The Big Flood
Chris - When did Britain become an island in its own right and for a long time that's been a big question but some scientists from Imperial college reckon they solved the puzzle earlier this year. One of them is with us this week and it's Jenny Collier. What do you think happened? How did we become the island that we're very proud of having?
Jenny - Well, it's been something that's been known about from other scientists for some time, particularly people looking at the human occupation of Britain. So we've been hearing a lot about very ancient ancestors of ourselves, humans. But humans first came to Britain about 700,000 years ago (at a very famous site of Boxgrove). Basically during this period, this is the quaternary, so the ice ages were coming backwards and forwards. Living in England was a bit marginal, if you like. Ancient man was coming backwards and forwards. Crossing to France and back into Britain basically as the ice came down and retreated. What the archaeologists have known for some time is that there's a big gap. Basically the humans were coming backwards and forwards for some time and then, for some reason, about 100,000 and 60,000 years ago there's no evidence for any humans at all. What we did in our group at imperial as a group of geologists was to go out and do a marine geophysical study in the English Channel where we literally measured the water depth. We found evidence for a catastrophic, large volume flood, which we think effectively cut Britain off. This helps explain why ancient man couldn't get back in.
Chris - Two questions, then Jenny, what do you look for on the sea bed that tells you there was a flood? What's the giveaway sort of signature of a flood?
Jenny - What we do in terms of telling a catastrophic flood from a normal fluvial process, as you might imagine, if you've got a very high volume of rapid-discharge water you get all sorts of scours and eddies, a whole group of landforms which you can put together and say they can only be formed by high volume water discharge and that means it's not a fluvial river. Frankly it's enormous. I've been working in a lot of exotic places and I never expected to find something quite so astonishing.
Chris - So a massive flood about, when? 500,000 years ago?
Jenny - Yes, we haven't dated it yet so from a geological point of view the only way we could date it - we call it the smoking guns. We've got the bit that's been carved out. The bit that's been found. We need to go and look towards the edge of the shelf to find out where the sediments that have been carved out had been dumped. We haven't done that yet.
Chris - Where do you think this massive flood came from and what triggered it?
Jenny - Well, that's right. So we've got this great big flood channel and basically it points to the straits if Dover and it's been well known. Again, mainly from people looking at animals and also plants that the straits of Dover was a land bridge that survived throughout these glacial fluctuations so man was happily trotting backwards and forwards across between the England and France across this land bridge which was just a geological structure. We made this hypothesis that the only way to get such a sudden water release would be to have a catastrophic breaching of the rock dam across the straits of Dover which would have releases basically a lake that built up in the southern North Sea.
Chris - And where did that Lake come from?
Jenny - Well as you imagine during a glacial period there's a lot of - we're on the edge of the glacier so there's all sorts of melt water coming from the ice itself. All of the rivers from the northwest of Europe (so for example the Rhine) use to go through the North Sea out into the Atlantic, would all get trapped and build up a big lake which would be dammed behind this straits of Dover rock ridge.
Chris - And that's what cut us off from France and made us country in our own right. Jenny thank you very much.