On a windswept hilltop just off the A637, midway between Wakefield and Barnsley, stands a shiny sculpture that seems utterly at odds with the rugged scenery which surrounds it. Robert Indiana’s Love (1966) is an icon of 60s and 70s Americana, as redolent of that hedonistic era as Robert Crumb’s Keep on Truckin’ (1968). It is the centrepiece of a bold new show at Yorkshire Sculpture Park, which sheds fresh light on this enigmatic artist, whose most famous image has overshadowed his more intense and thoughtful work.

Love made Robert Indiana’s name, but it was a name he never really wanted. “It was a marvellous idea, but it was also a terrible mistake,” he said. “It became too popular.” He never courted publicity, and he was uncomfortable with the mainstream fame which more extrovert artists of his generation enjoyed. He retreated to an island off the coast of Maine, where he could work in isolation, and it’s these later, darker works — more troubling, more enduring — which linger in the mind’s eye long after the easier appeal of Love has abated.

His early life reads like a John Steinbeck novel. Born in 1928 in Indiana, he was adopted as a baby by a Midwest couple called Earl and Carmen Clark, who named him Robert Earl Clark (it was only when he turned 30 that he changed his surname to Indiana). Then came the Great Depression, and steady work became hard to find. Earl and Carmen roamed from town to town, from job to job, taking Robert with them. They divorced when he was 10. Thereafter he lived with Carmen, who scratched a living working diners. During his childhood he moved house more than 20 times.

It was poverty which drew him into sculpture

Despite these handicaps, Robert did well at school. He had a lust for life a thirst for knowledge. He won prizes for English and Latin, and a bursary to study drawing at the local art institute. In 1946, aged 17, he joined the US Air Force. Stationed in Alaska, he edited the in-house newspaper. Stationed in New Mexico, he started an in-house newspaper of his own. Art was an abiding passion, but so was the written (and printed) word.

The GI Bill provided discharged servicemen with free college education, and Robert grabbed this precious opportunity with both hands. He trained at the Art Institute of Chicago, and from there he won a travel fellowship to Edinburgh University, where he studied philosophy and literature. In 1954 he returned to the US and rented a warehouse studio in Greenwich Village, the engine room of New York’s dynamic contemporary art scene.

Robert Indiana was part of this community, but he was never at the heart of it. His precise typography was a world away from the Abstract Expressionist paintings which were so fashionable at the time. His work was closer to Pop Art, but his affiliation with that genre was actually fairly superficial. He shared Pop Art’s interest in commercial iconography, but he didn’t share Pop Art’s fascination with celebrity and consumerism. He was more concerned with Civil Rights. Homosexual — in an era when homosexuality was a crime, and then stigmatised — he felt a natural affinity with oppressed minorities of every stripe.

It was poverty which drew him into sculpture. He started out as a painter and a poet (words were integral to his art), but though his work was admired by other artists, he didn’t make much money. He had to do day jobs to make ends meet. Unable to afford large canvases, he scavenged wooden beams from demolition sites and started painting those instead. He called these sculptures “herms’, after the decorative signposts sculpted by the ancient Greeks, but what they most resemble is Native American totem poles. Alluring and disturbing, they’re the best things in this thought-provoking show.

Remarkably, this is his first major British exhibition. Coming only a few years after his death, in 2018, aged 89, it’s a fitting tribute to an artist who kept his own counsel, followed his own path, and let his work speak for itself. “It says so much about its time, and all of it is still relevant,” says Clare Lilley, the director of Yorkshire Sculpture Park, and the curator of this show.

The Moores and Hepworths at Yorkshire Sculpture Park feel as if they’ve always been here

And for anyone who travels here, Robert Indiana is just one of many treats on show. The site of the Yorkshire Sculpture Park — 500 acres in the grounds of a historic stately home — is a wonderful place to see sculpture against the backdrop of the open sky. A wooded lake in a broad valley, surrounded by weatherbeaten moors, it’s a natural arena which contains a broad range of scenery, from formal gardens to wild parkland. Some sculptures are hidden in dense woodland, others stand exposed on barren hillsides; some seem like part of the living landscape, others stand in violent opposition to it.

“The thing that’s so extraordinary here is the earth,” says Clare Lilley. “It’s the trees that are growing here, it’s the sky above. That’s the raw canvas.” The pictures that are painted upon this canvas, by sculptors and curators, keep on changing. The weather and the seasons also play a powerful part. On a sunny summer’s day, it’s glorious, “a stage for the performance of heaven,” as Ted Hughes put it. On a stormy winter’s day, it’s intimidating, but exhilarating all the same. You can see how this muscular terrain inspired two of Britain’s greatest sculptors, Henry Moore and Barbara Hepworth, who were born a few years apart, a few miles away (the Hepworth gallery in Wakefield and the Henry Moore Institute in Leeds are both a short drive away).

The Moores and Hepworths at Yorkshire Sculpture Park feel as if they’ve always been here — atavistic, elemental, relics of an earlier, forgotten age. The Robert Indiana sculptures seem intrusive and incongruous by comparison, but as the clouds part and the sky clears, they suddenly make sense. Set against these big Yorkshire skies, they recall the roadside signs he saw so many times, in so many nameless places, on the highways and byways of his childhood, travelling from dead end job to dead end job, from one horse town to one horse town. Robert Indiana did what every great artist strives to do: he transformed the mundane world around him into something beautiful. As I leave, the last thing I see is his Love sculpture, framed by the blue horizon, and for a moment its simple message fills my heart with joy.

Robert Indiana: Sculpture 1958 – 2018 is at the Yorkshire Sculpture Park until 8 January 2023.