Geology and Ecology of National Parks

Glacier National Park

Geology and Ecology of Glacier National Park

Glacier National Park, nicknamed “The Crown of the Continent,” spans 1,583 rugged square miles in northwest Montana south of Waterton Lakes National Park in Canada, and together the two constitute the Waterton-Glacier International Peace Park. The geologic history of Glacier National Park reveals itself in stunning landscapes and large variations in elevation, climate, and soil type and supports a diverse ecosystems including rare and threatened species. Glacier National Park, is designated as a UNESCO world heritage site and an international biosphere reserve.

Pretty Glaciers

A view of peaks at Glacier National Park, Montana. Photo by Greg Pederson/USGS.

The geologic history of Glacier National Park stretches back nearly two billion years. Glacier National Park’s stunning landscapes are a result geologic processes including erosion, deposition, uplift, faulting, folding, and perhaps most notably, recent glaciation.


Sedimentary Rock Deposition

The geologic history of Glacier National Park begins in the Proterozoic Eon, the early part of Earth’s history before complex life inhabited the planet. Many rocks this old are not preserved at Earth’s surface today, having been eroded over time or been changed significantly by metamorphism. However, at Glacier National Park, hundreds of millions of years of sedimentary rocks are preserved in the Belt Supergroup.

Ariel image of glacial lakes and mountainsides

Aerial image of glacial lakes and steep, red mountainsides in Glacier National Park. The red rocks are part of the Grinnell Formation within the Belt Supergroup. 

(Credit: Robert Simmons, NASA. Public domain.)

The Belt Supergroup crops out at the surface in western Montana, Idaho, eastern Washington, and southern Alberta, Canada. The sedimentary rock layers in the Belt terrane were deposited in a massive (at maximum extent 300 kilometers wide) intracratonic basin environment. Differentiation within the kilometers-thick Belt Supergroup mark openings and closings of this inland basin over millions of years. The rocks of the Belt Supergroup are Precambrian in age, meaning that they were deposited before the explosion of widespread invertebrate animal life beginning in the Cambrian period. Accordingly, sedimentary layers were undisturbed animal life such as bioturbating worms, so many sedimentary structures have been  well preserved. Ripples, mud cracks, and even rain drop impressions are displayed within layers of the Belt Supergroup. One of the few pieces of evidence of early life preserved in the Belt Supergroup are abundant stromatolites within the carbonate units. The term stromatolite refers to laminated structures produced by algae. The presence of stromatolites within the rock record provides geologists with a window into the environmental conditions at the time of deposition. Almost all of the rocks within the Belt terrane are sedimentary or low-grade metasedimentary rocks, other than the Purcell Sill, a dark band of igneous rocks.


Approximately 150 million years ago, plates of crust began to collide with the western edge of North America, resulting in a series of mountain-building events known as orogenies. These events had a profound effect on the surface geology of the Glacier National Park. During the Sevier Orogeny, about 105 to 75 million years ago, sheets of rocks were thrust westward about 300 miles along a thin-skinned thrust fault, where just the upper layers of Earth’s crust were transported at a low-angle movement. Evidence of the Sevier Orogeny can be seen in the mountains of Montana in Glacier National Park. In contrast to the Sevier Orogeny, the Laramide Orogeny, which began in the Late Cretaceous, was thick-skinned, meaning it occurred along faults that had nearly vertical fault planes and displaced rocks thousands of feet thick. During the Laramide Orogeny, the Lewis thrust fault became the central plane of movement of the massive rock column of the Belt Supergroup. The Lewis thrust fault is perhaps Glacier National Park’s most famous geological feature. Before the Laramide Orogeny, the Belt Supergroup was buried below thick layers of Cretaceous rock, deposited during a time of rich prehistoric plant and animal life. During this event, the Belt Supergroup rocks were pushed up and over these Cretaceous rocks along the Lewis Thrust fault. Throughout this period of uplift, rock was folded and faulted forming interesting geologic features like synclines and anticlines. When the Belt Supergroup was uplifted the rock layers from the Paleozoic, Mesozoic, and Cenozoic above them were exposed and eroded away, and are no longer present in the park.

The Laramide Orogeny ended about 35 million years ago. After that, the fault system between the Pacific and North American plates began to grow, which triggered extensional deformation of the North American plate, including land extending to the northeast. In Glacier National Park, these events are evidenced by the presence of normal faults, in contrast to the thrust faults of the Laramide Orogeny.



Researchers studying Sperry Glacier in Glacier National Park

Scientists work on Sperry Glacier in Glacier National Park, MT. Public domain.

What is a glacier? Glaciers are large accumulations of ice, snow, and rock debris that form over time when winter snow exceeds summer melting and are massive enough to move under their own weight. The weight of years’ worth of snow leads to the compaction of the bottom layers, which turns snow into ice. This massive weight also causes the ice to become more flexible and viscous, so that it slowly flows downhill under the influence of gravity.

The Earth experienced a period of global cooling during the Pleistocene Ice Age, when huge ice sheets retreated and advanced over 10,000-year cycles. During the last major glaciation, which occurred approximately 20,000 years ago Glacier National Park would have been totally covered by glaciers. Glacier National Park sits along the Continental Divide, which, during the last glacial period, separated the Cordilleran ice sheet in the west from the Laurentide ice sheet to the east (

Clements Glacier, Glacier, NP.

Image of the Clements Glacier in 2010 by Ralph Thornton/USGS. Moraines at the base of the mountain display the extent of the glacier before retreat.  (Public domain.)

Glaciers have had a huge effect on the landscape of the park, leaving behind a variety of erosional features at Glacier National Park that can be attributed to its icy past, including U-shaped valleys, hanging valleys, arêtes and horns, paternoster lakes, moraines, cirques and tarns. Valleys formed by streams are generally a v-shaped, but glaciers produce a U-shaped valley. Rock debris is incorporated into the base of the glacier, and then the massive sheet of ice acts like a giant bulldozer carving out the valley. Hanging valleys are formed when erosion by smaller glaciers in tributary valleys doesn’t keep up with the erosion by the large glacier in the main valley. When deglaciation occurs, the smaller valleys are left hanging. Arêtes and Horns are the result of erosion by glacier of peaks on multiple sides. Maximum glacial extent is recorded by the presence of massive lateral and terminal moraine deposits: unstratified and unsorted sediment ground up by the glacier and deposited at the maximum extent of the glacier’s sides and end, respectively. Cirques are bowl-shaped, amphitheater like depression eroded into the head or sides of glacial valleys. Tarns are lakes that form in the basin of cirques after the glacier melts. Find an entire glossary of glacial terms here:


Glaciers Today

Looking out the mouth of Reynolds Glacier in Glacier National Park.

View looking out from inside the Reynolds Glacier at Glacier National Park. 

(Credit: Joe Giersch, USGS. Public domain.)

By about 10,000 years ago, the large ice sheets had retreated. Most Pleistocene Ice Age glaciers melted away during a Holocene warm period. Present-day glaciers at the park date back 7,000 years, and it is possible that a few survived the Holocene warm period making them even older. However, modern glaciers at the park reached their maximum extent at the end of the Little Ice Age, which extended from 1770 to 1850.

When the park was established in 1910, it is estimated that there were about 150 distinct glaciers at the park. As of 2015, there were just 26 true glaciers remaining. USGS scientists are working to better understand glacier-climate interactions at the park. In 2017, the USGS published a 50-year, time series analysis of named glaciers at Glacier National Park. The study used aerial photography and satellite imagery to document the perimeter of glaciers and found that on average, over the 50-year period, the area of glaciers had been reduced by 39% and some glaciers had been reduced by as much as 85%. Other tools USGS scientists use to monitor glaciers are seasonal mass balance measurements, area measurements, and remote sensing.

You can learn more about USGS research on glaciers and climate at the Glaciers and Climate Project.

Map of glacial locations in Glacier NP

2010 Map of the location of glaciers in Glacier National Park. USGS (Public domain.)

USGS scientists are also using repeat photography to document glacial change at Glacier National Park. The project, which began in 1997, pairs historic pictures of glaciers at the park with pictures of the glaciers today. The resulting visuals powerfully communicate one effect of climate change.

Since 1997, Grinnell Glacier has been photographed from the summit of Mount Gould multiple times.

Grinnell Glacier from the summit of Mount Gould. The glacier’s retreat and the enlargement of Upper Grinnell lake has been captured over the years through the USGS Repeat Photography Project. Photo: Lisa McKeon/USGS.  (Public domain.)

The loss of glaciers at Glacier National Park has a number of impacts. By storing ice, glaciers act as a bank of water that regulates stream temperature and streamflow in summer months, which affects agriculture, wildlife, and fire management. Earlier snowpack melt and more hot days will extend the fire season, which may increase risk to life and property and decrease air quality. The loss of glacial melt water may also cause the extinction several temperature sensitive insects at Glacier National Park, which indirectly affects fish populations at the park. The loss of glaciers will also impact alpine meadow ecosystems and increase the population of the invasive mountain pine beetle, which have adverse can have adverse effects on pine trees.

Kintla Glacier, 1901 - 2019, Glacier National Park

Kintla Glacier in 1901 and 2019 from the Repeat Photography Project. Over the 118 years between the photos, the glacier has decreased dramatically in size and vegetation has changed as a result of forest fire events.  (Public domain.)

For more information on the geology and glaciation of Glacier National Park:

USGS Geologic map of Glacier National Park

USGS History of Glaciers in Glacier National Park

USGS Glacier Retreat in Glacier National Park, Montana

USGS Glacier Research

USGS Glacier retreat in Glacier National Park, Montana (

USGS The Belt Series in Montana (1963)

USGS Video: Climate Connections: Questions from Glacier National Park, MT (episode 4)

NPS Geology of Glacier National Park

NPS Geologic Resource Evaluation Report

USGS Glacier Research

USGS Fact Sheet on Glacier retreat in Glacier National Park  

Science in Glacier National Park

National Park Service Glacier National Park Geodiversity Atlas


Protected for over 100 years, the ecosystems of Glacier National Park are relatively undisturbed, and a rich variety of plant and animal life thrive there. The park supports over 200 different species of birds, thousands of plant species, a variety of native fish, and over 70 unique mammal species including grizzly bears, lynx, big horn sheep, mountain goats, mountain lions, pikas, and wolverines. USGS scientists at the Northern Rocky Mountain Science Center, have provided research for over three decades to help manage and protect the unique plant and animal communities within the park.

Bighorn sheep at Glacier National Park

Photo of a bighorn sheep at Glacier National Park. USGS scientists at the Northern Rocky Mountain Science Center, are working to improve our understanding of bighorn sheep movements, approaches for monitoring bighorns, and habitat use in Glacier National Park.

(Credit: Tabitha Graves, USGS. Public domain.)

Seventy-one different mammal species make their homes at Glacier National Park. Some especially notable species include grizzly bears, lynx, big horn sheep, mountain goats, mountain lions, pikas, and wolverines. Grizzly bears can be dangerous, but tend to avoid humans. They are considered a threatened species, and there are a number of human-induced pressures, including climate change and habitat fragmentation, which pose risks to their survival. USGS scientists recently began a study determining the effect of climate change on huckleberries, which comprise over 50% of grizzly bear’s diet. USGS scientists are also working to understand how to best plan developments and conservation easements to allow grizzly bears to disperse over land area. Informed management, that connects grizzly bear subpopulations, helps bears build their resilience to land use and climate change. Researchers are also working to better understand bighorn sheep in and near Glacier National Park, which are vulnerable to disease. Just for fun, have you seen this video of a “dancing bear” in Glacier National Park?

Glacier National Park is also home to over 270 different species of birds. However, there are only a handful of reptiles and six amphibian species that live at the park; these low numbers are result of the relatively recent retreat of the Pleistocene glaciers and cool temperatures. Glacier National Park also has a unique aquatic ecosystem, which support a variety of native and non-native fish species and temperature-sensitive insects. Two species, the meltwater stonefly and the glacier stonefly, are found nowhere else in the world. These species have been petitioned for inclusion in the Endangered Species Act as their populations are extremely vulnerable to glacier and snow loss. USGS scientists continue to monitor the impact of climate change on these unique species. Threats to aquatic insects also impact fish species, who rely on them for food.

Another important aspect of the ecology of Glacier National Park is fire. Wildfire is required to maintain healthy ecosystems across landscapes of the western United States. Unfortunately, unnatural fuel buildup and developments near large natural areas can result in fires that pose risks to human life and property. USGS scientists are working to understand the importance of wildfires to plant and animal life, sustain natural systems and biodiversity, and mitigate risks. 

Panoramic image from Sperry Glacier in Glacier National Park showing smoke from forest fires.

Smoke as seen from Sperry Glacier. 

(Credit: Erich Peitzsch, USGS. Public domain.)