The journey from almost anywhere to the Tate St Ives is not unlike a pilgrimage. As you travel through time - possibly quite a long time, at a gruelling pace - both landscape and weather change, and you see many things: looming hills and plunging valleys; bleak moors and thick forests; glimpses of sea and secret coves. The Tate itself, nestled into a cliff above an impressive sweep of sand and sea, takes on the character of a shrine on a mount. Its cool, white exterior and apse-like entrance evoke the simplicity and monumentality of a Romanesque church, while inside the stained glass by Patrick Heron recalls the red and blue luminescence of a Gothic cathedral.
Although few of us these days walk such a journey (it may be quicker if we did), it seems a fitting prelude to the work of Richard Long currently at the Tate St Ives. Long, like his contemporary Hamish Fulton, makes his art by walking in landscapes, and he positions his work in the cultural history of walking - "from pilgrims to the wandering Japanese poets, the English Romantics and contemporary long-distance walkers".
It must say something about our times that the simple act of walking can be turned into art. But Long's involvement with landscape and nature is much more than mere nostalgia for a less urban age; there is something devotional to it, spiritual even. From Christianity to Buddhism, the idea of the path or way has great significance. On his journeys, Long makes sculptures using the landscape's elemental materials - stones, sticks, dust, mud, water - echoing Stonehenge or an isolated roadside shrine. And the walks themselves are conceptualised as sculptures - though these are about place instead of material and form.
For Long, as for Chaucer's pilgrims, the journey is more than a matter of getting from one place to another (indeed, some walks begin and end in the same place); rather, it is a way of experiencing, engaging and interacting with the world.
How, you might reasonably wonder, can this work be presented as art in a gallery? Well, Long records his walks in maps, photographs or text works (collections of descriptive terms) - what he calls the "distillation of experience". The text works on show, in particular All Ireland Walk, are a little like poetry, yet they are more like those children's puzzles where you are left to reveal the picture yourself by joining up the dots and adding the colour.
This is not the only way Long brings landscape into the clean, white spaces of the modern gallery. Early in his career, he was forced to confront the problem of exhibiting his work: in 1970, at the Dwan Gallery in New York, he pulled on his muddy boots and walked a spiral on the floor. Thus we have, in the current exhibition, two monumental mud works - Porthmeor Arc and Earth. In a matter of hours, Long slopped and slapped both works on to the Tate's walls using River Avon mud, China clay and his hands.
There is something minimalist and meditative about the text works, but the vigorous splashes, drips and swirls of the mud works suggest a religious fervour, and hit you with the force and drama of a huge altarpiece. While the former works "feed the imagination", as Long puts it, the latter feed the senses.
Despite the size and power of these mud-fests, Long shares the gallery with two large-scale ceramic installations by the Japanese artist Kosho Ito, as well as selected works by the Russian-born constructivist Naum Gabo. According to the Tate, these artists are linked by their status as "innovators in their particular field" and in the way they have "pushed the boundaries of what is possible in sculptural terms" - it sounds like the usual art-gallery puff, but the curators do seem to have some rationale in selecting such unlikely bedfellows.
Ito's installations strangely complement Long's work. Sea Folds consists of around 1,500 hand-folded ceramic pieces. These curving forms and the convulsing shape they give rise to recall not only the spiralling wave of a Japanese woodcut, but also the movement and gesture of Long's Porthmeor Arc. Meanwhile, Ito's Earth Folds, a craggy volcanic field that swamps the gallery courtyard, invites comparison (or perhaps contrast) with Long's semi-circle of Cornish slate on the roof terrace.
Clearly, an interest in natural materials and forms is a thread linking Long and Ito, one that also associates them with St Ives and its artistic community, past and present. But where exactly does Gabo fit in?
Constructivism is understood to concern the urban, the industrial, the modern - regular, geometric forms and man-made materials. "There was no such thing as rural constructivism," the art critic Robert Hughes has written. And yet, aspects of Gabo's life and work seem to challenge such assumptions. He lived in St Ives during the Second World War and had a significant impact on the community's landscape-loving artists - Peter Lanyon, John Wells, Wilhelmina Barns-Graham, as well as Ben Nicholson and Barbara Hepworth.
According to Gabo, space and time (themes not unfamiliar to Richard Long), are "the only forms on which life is built and hence art must be constructed". And he certainly looked to the natural world for inspiration: "Sometimes a falling star, cleaving the dark, traces the breath of night on my window glass and in that instantaneous flash I might see the very line for which I searched in vain for months and months." The cosmic traceries of Gabo's wood engravings, as well as the tactile Kinetic Stone Carving, the fossil-like Red Stone, and even Kinetic Construction (Standing Wave), point to his interest in the essential forms and movement of nature, and enter into a dialogue with the work of Long and Ito.
There may be vacuums in nature, yet no artist exists in one, as this ambitious exhibition asserts. The show's unique combination of works makes it well worth a visit. But be warned: if you do make the trip, you might be tempted to walk back.
Richard Long, Naum Gabo and Kosho Ito are at the Tate St Ives (01736 796 226) until 13 October