CLAIRE Midwood is crouched in the middle of the gallery with a lump of mud at her knees. She's in the dead centre of the airy exhibition hall, which is set on the rolling hills of the Yorkshire Sculpture Park, kneading the brown gunk into the shape of a large cannonball. It is both elegant and fetid, attractive in form and repellent in substance.
Every day the gallery's cleaners gather more mud, sweeping it up from the floor of this rural beauty spot and brushing it in Midwood's direction. And every day she adds another layer to the ball. Her task will not be over until January 2008. "My boyfriend says I stink," she says. "I have a bath every night."
The ball isn't the only thing making her smell. The enormous window that looks out at the Henry Moore bronzes scattered across the landscape has been caked with several layers of dung. With the sunlight on it, the excrement has the translucent texture of green fibres and gives the gallery the stench of an elephant house. There is a broad line of clear glass literally cutting the crap like a river winding its way from left to right, allowing us glimpses of the verdant hillside beyond.
This is what happens when you have Andy Goldsworthy for a boss. Midwood's mud ball and the encrusted window are the brainchild of the Dumfriesshire land artist, who is putting together a major celebration of his work in four sites across the park. For an artist who has created monumental sculptures from stone, snow and animal hair in a 30-year career, dung is merely one more raw material offered to him by nature.
Unlike Damien Hirst, Goldsworthy is working with such unpleasant material not to shock but to confront his audience with the reality of the landscape around them. By plastering a window with dung, interrupting our view, he refuses to let us treat the landscape as a picturesque idyll. Nature's beauty is built out of death, dirt and decay and Goldsworthy refuses to pretend otherwise.
"Nature is perceived by many as something that is romantic, pastoral, relaxing, peaceful and a backdrop for weekends before going back to the nitty-gritty of the real world which is the city. I would like to confront that. Nature is very harsh, difficult, rough, sometimes brutal, beautiful as well. There is this huge sense of energy, life, growth, death, decay. You can't walk very far without coming across something that is dead, decaying or dying, as well as being born and growing."
THE THEME CONTINUES on the walls of the Longside Gallery with a series of canvasses created near the artist's Scottish home. Laying each canvas on the ground, Goldsworthy positioned a mineral feed on top and invited the local sheep to tuck in. The result is nature's answer to a Jackson Pollock splatter painting: a medley of muddy brown footprints surrounding a moon-like circle where the mineral feed once stood.
But there's much more to Goldsworthy than sheep dirt, and the exhibition offers a sometimes breathtaking survey of his range. "This isn't a Tracy Emin," quips site manager Alan Mackenzie as we jump out of the Land Rover and approach a tent in a copse. "No, it's a godsend," says one of the workers sheltering from the wind as he takes a break from building a dry stone wall to Goldsworthy's specifications.
Entitled Outclosure, in punning reference to the Enclosures Act, the wall is a gorgeous circle of stone built just too high to see comfortably inside. Breaking the route of a path through the trees, Goldsworthy forces you both to consider the history of the land and to admire the skill of the craftsmen who have positioned some 120 tonnes of stone by eye.
We find the artist himself in the Underground Gallery beneath the visitor centre in the midst of more than 60 tonnes of wood. He is trying to figure out where to position the uppermost ring of sweet chestnut branches that are turning one room of the gallery into an upturned nest. There are more than 60 tonnes of wood in here, and the effect of being cocooned inside is overpowering.
The neighbouring exhibits are no less awesome. At the entrance he has built one of his cone-shaped towers of interlocking oak logs: 4.5m high, yet without a single nail for balance. There is also a room full of stone domes, thinly sliced layers of rock cut in diminishing circles like squat pyramids. Another room has been plastered with 20 tonnes of clay mixed with human hair and allowed to crack like a desert landscape.
Goldsworthy joins me by the most breathtaking exhibit of all: a curtain of horse chestnut stalks pinned together with blackthorns and straddling the width of a room. It looks like someone has suspended a sheet of Chinese calligraphy in midair. "The great thing about doing two or three rooms together is that one primes you for the other," says Goldsworthy. "Each room works your sensibilities differently."
The youthful energy that prompted him to get a tattoo of Elvis on his arm as a teenager has not deserted this friendly, articulate 50-year-old. Keen to press ahead with new work, he's reluctant to regard the show as a retrospective - "they're new works with their roots in old ones" - but admits it's something he couldn't have done in the same way at any point before.
"The idea of art reflecting an artist's life as they grow old is very important," he says. "I couldn't have made this work 10 years ago. To handle these spaces, to make connections with the work outside and with the social nature of the landscape, to understand not just particular trees and leaves but the overall context is something I've won over a long period of time."
Although Goldsworthy is the governing artistic force in all this, you can't help but notice that this operation is a communal affair. Whether it's Midwood with her mud ball, the stonemasons, the farmers, the tree fellers or the 16 volunteers who worked six days a week for three months on the clay wall, Goldsworthy's art is labour-intensive. "If there is a political side of the work it would be to say that people are an important part of the landscape," he says. "The British landscape is often read as being wild and untouched, and that is very wrong. People have worked the place and made it the way it is."
And of his choice of materials, he adds: "There's a subversive element about making art out of something that is very common and not often looked at."
There's also an interesting tension in the idea of making art that will rot, melt or crumble with time. Although Goldsworthy is a careful documenter of his work in the form of his popular photography books, he is frequently engaged in creations that are as ephemeral as the passing seasons. Despite this, he is fascinated by his place in time. He's not trying to make himself immortal, but engaging with the future.
"One piece I've made is a bed of rock where people can lie down in the rain and leave a shadow behind. That is a projection of a concept into the future. I love the sense of chance. Maybe it will be so successful that there will be queues of people in April showers waiting to lay down on the rock. I've no idea."
As he rushes back to work, I ask him if by working with the wonders of nature he could be accused of merely gilding the lily. That, he says, would be to miss the point. "I can never improve on what's there. As human beings we have to engage with the world. Why paint? Why write something? Because it engages our mind, it makes us look at the world. I don't feel I'm recreating the world, I'm just understanding it."
Andy Goldsworthy, Yorkshire Sculpture Park (01924 832 631), Saturday until January 6. www.ysp.co.uk