York Guildhall’s ancient seafloors

On the first Wednesday of each month, for York’s Hidden History, I lead a Guilded Tour of medieval York. The walk takes an hour, and we start and finish outside York Guildhall, but if there aren’t any events on, we might sneak a peek inside.

As a geologist, one of the things I particularly like about the Guildhall is that there are many different ancient seafloors hiding in the building. This is because a variety of marine limestones have been used during the building’s construction and conservation.

Ancient Seafloor 1 – Tadcaster Stone – 255 million years old

The 15th Century parts of the Guildhall are built from the classic York stone, which isn’t York Stone. It’s Tadcaster Stone, or – more properly – dolomitic limestone of the late Permian Cadeby Formation, laid down when Yorkshire was an arid, shallow, salty sea, much like the present-day Persian Gulf. If you peruse the Guildhall walls, especially where the stones have weathered, you’ll find an interesting mix of sedimentary and structural features. If you’re interested in exploring these in detail, just walk round the city walls: they’re built almost entirely from Tadcaster stones.

Ancient Seafloor 2 – Portland Limestone – 150 million years old

If you look above the main entrance to the Guildhall (see picture 1), you’ll see the stones in the upper part of the building don’t seem to be quite the same as those in the lower parts. They aren’t as weathered, and aren’t the same colour. This is because they are a different stone – late Jurassic limestone from Portland, Dorset – that was used in the mid-20th Century after the Guildhall was badly damaged in the Baedeker Raids.

Unlike Tadcaster Stone, which is formed of magnesium calcium carbonate, Portland Stone is made simply of calcium carbonate. If you look at it with a magnifying glass, you’ll often see very round shapes in the rock. These are ooidal (‘egg-shaped’) grains of limestone, formed by waves agitating small particles in warm, clear, shallow seawater. You can find them forming in the Bahamas today*. Portland Stone also has lots of Jurassic fossils in it, particularly bivalve shells, such as oysters.

(*you can also find ooids in Tadcaster stones, particularly those with a slightly pinkish colour.)

Other good spots in York to see Portland Stone and its fossils include the York Explore Library & Archive, the Virgin Money branch on Coney Street, and the NER War Memorial on Station Rise.

Ancient Seafloor 3 – Lincolnshire Limestone – 170 million years old

Step inside the recently renovated hall and the polished stone floor immediately catches your eye. Most of the floor is a creamy-coloured stone with a spotty or slightly swirly texture. Sold as ‘Jurassic malt‘, this is mid-Jurassic limestone from southern Lincolnshire. At first glance, the spottiness looks quite like the Portland oolites, but the Lincolnshire stone is rather coarser-grained, and the particles aren’t as regular or as rounded. Look closer, and you’ll find the stone is rich in fossil shells and skeletons, including at least one nicely preserved Jurassic coral.

I note that an ammonite fossil was discovered recently in Lincolnshire limestone at Lincoln Cathedral, so we should keep our eyes peeled! And if you want to get an idea how Lincolnshire limestone weathers when exposed to the elements, the late 19th Century frontages of St Michael-le-Belfrey and St Helen’s churches are both built from it.

Ancient Seafloor 4 – Purbeck Stone – 140 million years old

Inlaid on the floor between the larger blocks of Lincolnshire limestone, are smaller, darker (blue-grey) stones, which also polish nicely and are also full of fossils. These rocks are muddy limestones, early Cretaceous in age, and were formed in a shallow lagoon. They are quarried around Langton Matravers, Dorset, where the stone is known as Purbeck Grub. Geologically, it belongs to the Stair Hole Member of the Durlston Formation. If you’ve ever been to Lulworth Cove and admired the crumpled rocks, you might have seen it in situ.

Many years ago, as an undergraduate, I spent several weeks hunting Jurassic oysters along the Dorset coast, including in this very rock. As an older, much un-fitter individual, I’m pleased I can now revisit these formative days in a quiet, undercover space in my home town.

Bonus Ancient Seafloor – Rouge Royal Marble – 375 million years old

The floor and walls of York Guildhall are built of the four limestones described above. However, if you go to the reception desk and ask nicely (hi, Wendy!), you might be allowed to take a closer look at the staircase going up to the offices and council chambers. Its red, grey and white stone is rather gaudier than those we’ve already discussed, but it also tells a tale of an ancient seafloor, and one that is more than 100 million years older than the others.

Installed in the late 19th Century, this stone is known as Rouge Royal Marble, and comes from southern Belgium. Geologically, it isn’t a marble. Geologists only use that term for carbonate rocks that have been baked by contact metamorphism, destroying the original sedimentary structure (and any fossils the rock contained). To a stonemason, however, a marble is a hard carbonate rock that polishes well, which can include both limestones and marbles. And as stonemasons have been working with rocks far longer than geologists have, their terminology takes precedence!

Either way, there are definitely fossils in the Rouge Royal Marble, including brachiopods, bryozoans, and cephalopod molluscs. 375 million years ago, at the end of the Devonian period, southern Belgium (and indeed, south-west England) was part of a sub-tropical carbonate platform, with lots of patch reefs. There’s still some debate about why the rocks are red-coloured, but it may be due to sponges rotting as they were buried and turned to limestone.

The stone also many other interesting structures: the grey parts are often layered; the red parts can look clotted; the white parts are thin and jagged and cut right across the other units. They provide a nice case study in limestone diagenesis, the process by which sediments turn into sedimentary rocks. The grey and red layers are the sedimentary material (including stromatactis, about which I have to confess I know very little). The white jagged lines are mineral veins that formed when the rock was broken by tectonic forces, after it had turned to stone. They probably tell a tale of the Variscan Orogeny, but that’s another story (and one to be told by a structural geologist, not me!).

So there you go. Five ancient seafloors in one building, covering 235 million years of Earth history. And fossils galore! If you want to download a portable guide to the stones of York Guildhall, you can do so below. And if you spot anything interesting, do post me a picture!