In this month's Crag Notes, Jim Perrin takes us to Mowing Word - a crag that he developed back in the late 1960s. Nowadays its walls are adorned with classics, but back then it was something of a blank slate. " I had this marvellous cliff to try lines upon", says Jim. "This gleaming-orange, sunset wall of massively-bedded limestone's been my favourite cliff ever since".

I'd heard of the Castlemartin cliffs from John Rees Jones, first North Walian climber to repeat 1950s Rock & Ice routes. His family ran the chippy next to Joe Brown's shop on Llanberis High Street. John's a short, powerful Welshman, full of astute energy and insight. I learnt a lot from him, not least about Pembroke, where he'd made the first recorded climb (still a two-star VS) in 1962.

At Christmas 1968 Colin Mortlock and I were on wave platforms under Stackpole Head. Combers crashed in straight from the Atlantic, piled into the zawn alongside, towered above us, recoiled in a chaos of foam and noise. I knotted a bowline round my waist, set off into the unknown, and led Stackpole Grooves Direct (now E1, 5b). As introductions go, it was captivating. Afterwards I saw the west face of Mowing Word for the first time. Love at first sight! This gleaming-orange, sunset wall of massively-bedded limestone's been my favourite cliff ever since.

It had crack-lines, I was brought up on grit, itched to get on them. By 1969 I was working at the Oxford L.E.A. centre Mortlock ran in Glasbury-on-Wye. We went to Pembroke occasionally. Climbing with your boss is a bad idea when he's a fading force and you're a pushy young git. Tensions resolved two years later in a flaming clifftop row. But that's another story. This one's about how a Welsh sea-cliff classic evolved.

In those far-off days, the pioneering ethos was on sight, ground up, cleaning as you went, without abseil inspection, using minimal aid if any. Given how low achieved grades then were, it was a viable mode, and an adventurous one too. It held sway until the likes of Livesey, Drummond and Williams brushed it impatiently aside in the 1970s revolution.

Before that, I had this marvellous cliff to try lines upon. The first, in 1969, was a pleasant HVS chimney, Cormorant Flake, on the south face. Whilst wandering around on ledges beneath it I was drawn to the great square cave on the left – the heart of darkness! Waves thundered into it. Its walls were greasy and damp. I set off on a low traverse across its right wall. After 30ft, with a big cutaway behind my head, no gear, Mortlock well out of sight and tugs on the rope imparting a sense of his anxiety, I came scuttling back, having registered the wall's steepness. We went instead to look at the crack splitting the huge orange wall above the cave. If ever a crack winked, this was it. Balanced on the pinnacle at its foot (still there, astonishingly!) I reached into the crack. It was blocked with thick clay.

In August 1971 - Mortlock and I no longer on speaking terms - I returned to Mowing Word. John Greenland was an Oxford teacher, (now a cabinet-maker in Hay-on-Wye), who'd done very little climbing but was a natural. The weather was stormy-wet. After a couple of routes on the south face (where there's another tempting crack to which I'd later return), with John belayed off to the left I balanced again on the pinnacle. It trembled as rollers slammed into the cliff. Somehow, in the intervening years, the clay had washed out of the crack. I swarmed up, bunching and uncoiling on perfect locking jams with minimal gear. John seconded, we thought it maybe HVS, and were well pleased.

A day of rain, then we were back, soloing down Square Chimney (the usual descent in those early days), traversing under the south face on scrunching holds decorated with sea anemones, and continuing round the arête to the comfortable ledge from which, two years before, I'd headed into the cave. This time I made a few moves up a grey slab, reached the lateral corrugations that stretch across the face, and peered round. Horizontal cracks led onto the overhanging wall. You could jam along them, even fiddle in gear. There were enormous roofs above, an amazing sense of elemental drama below. The main anxiety was protecting John when he came to follow difficult moves just round the corner.

But he was a cool guy, coped without fuss in the most impressive situation he'd ever experienced. We continued to Dièdre Sud, finished up that. A cold grey mist was blowing in. The waves were huge, the atmosphere elemental. We were well pleased, but something still rankled. Two climbs over three days on a relatively unknown cliff, both with startlingly good sections, neither of them outstanding routes! I moved to London that autumn. In The Crown on Highgate Hill one October evening John Kingston suggested a quick trip out west. We went to Mowing Word, put the good bits together, saw how the flake was cemented to the face by flowstone.

Heart of Darkness/New Morning – I did it last with my son Will, not long before he died. There was a rope he told me not to use. "I had to chop a bit off that, Dad!" The climbing was way below his usual level, but he loved it. It's the best rock-climb I've ever done, is in the new Hard Rock. Maybe I'll do it again next year, for its fiftieth anniversary. I'll still feel anxious peering round the arête…