If Michael Jacobs had any doubt that he should undertake a perilous journey to the source of the river that flows from Colombia's mighty Andes to the tropical shore, it would not have survived the stare of the country's greatest writer. This book opens with a passage that freezes the reader as still as the gaze of Gabriel García Márquez immobilised its author: "his eyes as they were later that night, when they had turned into those of a caiman, opening every so often to look at me". Jacobs felt the maestro "offering his blessing to a journey I had already begun that night in my mind, up a river that was also a metaphor of memory".

Jacobs gets called a "travel writer"  as well he might after his epic voyage through the world's longest mountain range to write Andes  but this is a historical and archeological portrait above all. And now he moves towards other planes, deepens the level  the land and landscape are triggers for metaphysical reflections that often border on the psychedelic. Jacobs was once a bright and rising star at the Courtauld Institute of Art (until his loyalty to the brilliant art historian and Soviet spy Anthony Blunt cost him an academic career), and his writing here deploys a poetic ear and painterly eye to achieve ruminations more akin to those of Coleridge wandering Lakeland than the outward-bound adventurism of a "travel writer". The principal plane to which one is transported is that of time, specifically memory. "I remember absolutely everything about the river," García Márquez had said to Jacobs. But the memory is not just that of either writer, it is that of the river itself, a timeless inward eye.

Jacobs's aged mother is a major character in the book  along with his mobile phone. Mrs Jacobs, back in London, is suffering from Alzheimer's, and her deterioration infuses the son's adventure with guilt and filial powerlessness. Every so often, when a signal is available, the phone rings  Jacobs's mother "desperately asking for me. Then the line went dead", leaving the author to "speed down the hidden inlet, past banks of herons and into the epic expanse of the Magdalena". Another call received en route for places with names like Whirlpool of Death informs that Mrs Jacobs has "managed half a yoghurt. She had then tried eating a grape but had given up".

Jacobs is attempting to defy modernity with this voyage. After many adventures on the river he is "little more than an hour's drive" from his point of departure but there was "no point in thinking in those terms. We had moved into another sphere of existence".

The news from London ebbs and flows like the waters until Jacobs finds himself yearning "for the days when the only way of being contacted abroad was through a letter sent to a poste restante", or indeed "the days of Humboldt, when something terrible could happen to a member of your family and you wouldn't hear about it until a year or two later".

Meanwhile, the encounter with breathtaking nature itself is often charged not with the travel writer's usual yearning for synthesis with the wild but apocalyptic destruction by mankind, antagonism between man and earth: at one point the river is "swarming with iguanas". Jacobs's crew is engaged in a "pointless killing" of the animals, in which the writer is indirectly complicit, leading him to sense "that this environment I had violated would eventually demand its revenge".

A turtle is brutally chopped in half, moving Jacobs to reflect on how a friend's pet turtle would outlive her; a riverboat captain is "engaged in a quixotic struggle to save the wildlife of the Magdelena. He said if you killed off the river's animals, the whole world would die"  which, after a fashion, it would, and is.

So we reach this point that no travel book is supposed to reach, whereby the snake eats its tail: instead of breaking free into the wild he adores and does often find, Jacobs gives the lie to travel writing in this shrunken world conquered by mobile phones, contempt for nature and the plague of tourism.

When Jacobs reaches the mystical "Wood of Statues", a "hallucinatory world of suns, moons, phalluses, eagles, clawing snakes" in strange and ancient carvings, he is distracted by "the adjoining cafe and visitors centre the demands of modern tourism like a harness keeping in check elemental forces". Then there is the moment that turns the book's climax into a tragi-comic farce, which occurs just after Jacobs has drunk from the river's source  "the spring of memory, the fountainhead of creation, the entrance to the underworld". As soon as this rite is achieved, Jacobs and his companions meet the shadow that has been cast over the entire journey: Colombia's violent recent past. Jacobs is apprehended by a platoon of Farc guerrillas. "We have a problem," says the Farc deputy commander. Jacobs considers himself a dead man. But three things are happening, all utterly surreal.

First, Jacobs finds himself befriended by a young female guerrilla, who prepares him food. Second, as though to converge the book's themes of a world surrendered to kitsch, our feared Farc commander wants only to lecture Jacobs on Lenin before  more importantly  seeking his counsel on how to develop tourism in the upper Magdalena region, and set up a further visitor centre at the river's source. He promises Jacobs that if only Farc could implement its new aim to promote tourism, the Marxist-Leninists would take him to even more remote and compelling attractions. Thirdly, after Jacobs is released, the guerrilla redoubt is bombed by the Colombian air force, with all Jacobs's unexpectedly affable new acquaintances probably killed. We are not told whether he sees his mother again.

So as Jacobs navigates the mysterious river and distant land, he also explores great questions about our time, lives and deaths, and about banality, the end of adventure, "the utter pointlessness of it all". Jacobs loves this land and its people. Like Colombia itself, the river had become "with all its beauty, dangers and violent history, like a drug to which  I had become addicted". Yet it is with relief that he realises he had drunk not from the waters of Mnemosyne, goddess of memory but of Lethe, deity of forgetfulness, thus condemned to "return to earth, to learn all over again".