Finally, after four years living in County Durham, my Taller Half and I have started volunteering with the Durham Wildlife Trust. Today we had a geology training day, led by geologist David Lawrence. Half classroom, half field trip, we learnt all about the rocks under our feet and how they affect Durham’s people and wildlife.
First things first. There are three basic types of rock:
- sedimentary: layers of sediment that settle on the ground and get stuck together over time
- igneous: molten rock forced into or through the earth’s crust that form into hard crystalline structures as it cools
- metamorphic: rock that is transformed from one thing to another when heated or put under great pressure (sounds like alchemy to me)
County Durham sits mainly on sedimentary rock – limestone, sandstone, mudstone and coal – but it does have igneous and a tiny bit of metamorphic rock too.
Igneous rock is formed by volcanoes or when molten rock is pushed through fault lines in the earth. This latter process is responsible for the igneous rock in Durham. Long narrow bands of dolerite form the Whin Sill. This is famous for two reasons – Hadrians Wall in Northumberland is built along a sharks-tooth ridge of Whin Sill, and the Teesdale waterfalls High Force, Low Force and Cauldron’s Snout are carved out of it. As the molten rock forces itself through faults in the sedimentary layers, tiny bands of metamorphic rock are also formed. At High Force you can see all three types of rock if you know what to look for.
There might not be much metamorphic rock in Durham, but it plays a very important role. In parts of Teesdale the heat from the molten Whin Sill produced a sugar limestone – granular white crystals that look like sugar and while very fragile, creates the perfect habitat for the famous Teesdale Assemblage of arctic-alpine plants, including the spring gentian. You can read about Moor House nature reserve and the sugar limestone here.
The oldest rocks lie to the west. Known as Namurian, they are mainly limestone layers with rich mineral seams. Lead mining was serious business in days gone by, and in some places you have to take care not to fall into old limestone quarries.
Further east the rocks get younger. The Durham coalfield if formed of mudstones, sandstones and of course coal, and takes up most of the middle of Durham. There are few obvious signs left of Durham’s mining heritage, and nature has largely reclaimed many of the sites. It will take generations for the people to forget though, and the Durham Miners Gala is a significant annual event.
Our youngest rocks hug the east coast. Still mainly limestone, this limestone is special, as it contains not only calcium but magnesium too. Magnesium limestone is a North East speciality, and the majority lies in County Durham. The magnesium, just like the sugar limestone in the west, attracts its own unique array of flora and fauna. Much of it has been quarried out, but it turns out this isn’t a bad thing, as the quarries have been colonised by limestone specialists that struggle to compete on the loamy soils that have built up over the top.
Finally, we reach the sea, and this is where we spent the afternoon. Blackhall Rocks, part of the Durham Heritage Coast, was once the place where all the spoil from the coastal mines was chucked. There’s still some spoil left, trying to turn itself into a manmade sedimentary layer, although the sea has other plans. We are standing on Permian rocks and Quaternary boulder clay. Deposited by glaciers during the ice age, boulder clay is actually found all over Durham and is responsible for much of our soil – it’s like the icing on top of a layer cake. If you ever find yourself in a stony field wondering why there is so much builder’s rubble in the soil you are probably standing on boulder clay.
The cliffs at Blackhall are formed from an ancient coral reef sitting behind a ridge of dolomite limestone. Unlike the quarried limestone further inland, this is not a uniform slab of rock. It is formed of many layers, or laminates, laid down by successive growths of algae when this area was a warm shallow sea, and is the kind of thing that gets a geologist really excited. More information is in this Durham Wildlife Trust leaflet.
The limestones in this area have other interesting qualities. Cannonball rocks are balls of rock growing out of the base sediment. Known as concretionary structures, other patterns look like fossil corals, but they are in fact created from some kind of chemical reaction, presumably triggered by the warm, mineral rich waters in which the sediments were laid down.
All too soon it was time to leave the cliffs and head back to the minibus and home. There’s much more to Durham’s geological heritage than I can tell you here, but I hope I’ve given you a taster. What do the rocks in your area tell you?
- The North Pennines is a Global Geopark – the first in the UK.
- Photos from the day are on the Low Barns DWT volunteer pages.
ps: please don’t take my word for any of the geology stuff, and if I’ve got anything wrong I’m happy for you to point this out in the comments
pps: I’ve chosen my links to give you more info if you want it – so if you want to find out more please click on