The LibDem-led authority said it was in the process of assessing the extent of the damage and coming up with a plan to repair it.
Thornton said this was “not something we could see coming” and said water got into ground around the road and “washed it away”.
“It is a very big job,” he said.
The A592 is the main route to Patterdale from the Ambleside and Windermere areas.
While a diversion will be in place via the A6, Thornton said he knew local people would be using a road known as The Struggle.
“It’s called The Struggle for a reason – it’s narrow, it’s steep and it’s difficult,” he said.
But he added the council would ensure it had an “enhanced plan for gritting” to keep it open over winter.
