John Christopher has written several novels of global catastrophe,of which this is certainly the best.
The basic premise is that of extreme earthquakes on a worldwide scale, which reduce towns and cities to piles of rubble and plunge the survivors straight back into the Stone Age. Much of western Europe is drastically uplifted, transforming the English Channel into a muddy desert overnight - whist elsewhere, lands are thrown down and drowned under inrushing seas.
The cataclysm and its aftermath are seen from the viewpoint of Matthew Cotter, a Gurnsey horticulturalist who finds himself one of a handful left alive on the former island. The future they face, attempting to begin life again with what they can scavenge amid the devastation, seems hard and uncertain enough.
Matthew then treks across the empty seabed to England, in the faint hope that his student daughter has also survived. He finds the situation far worse in a wider land, with many competing bands of scavengers. Pillage, rape and murder are now the norm as mankind revets to utter barbarism.
The actual scientific likelihood of such immense convulsions in the Earth is very doubtful, and the author's explanation - as a new mountain-building episode - is certainly nonsense, since such events take tens of millions of years. The sheer dramatic impact of a global earthquake, however, makes this book greatly entertaining for all but the most pedantic.
Its central emphasis is on the reactions of people, totally unprepared, who see their world turned (almost literally) upside down and everyone they knew destroyed. While some find natural strength and determination, even leadership, others respond with violence, with apathy and despair, or retreat into lunacy. John Christopher displays a subtle and far-ranging mastery of characterisation. He has created a stark and very believable vision of human struggles to survive in a world made suddenly strange, lawless, primitive and hostile.
It might have been even better to see Matthew Cotter and others ten or twenty years on, after the barbaric majority had mostly starved or slain each other and nature had begun to reclaim the shattered country. Would naval vessels have survived in mid-ocean and acted as nuclei for new communities? Or would the fallout from wrecked nuclear power stations have caused widespread cancers, sterility, mutations - and ultimately lethal new diseases, which would finish off the human race?
This is, surely, the essence of "thought-provoking" literature.
Regardless of unanswered questions, I would rate "A Wrinkle in the Skin" as being among the finest pieces of speculative fiction I have read.