The extreme weather conditions caused by Storm Callum meant around 300 tonnes of debris reached the thoroughfare.
A further 50 tonnes had also blocked the nearby Old Military Road, according to Bear Scotland, the trunk road maintenance firm.
Both roads have been closed since Tuesday, forcing motorists to make a diversion of nearly 60 miles, but transport secretary Michael Matheson said a “safety first” approach was paramount.
He said: “The landslides at the Rest and Be Thankful this week have been some of the most challenging in recent memory, both for the community and for the teams working in difficult conditions.
“I saw for myself this morning the size of the challenge, with secondary slips complicating matters and delaying use of the Old Military Road diversion route.
“It is a dynamic situation on the hillside.
“There are warnings of further heavy rain and while every effort is being made to get traffic moving, today’s events reinforce the need for a safety-first approach.
Elsewhere, Storm Callum continued to cause disruption across the road, rail and ferry network as rain and heavy winds battered much of western Scotland.
A spokesman for Glasgow Airport said five outbound flights and five arrivals – all of which were on domestic routes – were cancelled yesterday.
Several Caledonian MacBrayne services were cancelled, including crossings from Oban to Castlebay, and Mallaig to Armadale, while train services between Kilwinning and Largs and Ardrossan were suspended due to waves crashing over the sea wall at Saltcoats.
Skye and Dornoch bridges were temporarily closed to high-sided vehicles, while high wind warnings were in place at the Kessock and Erskine bridges.
Heavy rain also added to early evening congestion on the M8 and M90 Queensferry Crossing.
SP Energy Networks said some of its customers in Girvan were affected by power cuts, but its engineers had restored the supplies by late afternoon.
A Met Office amber warning for periods of heavy rain across swathes of the Central Belt and south-west Scotland is in force until 6pm today. As of yesterday evening, Sepa had two flood warnings in place for Corpach and Caol near Fort William and the Churchill Barriers in Fort William. A further ten flood alerts were in place across the country.