How I hang on to hope in a tide of fear

If you stroll along the "infinite shingle" of Chesil Beach in Dorset, as Ian McEwan did while composing his new novel, you will find that millennia of tides and winds have "graded the size of pebbles" along its 18-mile length, "with the bigger stones at the eastern end". The writer went to check this out, and felt - as he weighed the pebbles in his palms - that it was true.

Already, critics have lauded On Chesil Beach as a major achievement from a painstaking micro-historian of the inner life. Edward and Florence, its loving but fatally innocent couple, stumble into a wedding-night disaster in the "buttoned-up", respectable England of July 1962, the victims not merely of "their personalities and pasts" but of "class, and history itself ". Yet long-haul admirers of McEwan will detect some even deeper rhythms at work here. Once again, he traces the ominous crossing of a threshold from one human state to another: a step into the dark framed - as often in his fiction - by the inexorable onward movement of maturing and ageing bodies, of biological evolution, of climate and even geology itself.

We talk in a restaurant in Fitzrovia, a short walk for McEwan from the handsome house in a Georgian square that he fictionally lends to the neurosurgeon Henry Perowne in Saturday - another novel that pivots on momentous changes, all the way from the medical to the military realms. Upstairs, there seems to be a meeting of the revived Bonzo Dog Doo-Dah Band, exactly the kind of wacky pop pranksters that Edward, in the lonely hippie-era limbo where McEwan's epilogue leaves his stubborn hero, might have promoted in his Camden record shop.

Outside, the sunshine signals another kind of transition, from winter into spring. And McEwan, a model of quietly spoken exactitude with words and ideas alike, stresses that On Chesil Beach aims at more than just the scrutiny of that early-Sixties cusp of change between - as Philip Larkin and almost all the reviewers have put it - "the end of the Chatterley ban/ And The Beatles' first LP".

For all the pin-sharp evocation of a time when "youthful energies were pushing to escape, like steam under pressure", this last gasp of British sexual inhibition gave his story a starting point and not a terminus. "I never really thought of it as a historical novel," he explains, "because I was interested in another aspect: which is when young people cross this line - the Conradian shadow-line - from innocence to knowledge. You're also dealing with a human universal. So I was rather interested to discover what young people would make of this. And I was quite relieved, for example, that my sons took to it avidly - even though they're living at a time when they not only have girlfriends, but they have lots of friends who happen to be girls: another world."

The book also survived a test-run beyond McEwan's family (his wife is the journalist and author Annalena McAfee, and he has two early-twenties sons from his first marriage). He read an extract at Hunter College in New York, to the sort of student body who might have been forgiven for failing to sympathise with the bedroom blunderings of a pair of virginal Home Counties 22-year-olds in the summer before the Cuban missile crisis. "This is a community college," the author says, "and the kids are - tough is not the word, they're really lovely, but they're not protected. They 've clearly been out there." Would this street-smart audience think: why don't Edward and Florence "just get on with it? What's the problem? On the contrary: they seemed deeply engaged.

"So there have to be two elements running side by side," McEwan continues. "One is that, this is particular: these are characters frozen in history, limited by psychology, by class, by private experience. But on the other hand, this is a universal experience that is differently dressed up by different people at different times." Youth always has to cross that line, even if it would no longer run through the starched sheets of a marriage bed in a dowdy Dorset hotel.

Always the punctilious realist, Mc-Ewan nonetheless skirts the seas of parable, or myth. Yet for this, the 12th work of fiction since his 1975 debut with the luridly memorable tales of First Love, Last Rites, he wanted to avoid wading in too deep. "This particular beach offered so many metaphorical possibilities," he says. "They could kill the novel! So I really had to row back quite hard on that. The fact that impersonal forces have created order; the fact that the last scene is played out on a tongue of shingle, so you're stranded on both sides; the sense that they sit down to dinner on an evening when they both hope to gain knowledge, which clearly relates to being on the edge of the known world... It was so rich, that I had to keep the volume down."

McEwan's fiction strikes so hard and lingers so long in the imagination precisely because he keeps the interpretative volume down. "Readers will rebel," he believes, "when they spot an overriding, determining metaphor." Or, perhaps, a determining cause. On Chesil Beach hints at a specific reason for Florence's "visceral dread" of sexual experience, one that throws a line from this work back to the toxic households of those earliest stories. Her creator reveals that "in an early draft, it was all too clear". The finished work allows more space for the reader: we can join the dots through the past ourselves, just as we can fill in the futures to be enjoyed or endured by both after the act, or failure to act, that will mould them. Edward, the promising historian, now seems headed for a life of amiable counter-cultural drift; Florence, the driven violinist, stands on the brink of a solitary musical destiny.