At www.theotherside.co.uk/tm-heritage/background/channelform.htm … what made the 22 mile gap between Dover and Calais?
There are geological faults along the line of the Channel and they are thought to be several million years of age. They mark where part of the crust has slipped – see above.
Earthquakes occur on the faults – one of them as recently as 1995. During the building of the channel tunnel it was important to know what the geology was at the Straits of Dover
the tunnel follows the Lower Chalk formation most of the way but the fault lines presented a hazard as the Lower Chalk had dropped at the fault. There were buried valleys on the sea bed filled with mud and sand rather than solid rock and if they had struck one of these the tunnel would quickly have flooded.