The 1.5-Billion-Year Geology of Glacier National Park

The Belt Supergroup: 1.5 Billion Years of Sediment

Almost every rock you see above treeline in Glacier National Park belongs to the Belt Supergroup, per NPS.gov. This sequence of sedimentary rock accumulated over roughly 100 million years in a shallow inland sea — the ancient Belt Sea — that covered what is now eastern Washington, northern Idaho, and western Montana around 1.5 billion years ago.

The Belt Supergroup built up to a staggering 18,000 feet of sediment layers. The main rock types are quartzite, argillite, mudstone, limestone, and dolomite. Most of what you observe on Glacier's cliffsides, canyon walls, and ridgelines is this ancient marine deposit, compressed and uplifted over geologic time.

The park's rock record is so well-preserved because the Belt Sea was remarkably calm, producing flat-lying sediment layers with almost no volcanic disruption. This made Glacier's rocks internationally significant for understanding Precambrian Earth — geologists travel from around the world to study them.

Why Are Glacier's Rocks Red, Green, and Purple?

The striking colors of Glacier's argillite — a fine-grained sedimentary rock — directly reflect oxygen conditions in the ancient Belt Sea, per NPS.gov. The color pattern follows a simple chemical rule.

This color-coding makes Glacier's cliffs almost like a geological timeline you can read with your eyes. Red layers above green layers typically indicate the ancient sea was retreating, exposing sediment to the atmosphere. Green layers beneath red ones suggest the sea was deepening, cutting off oxygen at the seafloor.

The most accessible place to see this color variation is at the Going-to-the-Sun Road cuts, where the road sliced directly through exposed Belt Supergroup strata. Logan Pass at 6,646 feet offers panoramic views of multicolored ridge faces that display the full argillite spectrum.

The Purcell Sill: A 780-Million-Year Dark Stripe

Running horizontally across several of Glacier's highest peaks is a distinct dark band known as the Purcell Sill — a roughly 100-foot-thick layer of dark igneous diorite intruded between the lighter Siyeh Limestone layers approximately 780 million years ago, per NPS.gov.

Unlike the surrounding sedimentary layers, the Purcell Sill formed when magma forced its way horizontally between rock strata without erupting at the surface. The intense heat of this intrusion recrystallized the surrounding limestone into white metamorphic marble, leaving a visible white band immediately above and below the dark diorite stripe.

The Purcell Sill is one of the easiest geological features to identify without a guide. On Mt. Siyeh and Mt. Cleveland — Glacier's highest peak at 10,466 feet — the dark horizontal stripe breaks up the otherwise pale limestone faces in a way that's visible from miles away. The Siyeh Pass Trail offers one of the best close-up views of this formation.

The Lewis Overthrust: When Mountains Travel 50 Miles

The single most dramatic geological event in Glacier's history was the Lewis Overthrust — a collision between tectonic plates that pushed an enormous stack of ancient rocks approximately 50 miles (80 km) eastward over rocks that were 70 million years younger, per NPS.gov.

This process began approximately 150 million years ago and continued for 90 million years, with movement occurring in millimeter-per-year increments during earthquake episodes. The result is one of the most famous geological anomalies in North America: older Precambrian rock sitting directly on top of younger Cretaceous rock — the opposite of the normal sequence where older rocks lie below younger ones.

Chief Mountain, at 9,080 feet near the park's northeast corner, is one of the most striking examples of the Lewis Overthrust. The mountain is an "outlier" — an isolated remnant of the overthrust sheet that erosion has cut away from the main block. It sits on top of much younger shale, creating a geological monument visible for miles across the plains.

How Glaciers Carved the Park's Iconic Terrain

Although the park's rock dates back 1.5 billion years, its dramatic topography is far more recent. The Pleistocene Ice Age, which began approximately 2 million years ago, sent massive glaciers flowing through the region, carving the park's signature terrain before finally retreating about 10,000 years ago, per NPS.gov.

The key glacial landforms visible throughout the park: U-shaped valleys — wide, flat-bottomed valleys carved by glaciers moving through river canyons (McDonald Valley and the Many Glacier valley are classic examples). Cirques — bowl-shaped basins at the heads of glaciers, often containing a lake (tarn); Grinnell Lake and Iceberg Lake sit in glacial cirques. Arêtes — knife-edge ridges formed where two cirques eroded toward each other from opposite sides; the Garden Wall along the Continental Divide is one of the park's most famous arêtes. Horns — pyramid-shaped peaks created by three or more cirques eroding toward a central point; Mt. Reynolds near Logan Pass is a textbook horn. Moraines — ridges of rock debris deposited at the edge or end of a glacier; the terminal moraines at the foot of Grinnell Glacier mark maximum ice extents from the Little Ice Age.

The vertical relief in Glacier — with elevations ranging from 3,150 feet at the lowest valley floors to 10,466 feet at Mt. Cleveland's summit — is a direct product of glacial erosion accelerating the uplift that had already occurred through the Lewis Overthrust.

How Many Glaciers Does Glacier National Park Have Today?

When Glacier National Park was established in 1910, it contained approximately 80 named glaciers. As of 2026, approximately 24 named glaciers remain, per NPS.gov.

The current glaciers are relatively young — they likely formed during the last 6,000 to 8,000 years, grew substantially during the Little Ice Age (roughly 400 to 500 years ago through 1850), and have been retreating since. Retreat accelerated significantly during the mid-to-late 1920s and has continued through the present day.

To qualify as an "active glacier" under standard glaciological definitions, a body of ice must be at least 25 acres (0.1 km²). Several of the park's named glaciers are near or below this threshold. Grinnell Glacier, Harrison Glacier, and Sperry Glacier are among the most-studied. The Many Glacier area remains the best hub for glacier viewing.

Stromatolites: Fossils From 1.5 Billion Years Ago

Glacier National Park contains some of the best-preserved stromatolite fossils on Earth — mound-like structures created by blue-green algae (cyanobacteria) that lived in the Belt Sea approximately 1.5 billion years ago, per NPS.gov.

Stromatolites are found in the Altyn Formation limestone and dolomite throughout the park and appear as rounded, layered structures in cross-section. Living stromatolites exist in only a handful of places globally — including the tidal channels of the Exuma Cays, Bahamas — which shows just how little these organisms have changed in over a billion years.

In addition to stromatolites, the park contains Horodyskia moniliformis — a string-of-beads fossil that may represent an evolutionary link to eukaryotic (complex-celled) life. Cretaceous fossils including mollusks, bivalves, and plant material also occur in the park's younger rock formations near the eastern foothills.

Where to See the Best Geology in Glacier National Park

Entrance fee: $35/vehicle (valid 7 days), $20/person on foot or bike, $30/motorcycle, per NPS.gov.