Learn more about the geology of Glacier National Park.
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.
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.
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 100,000-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.
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.
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.
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.
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.