He had long shoulder-length hair and a droopy moustache. He was dressed all in denim. Very Seventies. He might have been a roadie for the Allman Brothers Band. Or – he had a mean kind of look in his eyes – a member of the Provisional IRA which had recently broken away from the mainstream of the republican movement. But Melvin Webber was not in California or the Falls Road. He was stood in an ill-painted backyard next to a couple of beer barrels in the Welsh Valleys. Merthyr hard man, the caption to the photograph read. Not to be messed with.

Or there was Mad Malcolm, who we learn was a prolific consumer of illegal chemical substances. He could have been a member of Thin Lizzy, his hair as curly wild as Phil Lynott's, a louche lace-up neck to the T-shirt he wore beneath his black leather blouson.

There were other indications of the era. The long hair of the 10-year-olds posing with a football stitched from leather panels in the old style. The TV dinner in its foil compartmentalised tray, very modern, on the table where the photographer's Aunt Rhoda is leaning dotingly over his younger brother Martin. And there was the studied self-conscious catatonia of the bored young gravedigger, with sweeping locks and bandito moustache like those of The Eagles in their Desperado phase, taking a break in their coal-warmed hut at Cefn Cemetery while his older boss, Tommy Gravedigger, sat just as quietly but with the composed stillness of the years about his set features.

That generation gap is, in its way, the subject of the exhibition of portraits currently on display at the Light House Gallery in Wolverhampton. For those signposts of the Seventies stand like lonely salients among the portraits of an extraordinary collection of characters who, the photographer Robert Haines says with hindsight, "look as if they have blown in from another century".

The hindsight may now be 20/20 but three decades ago, when these photographs were taken – between 1971 and 1972 in and around his home in the village of Heolgerrig and nearby Merthyr Tydfil – Haines already had an inkling that this world was on the cusp of extinction. Merthyr was no longer the Iron Capital of the world. And the older generation among whom Welsh was the first language, and whose men had spent their working lives underground and the rest of their time in the pub and occasionally the chapel, were the custodians of a dying era.

Haines had grown up among them but gone away to London to study photographic arts. In the vacations he wandered his childhood haunts with his camera. The middle classes and the younger generation knew about cameras. But among the older, poorer folk they were still a rarity. Take my picture, lad, they instructed. He did and they were satisfied. It made them feel important.

The resulting portraits tell of more than a century vanished. Some are timeless and universal. The steady dignity of Nan Haines, maker of the world's finest Welsh cakes. The perky comedy of Dai Passmore and his terrier on the slagheaps above the village. The muffled anguish of the old woman crying, dressed in black, as pit widows had been for generations. The bony features of Martha Lewis, paring vegetables, as her husband gazes into the hearth, demonstrating that the art of meditation is not some New Age import to these shores. An old woman shovelling stones after a flood or another knitting on the doorstep of her terraced home. Retired colliers who loved rabbiting and foxhunting, riddles and rakes, feathers and fishing.

There was a directness and an innocence to it all. No one questioned the motives of the photographer. They just made eye contact with him and sat, comfortable within their own skin, with an extraordinary dignity, each enjoying their little moment of history. Old Man Jenkins looking like an 18th-century East European rabbi. Dai Llewellyn the gurner. The barmy smile of Eddie Abrahams who didn't go out much but when he did always got blind drunk and invariably ended up lying on the floor, showing off his technique for cutting coal. Bill Baldy, with all his worldly possessions, sitting matter of factly, smiling slightly, on the bare mattress in the damp-patched room he shared with five others in his lodgings in the Castle Hotel – a "bloody doss house" as he described it, without shame or self-pity.

The generation of middle-aged folk between them and the youngsters has a dated feel about it. Their leisure too was from a bygone era, shaped by the cinema not the television. Billy Diana earned his nickname by gyrating his hips, Presley-style, when he'd had a few, serenading the women with his signature rendition of Paul Anka's 1950s classic, "Diana". Bryn Dan, a ladies' man, with his pomaded backcomb, and his succession of finely carved walking sticks, all his own work. Or Tex Jones, who lived on the Gurnos Estate but who was crazy about the Wild West, who hand-worked his own leather gunbelts and made a stool out of a cowboy saddle on which he would perch, eating cold baked beans out of a tin.

It was a hard life and these were people hardened to it. And yet they were cultured. Dick the Rock, the stone mason, wrote Welsh poetry. Art was encouraged and admired by all. At gatherings everyone without exception would sing. When Haines' grandfather died he took to the grave a score of unrecorded old Welsh songs. His great-aunt told him that her son was going to waste his life because he had become a barrister when he should have been a musician or an artist.

They have gone now, these ancient silicotic colliers. Slowly expired, their last years spent chained to an oxygen cylinder. But then so have many of the youngsters too. The Merthyr hard man, Melvin Webber, lived hard and died hard, after being blasted with a shotgun. Mad Malcolm died after hitting a lamp post at high speed on a motorbike while fleeing the police. But if the captions are all retrospective, the photographs keep something wonderful alive, the fag-ash dangling timelessly but never falling.

'Once Upon a Time in Wales' by Robert Haines is showing at the Light House Gallery, Wolverhampton, to 9 May, then touring. A book of the photographs is published by Dewi Lewis, £14.99

Link: The Independent