Here's a fantastic talk by Dr Jenny Collier explaining how Britain became an island. (The video should auto-skip to 1m 52s, skip there yourself if it doesn't cos that's when things actually start.)
To begin with, Britain was a peninsula of mainland Europe, connected by a wide lowland area criss-crossed by low hills, much like the landscape of Kent and northern France. A chalky rock ridge separated Dover and Calais, but chalk is weak and erodes easily.
Major rivers such as the Rhone and the Rhine flowed west from Europe and into the Atlantic, but encroaching ice put a stop to that. Their paths blocked as the ice expanded southwards over most of Britain and the North Sea, the rivers began draining into the lowland area just north of the ridge - the area now between north Kent and Essex on our side, and Belgium and The Netherlands on the European side.
A lake formed here. It grew.
As it grew, so did pressure on the ridge. Eventually (and possibly triggered by an earthquake - they do happen in these parts), the ridge fractured, and the lake plunged outwards and south west-wards, carving out the beginnings of English Channel. It was what geologists call a "catastrophic event", which for you and me means a natural disaster.
It would have been quite a sight: the ridge collapsing to its southern side, and huge volumes of lake water whooshing over the lip, widening it in the process. The force of the water gouged out holes in the land at the base of the ridge - they're still there, Jenny's sonar picks them out very clearly.
The floodwater roared across the low-lying land between southern England and northern France. Existing river systems were overwhelmed. New islands were formed, but also a new barrier. Presumably (Jenny doesn't mention this in her talk, but this is my assumption), when the ice sheet melted later, the floodwater widened the gaps further. The Channel we know was created.
Thanks to Geology in the West Country.
Filed under: notes
(14th June 2013)