What tunnelling turns up for archaeology
Nowadays, we have the technology to build massive tunnelling projects, but the ground beneath our feet also contains the archaeological record of our cities. What might we be destroying by digging through our history? Craig Halsey is an Archaeological Project Manager from the Museum of London Archaeology and is involved in preserving and studying finds from tunnelling projects like this. He spoke to Dave Ansell.
Dave - Hi, Craig. Having listened to the descriptions of these tunnel boring machines, is there any archaeology left when it comes out of the other side?
Craig - Well, absolutely there is because, as been mentioned, they're tunnelling through London clay. London clay is about 60 million years old. It dates to the Eocene period. So, it doesn't actually have any archaeological interest to us. Where the tunnel do much of the impact when it's things like the actual stations or the shafts they need to dig touch to get the tunnel boring machines in. So, it's not the tunnels themselves. It's all the other associated infrastructure that needs to be done to make development work basically.
Dave - So, what sort of things do you find in these places?
Craig - Well, it really depends on where you are actually excavating. Most of the work that I've been involved with has been mentioned actually, out near the docklands out near Canary Wharf, parts of Crossrail. And beneath the present landscape there, you've got a very thick sequence of alluvial and peat deposits which date back about 10,000 years, what we call the Holocene. And there's various Paleoenvironmental Proxy Indicators as we call them. So, things like pollen, that can tell you about the past paleoecology and also how people have had an impact, the way the forest has been cut down and so forth. There's other environmental indicators such as diatoms and ostracods: diatoms are like a silica algae, ostracods are bivalves. We can use these to actually look at things like sea level change, changes in the actual river. Although it's not necessarily associated with actual occupation, it can set the background for within which Bronze Age populations used to exist. You can look at settlement patterns and things like that as well. Occasionally, you do get archaeology with these deep sediments. You can get things like Mesolithic flint scatters on the surfaces - the gravels or if you're particularly lucky, you can get things like timber trackway. So, that's the sort of where I've mainly been involved with, but many of my colleagues have worked more in Central London where the deposits are a little bit shallower. They have found Roman burial grounds, post medieval burial grounds, remains of Roman buildings and medieval buildings. So, it really does just depend on where you are and where you're digging the holes.
Dave - Are there any challenges with doing archaeology in the kind of environment of people building tunnels around you?
Craig - Absolutely. It's a question of, they're under a quite difficult time constraints. So, the last thing they really want is a huge amount of archaeology to deal with. But all these things can be programmed. We make an assessment over the site way before the works actually start and one of the things we'll do, we'll do a desk space study. So, we'll look at what archaeology has been found around a particular development site. We can look at historic maps. So, we've generally got a good idea about what might be there and also, we'll do some evaluation work as well, so that could be test pits or boreholes and just to get an idea about what's there before they actually start digging the hole. So really, we're there to try to minimise their risk. So, the last thing they want is when they start to undertake these works and if they find something completely unexpected that slows the whole process down, that's what we try to avoid.
Dave - Is it at all scary digging in these really deep holes as well or does that cause you any problems?
Craig - Well, what's different about this type of work and working with tunnels is that, it actually gives us an opportunity to look at sediments that we wouldn't normally get access to because some of these shafts can be quite deep. If we were dealing with them in more traditional methods, just using trenches, there's lots of problems you have. Water ingress is the main one, but when they put in these shafts, I think it was mentioned, these concrete caissons, they would normally de-water the site. That actually enables us to get these deposits which we wouldn't normally be able to look at. So, in some ways, it's a benefit. We can get more information from it than we would normally expect.
Dave - Brilliant! So, we get to understand how London used to be. Thank you very much, Craig Halsey who's from the Museum of London Archaeology.