The boulder stands above North Gully on the East Face of Tryfan. Photo: Chris Lloyd/Ogwen Valley MRO

Walkers and climbers are being warned of a potentially hazardous boulder on the slopes of Tryfan in Snowdonia.

A large stone block was observed by mountain rescuers during a recent incident on the East Face of the mountain.

Members of Ogwen Valley Mountain Rescue Organisation were called out on Saturday to aid two walkers who were stuck near the bottom of North Gully. The pair had missed the safer route into Little Gully and were brought to safety by rescue team members.

During the incident, the team members made a note of a large boulder just below the junction of the two gullies. It is estimated it weighs about four tons.

The boulder lies a short distance beyond the point at which the path to Little Gully, on the right, leaves North Gully. Photo: Bob Smith/grough

The stone block is perched precariously on rocks and soil above North Gully and Heather Terrace. It is thought recent heavy rain may have eroded some of the soil.

A spokesperson for Ogwen Valley Mountain Rescue Organisation said it had passed on information about the boulder to the National Trust, which owns the mountain, and to the Snowdonia National Park Authority.

North Gully and Little Gully combine in a scramble route that can be used to ascend or descend the East Face and emerges close to the summit of the North Ridge.

The boulder and debris from an earlier fall in the Western Gully. Photo: Chris Lloyd/Ogwen Valley MRO

Earlier this year, a similarly sized boulder rolled down the opposite side of the mountain. The rock left marks on the footpath in Western Gully during its 75m journey down the hill.

In June, grough accompanied Chris Lloyd of the Ogwen Valley rescue team on an ascent and descent of Tryfan to highlight the potential pitfalls of routes on the mountain.