Jobs at a Gwynedd slate quarry are under threat after half a million tonnes of stone fell off a rock face, it has emerged.
The Health and Safety Executive (HSE) is investigating last month's incident at the Penrhyn quarry in Bethesda.
Parts of the quarry have closed, and around 90 of the 200 workers have had hours reduced, with others redeployed.
Welsh Slate said it was looking for volunteers for redundancy but hoped to avoid compulsory job losses.
A HSE spokesman said the landslip on 27 April was reported to have happened at 0530 BST, with no injuries resulting from it.
The Penrhyn quarry produces roofing slate and construction and industrial aggregates which it exports around the world.
The company was in the process of re-aligning the quarry to exploit new seams and extend the life of the quarry, and said the landslip did not affect those plans.
A spokesman said not all staff's working arrangements had been affected by the rock fall.
In a statement, managing director Chris Allwood said: "A rock fall that took place overnight between April 26 and 27 has reduced the amount of material currently available for processing at Penrhyn Quarry. This has had an impact on our roofing slate output.
"We are taking a series of measures to minimise the impact on our workforce and are redeploying people to other parts of the business where possible."
He said some slate production was being secured from other locations and an extra shift was being added to the firm's crushing operation.
"In discussion with our workforce, an option of short-time working in the affected departments was balloted amongst Unite members and passed on May 10, with a majority opting for a four-day week.
"Personnel from the affected departments will switch from a five-day week to a four-day week starting on May 21."
Mr Allwood said managers were aiming to avoid compulsory redundancies, and would consider volunteers for redundancy from all areas of the business.
The Unite union told the Daily Post they were working with the company to minimise the impact on workers, adding the consequences of the fall could have been much more serious had it happened in working hours.