A massive avalanche, one not seen in Glacier National Park in decades, roared down the "Garden Wall" that helps mark the Continental Divide and took out nearly 550 feet of stone retaining wall along the Going-to-the-Sun Road.

Born in the upper reaches of the rim of rock that towers above the Sun Road on its west-to-east climb to Logan Pass, the slab of snow slid an estimated 4,000 vertical feet. Given a "Class 5" rating, the highest available, the avalanche was thought to be one in a series of avalanches that slid down and across the west side of the Sun Road, most likely in January, say park officials.

The section of the Sun Road that sustained avalanche damage was in the vicinity of Alder Creek, approximately 2.5 miles above "The Loop." Park officials guess the avalanche occurred on or about January 8, as heavy rain had fallen on top of a large accumulation of new snow. Avalanche specialists suggest that the heavy rainfall percolated through the snowpack down to the ground/snow interface. This caused an avalanche that started just below the Continental Divide along the Garden Wall.

The two slide paths crossed the upper Sun Road at Alder Creek and again near the Packer’s Roost Road. This location was not previously identified as a known avalanche zone along the Sun Road.

According to park avalanche specialists, it is unlikely that an avalanche nearing this scale and magnitude has occurred in the park for many decades. Debris that consists of trees and rocks was deposited on the Packer’s Roost Road at a depth of 15-20 feet deep.

National Park Service and Federal Highway Administration engineers documented that the avalanche sheered away portions of stone masonry from the Sun Road at Alder Creek. A 95-foot section of stone masonry wall installed last year was lost and approximately 450 feet of 1920s' era historic stone masonry was also taken out by the avalanche. There is also approximately 200 feet of asphalt damage.

Despite the damage, Glacier Superintendent Chas Cartwright is confident the Sun Road will open for traffic when current snow-plowing operations wrap up.

“At this point, we see no reason why the entire Sun Road won’t open to vehicle traffic when plowing is completed… and when road and weather conditions permit the entire 50-mile long historic landmark roadway to open across Logan Pass," he said.

At this time, nearly two-thirds of the entire road is open for vehicles. Spring plowing operations have included clearing the debris from both sections of the Sun Road and the Packer’s Roost Road.

Park plowing crews only recently cleared snow from the upper roadway in the vicinity of Alder Creek and discovered the damage.

“It is very unfortunate that we lost so much of the historic masonry wall, but given the scale of this avalanche, we are fortunate that damage to the Sun Road was not worse than it is," said Superintendent Cartwright. "Ironically, the avalanche came through the current road rehabilitation zone where the roadbed has not yet been rehabilitated.”

Engineers and park officials will be assessing options for repairs that will include installing barrier rocks or other removable guardrails when the outer lane is repaired.