On one lofty wall of the beach- facing gallery at Tate St Ives, David Nash has made a monumental charcoal drawing. It traces the epic, 26-year journey of his Wooden Boulder from its origins high up in the Vale of Ffestiniog down to the Irish Sea. Having used a chainsaw to hew the boulder from a stricken oak tree, Nash followed the progress of his wayward sculpture as it tumbled over waterfalls, rested in pools, floated down the main valley river and then moved restlessly around the Dwyryd Estuary. After becoming stranded on a sandbank and then loitering in a salt marsh, it finally disappeared in April 2003.
Nash had known the boulder would vanish one day, but he is still searching the estuary creeks in the hope that it might reappear. The lengthy peregrination of his sculpture is recorded in a film at Tate St Ives, tracing the meandering path it took through the Welsh countryside. The aftermath of the boulder's disappearance is melancholy, but Nash's drawing does not rule out the sculpture's re-emergence in the future. Watching the waves advance on Porthmeor Beach outside, visitors can imagine his boulder bobbing through the surf one day and ending up on white Cornish sand.
Although Nash has lived and worked in Blaenau Ffestiniog since the late 1960s, when he purchased the Victorian chapel where an impressive collection of his sculpture is still housed, St Ives provides a felicitous context for his work. The act of carving, no less than the concentration on fundamental geometric form, links him immediately with Barbara Hepworth. Like her, Nash has made his home in a remote part of Britain. He also shares Hepworth's early commitment to the importance of "carving direct", as well as her passion for simplifying the language of sculpture. The cube, the sphere and the pyramid dominate this exhibition, but there is nothing dry or limiting about Nash's geometric abstraction. Instead of letting it become rigid and predictable, he always allows these forms to be shaped by his profound understanding of wood.
Nash always uses unseasoned wood from fallen or uprooted trees. After he has sliced it with his chainsaws, the material begins to warp, wrinkle and crack. The changes are as inevitable as they are unpredictable. Dramatic black fissures run up the side of Coil, where he has also retained a strong sense of the original oak trunk. It curves and bulges, asserting the vitality of organic life before suddenly opening up to reveal a shadowy cavity within.
Coil began with a mistake - a "wrong cut". Nash is attentive enough to the character of wood to benefit from happy accidents of this kind, yet he would never let any Hepworth-like belief in "truth to mat-erials" push him to be passive. Reserving the right to impose his own imaginative priorities on the material, he created the tall cluster of lime called Sheaves by cutting vertical lines into the pale wood. Viewed from the front, they possess a stern and unyielding authority, but from the side, Nash still allows the tree's original identity to assert itself more strongly.
Hepworth was the first British sculptor to cut right through her block, implying that the inner void had a vital role to play in the finished carving. Nash followed suit when he made Elm Frame, Fourteen Cuts. As well as administering 14 equidistant slices to the wood all the way along its surface, he opened it up from front to back. So the conventional notion of sculpture as a three-dimensional bulk is challenged.
The rival claims of strength and fragility also nourish his work. In an upstairs room, we find Crack and Warp Column stretching towards the ceiling. Carved out of lime, its bleached colour lends the work a sense of vulnerability. So does the incessant horizontal cutting, administered at regular intervals from base to summit. The work resembles a column of plates, stacked one on top of the other, ready to crash. For all its potential frailty, however, Crack and Warp Column suggests a proud inner resilience.
Nash finds the textures, ages, colours and structures of wood inexhaustibly fascinating. He makes us return to them over and again, appraising the intrinsic qualities of oak, beech or elm as if for the very first time. But he is also prepared to expose his material to ordeal by fire, turning wood into carbon. The final room for his Tate show is inhabited by a trio of large burnt forms: a cube, a sphere and a pyramid. Ranged in a row, these dramatically blackened works recall Plato's idea that we all know geometric forms from our experience of the spiritual world. They look utterly removed from human comprehension, and implacably bound up with death.
In this respect, Nash's charred carvings seem timely as well as primordial. They reflect our troubled 21st-century world, where the urge to burn and destroy has taken on a new vehemence. At the same time, the resilience of the cube, sphere and pyramid is oddly heartening. They have undergone the fury of the flames only to emerge defiant, intact and immutable.
"David Nash: making and placing abstract sculpture 1978-2004" is at Tate St Ives, Cornwall TR26 (01736 796 226) until 26 September