A "substantial landslide" has forced the emergency closure of road.
Part of the road between Saron and Cyffylliog, near Ruthin, slid down the embankment.
Denbighshire council officials visited the site on Tuesday to assess the extent of the damage.
A spokesman for the council said the road was expected to remain closed for some time while repair work is carried out.
Llandyrnog councillor Merfyn Parry said his daughter came across a branch in the road, stopped to move it, and noticed the road below had collapsed and fallen away.
"Had she not have stopped she would have driven down the bank," he told the Local Democracy Reporting Service.
"I notified the council's out of hours team who responded very quickly in getting the road closed.
"It's a warning to drivers using our rural roads to drive carefully and be observant of road conditions more so now with the darker nights and the winter ahead of us."
Local councillor Joseph Welch said he would be asking the council to repair the road as soon as possible as it was "well-used road".