A Wrinkle In The Skin

A Wrinkle in the Skin (aka The Ragged Edge) is a 1965 post-apocalyptic science fiction novel written by the British author Samuel Youd under the pen name of John Christopher.
A massive series of earthquakes on a worldwide scale reduce cities to rubble, plunging survivors into barbarism. Most of western Europe is dramatically uplifted, transforming the English Channel into a muddy desert, while elsewhere lands are plunged below sealevel & flooded.
The protagonist is Matthew Cotter, a Guernsey horticulturalist who finds himself one of only a handful of survivors on the former island. Cotter decides to trek across the empty seabed to England, in the faint hope his daughter has somehow survived. He finds the situation on the former mainland has descended to barbarism, with competing bands of scavengers preying on survivors. He & a young boy finally make their way to the borders of Sussex, where his daughter was staying, only to discover that the land has slipped beneath the sea. Cotter, along with some survivors from the mainland, eventually returns to Guernsey.