Rock Glaciers: Reminders of a Glacial Past and Dynamic Landforms in a Warming Future
by Matthew Morriss
An oblique aerial photo of the Gad Valley rock glacier at the Snowbird ski resort in Little Cottonwood Canyon, Utah. Note the steep front on the left, indicating the potential for ice cementation of sediments. The rumpled surface texture is known as “ridge and furrow” topography, emblematic of rock glacier movement.
Utah is a state of dramatic landscapes that include red rock canyons of southern Utah, wide open desert valleys of the West Desert, and the high alpine environments of the Wasatch Range and Uinta Mountains. These alpine zones owe their modern form to ice glaciers that occupied this region multiple times over the past 2.5 million years. These glaciers carved and widened valleys, and generally sculpted the region into the landscape seen today in canyons like Little Cottonwood and Big Cottonwood of the Wasatch Range. As geologists, we study these glacial landscapes and the piles of debris they left behind, known as moraines. However, there are still glimmers of active ice glaciers here in Utah seen in landforms known as rock glaciers.
Rock glaciers form in high elevation alpine environments. They can contain large accumulations of ice, insulated from warmer temperatures by a carapace of fallen rock called talus. Talus also hides the true interior structure of the landform. However, with the help of geophysical imaging techniques like ground penetrating radar, geologists can find out how much ice there is and where it is distributed within the rock glacier. Scientists think that rock glaciers formed during the most recent geologic period known as the Holocene (approximately 11,700 years to present) and not from remnants of glacial ice from the last glaciations. Rock glaciers form from the accumulation of snow in steep, shaded, north-facing areas at high elevation that collect and protect a winter’s snowpack from melting through the summer. Several colder years with more snow accumulation could yield a permanent snowfield, which through a colder decade or two would transition to ice through its own weight and internal pressure. Then with a drier and warmer spell, that snow becomes buried and insulated by rock and rocky debris.
Cross-sectional diagram of a rock glacier, depicting the internal ice mass or permafrost core overlain by rocky debris. Meltwater sources contribute to both surface runoff and groundwater recharge into underlying aquifers.
A repeat of this process over thousands of years leads to the growth of a rock glacier. Rock glaciers are distinctly different from glaciers in that they have few to no crevasses and rarely have visible ice on their surface. Rock glaciers cover relatively small areas compared with their older ice glacier relatives. In the Wasatch Range, rock glaciers have an average area of approximately 74 acres (0.1 mi2 [ 0.3 km2 ]). Their overall volume remains more enigmatic. Determining volume requires measurements of their thickness which is hard to constrain without methods like ground penetrating radar, an actual drill core through the rock glacier, or other geophysical imaging methods.
Utah is the 5th most mountainous state in the conterminous United States with numerous high elevation, mountainous areas. So, with time and climate oscillations through the Holocene period, more than 800 rock glaciers have developed throughout the state. These rock glaciers are mostly located within the Uinta Mountains, Wasatch Range, and La Sal Mountains. Other pockets of rock glaciers are in the Tushar Mountains, Fish Lake Plateau, and a couple in the Henry Mountains.
In addition to their widespread presence, rock glaciers may play an important role in Utah’s water systems. They are found in the headwater reaches of watersheds we rely on for drinking and irrigation water. Local to Salt Lake City, this includes Little Cottonwood Creek and Big Cottonwood Creek. These same high elevation regions at the headwaters have experienced more rapid warming due to climate change than lower elevation areas, which is a phenomenon known as elevation-dependent warming. This makes rock glaciers highly sensitive to warming temperatures.
To better understand how a changing climate impacts rock glacier meltwater, the UGS is involved in several studies of alpine regions of the Wasatch Range near Salt Lake City. The UGS began a pilot study in 2023, in partnership with researchers at Utah State University and the Water Resources Division of the Utah Department of Natural Resources, to evaluate the amount of water that may be sourced from rock glacier melt during late summer. This work builds on a recent study in the Uinta Mountains that showed meltwater from rock glaciers could make up to ~25% of late summer runoff in alpine catchments. This topic is particularly important to a dry state like Utah, which has historically relied on snowmelt for our water supply through the summer. Moreover, given that about 25% of Salt Lake City’s water supply comes from Big and Little Cottonwood Canyons and these two catchments have around 70 rock glaciers, a previously unaccounted part of Utah’s largest city’s water supply could come from these rock glaciers.
Distribution of rock glaciers (shown in blue) across Utah. Most rock glaciers are located in the Uinta Mountains, Wasatch Range near Salt Lake City, and the La Sal Mountains.
Climate change has already had an effect on rock glaciers, and the mountainous areas of Utah will continue to experience accelerated warming compared with the lower elevations. This warming could lead to destabilizing melt within some rock glaciers. Several rock glaciers in Europe have either collapsed into large landslides or triggered debris flows through rapid melting. A recent 2022 landslide of a rock glacier-like feature at high elevation in Rocky Mountain National Park, Colorado, has the UGS on the lookout for similar types of destabilizations in the Wasatch Range. To better understand the potential hazard, water resource questions, and make better, more up-to-date geologic maps, the UGS has started to conduct annual lidar (light detection and ranging) drone flights of the Gad Valley rock glacier in the Snowbird ski area to monitor its movement. Our goal is to better understand if Utah’s rock glaciers are accelerating or slowing with increased warming, or showing other signs of instability that could threaten the infrastructure in Little Cottonwood Canyon.
The study of rock glaciers in Utah is a multifaceted endeavor, representing both a window into the state’s glacial history and a preview of how these landforms may respond to future climate changes. As temperatures continue to rise, understanding the potential for destabilization, changes in water contributions, and overall behavior of rock glaciers will be crucial. Rock glaciers are an anachronism; they are smaller glacial-esque landforms reminding us of large alpine glaciers that inundated our mountains across Utah. Rock glaciers remain dynamic alpine features, prompting the UGS and other researchers to explore their profound impact on Utah’s mountains and hydrology through cutting-edge research.
ABOUT THE AUTHOR
Matthew Morriss
is a mapping geologist at the Utah Geological Survey, working on a broad array of projects around the State including making geologic maps of the Vernal NE and Blanding North quadrangles. His background is geomorphology with 2.5 years at the USGS Utah Water Science Center, earning a PhD from the University of Oregon; MSc. from North Carolina State, and a B.A. from Whitman College. He’s actively engaged in the research of rock glaciers throughout Utah and the intermountain west. Originally from Austin, Texas, Matthew has explored geology in the west and east and as far away as Thailand and Mongolia; he’s happy to have called Salt Lake home for 5 years and loves reading a book on the couch and spending as much time as possible in our local mountains with his partner Sarah and dog Hauk.
