a country with a huge heritage in stone quarrying, stone masonry, stone carving, and absolutely spectacular marbles and sandstones that are used in structures in Rajasthan and Delhi: a really fantastic stone heritage. And with a little bit of reading of the local geology, you can find out about these stones. And by far and away, I’m not the only person who does urban geology walks. I have my equivalents in Washington, in Edinburgh, in India. There are people that have written on the building stones of these cities. If you’re visiting a new city, Google “is there a building stone walk for this city?” and you probably will find something.

Oliver Strimpel: Ruth Siddall, thank you very much.

Ruth Siddall: Thank you, Oliver. Thank you so much for inviting me to be part of this podcast.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you’ll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.