What is the most possible reason for the English Channel to emerge?

M: Mary, can you tell why Britain is separated from the continent?
W: I guess I cannot. Maybe millions of years ago, at the end of the Ice Age, a great flood washed over a wide, low plain, and created the English Channel. From then on, the island of Britain was isolated from the rest of Europe,
M: I know there have been many people who have wanted to swim across the Channel, and some of them have succeeded.
W: What’s more, quite a lot of people have thought of building a channel since the 18th century, but no one thought this dream would come true. Now however, the Channel, the longest underwater passage in the world, has been built.
M: When did such a miracle occur?
W: A British team started to drill southeast from Dover and a French team started to drill northwest from Sangatte in 1987. The two teams met under the Channel in December, 1990, and they celebrated the completion of the 23-mile shaft. The tunnel was not open to the public until 1993 and now it is able to carry up more than 30 million passengers a year.
M: How long will it take to cross the Channel on the trains?
W: Only 26 minutes. In the old days, people had to spend hours on the ferry in rough seas.
M: It serves to strengthen the tie between Britain and the continent, further promote their mutual understanding.
W: You are right.