When I read about the Great Hedge of India in the book of that title by Roy Moxham, I assumed it to be a great leg-pull. That there had been a comparable landmark in India to the Great Wall of China, 2500 long and that nobody had heard about it, was remarkable.

Upon delving into it, it transpires that it indeed was a fact. For much of the nineteenth century, there indeed was a Customs Line, all of 2500 miles in length, designed chiefly to protect the government monopoly of salt in British India by keeping out salt smugglers from the princely states. In its prime it included more than 1700 guard posts, manned by 13000 Indians under British officers, and hundreds of miles of it really made of an impenetrable hedge, fourteen feet thick and as many feet high, consisting mostly of thorny brushwood.

It seems that this astounding historical revelation has been buried away all these years in departmental reports and forgotten memoirs, unobserved by scholars and travel writers alike and it has taken the author of the book, Roy Moxham, to resurrect it. He first read about it in a footnote of a second-hand book he had picked up in the Charing Cross Road, London, and he apparently spent years thinking about it, talking about it and researching it, before flying off to India to discover for himself any last remains of the Great Hedge of India.

It has to be said that the most exciting thing about his enterprise is the discovery of the Customs Line. Moxham, the author's writing could be expected to be dramatic, is in fact quaint and curious. He is an amateur researcher and his prose is endearingly amateur - rather like those round robin letters that novice travellers send to their relatives at home. He is by no means a novice traveller himself, having been a planter in East Africa, but his obsessional visits to India in search of his Hedge are described in his book with an innocent honesty and earnestness.

He was relatively new to India when he started on the quest. He had first gone there some years before and his original responses had been every traveller's first responses to India's immediate horrors. He had read everything there was to read about the Customs Line in the libraries of London, had studied maps by the hundreds and learning Hindi, had made many Indian friends, knew how to deal with rickshaw men and had even mastered the use of a satellite navigator.

Much of his book is a memoir of his indefatigable searches across the heart of the subcontinent, by train, by motor-rickshaw, by tonga and all too often on foot. He stayed in village huts, in small town hotels or with Indian families. He followed every clue, he obeyed every instinct, he questioned everybody he met, rich or poor, scholar or illiterate, in his fanatic determination to find remnants of the Hedge.

Hardly anyone had heard of it, but by Moxham's account nearly everyone was happy to help him on his way, and one Indian acquaintance in particular seems to have gone well beyond the call of friendship in sticking by him through thick and thin, prickly pear and bandit danger. In fact nothing very startling happened during these many friendless journeys, but there was always the possibility of sudden death by violence, disease or, one should imagine, sheer exhaustion, and at least he once found himself sharing a sleeping compartment with a chained and handcuffed criminal (who had to be unshackled in the middle of the night for him to go to the lavatory).

It would make a slight if charming travel book, but fortunately throughout the work Moxham returns to the fascinating subject of the Hedge itself. He had liked the sound of it when he started to tell us, as a sort of eccentric British folly, but he came to think of it as "a monstrosity, a terrible British instrument of British oppression." His view of the Raj in general seems to be conventionally disapproving - the Indian Mutiny for instance, always gets inverted commas - but he is particularly against the Customs Line because he says it denied millions of Indians the cheep salt they needed not just for their happiness, but for their health and being.

He is probably right, although be appears to have forgotten that in his original Charing Cross Road footnote written in 1893, a bigwig of the Raj, Sir John Strachey, himself denounced the line as "monstrous system," to which it would be almost impossible to find a parallel in any tolerably civilised country." Never mind: the Indians have forgotten it, the British have forgotten it and only Roy Moxham is left in mild fulfilment. And even he, when at the very end of his book discovers a last brief stretch of the Hedge (thorny acacia), cannot resist claiming it in retrospect to be the greatest hedge the world has ever known.