Pretty Rocks Landslide

Learn and Explore

Updated: August 24, 2021

The Pretty Rocks Landslide, which underlies the roadbed at Mile 45.4 of the of the Denali Park Road, traverses a precipitous slope near Polychrome Overlook. It is one of several known landslides in the area and one of the more than 140 mapped unstable slopes along the entire Park Road. National Park Service (NPS) monitoring data indicate that the rate of movement in this area has increased dramatically in recent years.

Climate change, with its associated warmer average temperatures and increased precipitation, has taken a problem previously solved by minor road repairs and made it difficult to overcome with short-term solutions. As the area becomes more unstable, traditional road maintenance methods have become inadequate. This has resulted in road closures in the Pretty Rocks area to protect public safety.

Park managers are weighing potential engineering solutions that will allow for long-term access west of Pretty Rocks. The NPS and Federal Highway Administration (FHWA) are collaborating to keep the road operational, resilient, and safe for travel under dynamic and changing conditions.

 
timelapse of a mountainside with a gravel road on it slowly sliding downhill
Time lapse of the Pretty Rocks slump, from July 21 to August 25, 2021. In this time, the road displacement was ~6.5 meters (21 feet)

NPS Geology Team

 
 

Pretty Rocks Landslide Comparison (November 2019 vs January 2020)

a man standing on a dirt road, on a portion lower than the main surface of the road a man standing on a dirt road, on a portion lower than the main surface of the road

Left image
Late November, 2019. The area had slumped around 10' since September.
Credit: NPS Photo

Right image
Early January, 2020. The area had slumped significantly further since late November.
Credit: NPS Photo

The vertical offset between the two points moved from 10' in November 2019 (left) to 15 feet in January 2020 (right). The center of the road is near the left side of each photo. 
 
The rate of movement in 2020 was similar to the observed rate for 2019. In March 2021, NPS road crews discovered a total offset (slump) of 18 feet.

 

Background

The Pretty Rocks landslide is more accurately described as a rock glacier. Rock glaciers are closely related to glaciers, but they contain much more rocky debris with ice in the pore spaces. The average annual temperature shift of the area to above freezing in recent years is believed to be causing the rock glacier to thaw, resulting in the recent acceleration of the landslide.

The landslide at Pretty Rocks has been active since at least the 1960s, but has evolved from a manageable maintenance concern into a much more extensive maintenance challenge. Before 2014, the landslide only caused small cracks in the road surface and required maintenance every 2-3 years.

Between 2016 and 2017, the full width of a 100-yard (90-m) section of road slumped up to 0.2” (0.5 cm) per day, steepening the road gradient and limiting sight lines for drivers approaching the sharp turn. but not warranting long-term closures.

However, by 2018 the slumping increased to almost half an inch per day, and then to three and a half inches per day by August 2020. Early August rains in 2021 appear to have triggered the rate to increase significantly, with much of the landslide currently moving downhill at over ten inches per day.

NPS road crews maintained a safe drivable surface for most of the summer of 2021 by importing as much as 100 dump-truck loads of gravel per week to fill the slump. On August 24, park managers recognized that this solution was no longer tenable or safe and enacted a road closure to the east of Pretty Rocks. As the area becomes more unstable, traditional road maintenance methods have become inadequate. The park’s ability to adapt to future conditions will require transitioning to more expensive and novel construction methods to maintain road-based access.

 

Next Steps

Last updated: August, 2021

In May 2020, NPS and FHWA partnered to conduct an Expert-Based Risk Assessment (EBRA) that evaluated the geotechnical attributes of several alternatives, including north and south re-routes and a mainline option. Informed by the EBRA, a value analysis was conducted in July 2020 to determine which alternative provided the greatest value.

Based on the outcomes of the EBRA and value analysis, FHWA recommended the mainline option that would retain the existing road alignment. In collaboration with FHWA, the NPS will address maintaining road access west of Polychrome Mountain in two stages:
  • Short Term Plan: The NPS will strive to sustain temporary access at Pretty Rocks by filling the Pretty Rocks slump in the spring and continuing road maintenance efforts throughout the summer.
  • Long Term Plan: NPS and FHWA are moving forward with the design and environmental analysis of the mainline option to retain the existing alignment. This option includes building a bridge over the Pretty Rocks landslide, as well engineered solutions to address several additional geologic hazards along the Polychrome section of road. Possible alternatives to the proposed action will include continuing to fill the slump. The environmental compliance process, which includes public participation, will be conducted in 2021 and 2022 and implementation of a solution could begin as early as 2023.
 

History of the "High Line"


The Alaska Road Commission (ARC) began to build the trans-park road into Mount McKinley National Park in 1922. Over the course of 16 years, crews carved a road out of wilderness. During this time, the National Park Service decided to place part of the road along the side of Polychrome Mountain—the "High Line."

Learn more about how and why the High Line became the park road route.
 
 

Pretty Rocks Landslide Comparison (September 2018 vs March 2019)

woman and child sitting on a dirt road with a large crack in the middle of it woman and child sitting on a dirt road with a large crack in the middle of it

Left image
The eastern landslide scarp through the road on September 29, 2018. The road was last graded on September 14th and closed to most vehicle traffic on September 19.
Credit: NPS Photo

Right image
The eastern landslide scarp on March 22, 2019. Survey rod held by park employee is 6.5 feet (2.0 m) tall and is placed near center-line of the road. Note employees had recently shoveled dirty snow in foreground to aid in measuring the scarp.
Credit: NPS Photo

Last updated: August 27, 2021

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Denali Park, AK 99755

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