NPS photo. The impressive mountains and valleys within the park were formed over 1.4 billion years by a number of geologic processes including erosion, sediment deposition, uplift, faulting, and glaciation. To understand the formation of Glacier National Park's geologic wonders, there is a simple way to remember how they formed. Just remember: silt, tilt, slide, and glide. The SettingOver 1,800 square miles (4,660 square km) of the rugged Rocky Mountains are found within the boundaries of Waterton-Glacier International Peace Park. Two mountain ranges, the Livingston Range and the more easterly Lewis Range, run from northwest to southeast through Glacier. The Continental Divide of the Americas follows the crest of the Lewis Range. Elevation within the park varies from a low of 3,150 feet (960 m) at the junction of the Middle and North Forks of the Flathead River, near the Lake McDonald valley, to a high of 10,466 feet (3,192 m) on Mt. Cleveland. The geologic processes that shaped this dramatic landscape happened in four stages:
NPS photo. Ancient Sediments–1.5 billion years agoMany of the rocks that form Glacier's mountains are the result of the deposit of sediment and silt into an ancient inland sea that existed during the middle Proterozoic Era (about 1,500 million years ago). Known as the Belt Sea, it covered parts of present-day eastern Washington, northern Idaho, western Montana, and nearby areas in Canada. Streams and rivers carved into higher-elevation areas and carried sand and silt into the sea. Year by year and layer by layer, these sediments built up into an immense thickness with the seafloor sinking underneath the weight of the accumulated sediment. Over roughly 100 million years, the thickness reached 18,000 feet (5,500 meters)! In a few places, pillow basalt formations (volcanic igneous rock that has a pillow-like structure) formed after lava erupted onto the sea floor. These can be seen today in the Granite Park and Boulder Pass areas of the park. Over millions of years, as sediments accumulated in the Belt Sea, mounting heat and pressure compressed the sediment into rock and created layers of quartzite, argillite, mudstone, limestone, and dolomite. Together, these are known as the Belt Supergroup and can be seen above treeline in much of the park. The sedimentary nature of these rocks and their history as part of a vast inland sea can be seen in preserved mud cracks, ripples, and layers—just like what you might see on a tidal mud flat, a beach, or a sea floor today.
From the pebbles in Lake McDonald to the faces of entire mountains, perhaps the most eye-catching feature of Glacier's geology is its varying colors. Different layers of rock can be dramatically diverse shades, and their color can tell us a great deal about their history. The process that created these striking colors centers around one element–iron. The argillite in the park contains significant amounts of iron, which is a reactive metal. Much like the bumper on an old car, iron will oxidize (or rust) and turn reddish/orange when exposed to oxygen. The red and purple colors in our rocks formed the same way. As the Belt Sea began to retreat, the iron within sediments were exposed to air, allowing oxidation to occur. Conversely, the green and blue colors found in Glacier’s rocks are a result of the rocks forming in an environment without oxygen. Underwater on the floor of the Belt Sea, iron starved of oxygen goes through a process known as reduction, with the iron bonding to silica compounds. Under heat and pressure, the iron-silicate minerals were converted to chlorite, a mineral which produced the green and blue colored rocks found in the park today. The chemical composition of these rocks—in addition to them forming in shallower versus a deeper water environment—is largely responsible for the variation in color. 
NPS photo. An Intrusion of Magma–780 million years agoWhile most of the rock in Glacier is sedimentary in nature, there are also igneous rocks formed from ancient magma. This liquid, melted rock, called magma, forced itself between layers of Siyeh Limestone while still underground. The magma formed a layer called the Purcell Sill, a dark band of igneous rock (called diorite) about 100 feet (30 m) thick. The heat of the intrusion recrystallized the surrounding limestone into white metamorphic marble. The Purcell Sill can be seen on Mt. Siyeh and Mt. Cleveland in Glacier, and Mt. Blakiston near Red Rock Canyon in Waterton. Despite the name “Granite” Park Chalet, there is no naturally occurring granite in Glacier National Park. The area around the chalet was named after exposed igneous rock. However, the rocks were misidentified by early prospectors—they were either looking at the pillow lavas, which are basalt, or at the Purcell Sill.
NPS photo. Faulting and Uplift–150-60 million years agoApproximately 150 million years ago, tectonic plates collided on what was then the western edge of North America. That collision began the tilt of Glacier's mountains and the process of mountain building that would continue for nearly 90 million years. In the area that would become Waterton-Glacier International Peace Park, massive forces folded, uplifted, and pushed a stack of rock thousands of feet thick along what is known as the Lewis overthrust. That stack of rock is the colorful sediments that were slowly moved in the tilt stage and are roughly 1.5-1.4 billion years old. Tectonic forces pushed the whole stack up and over much younger rocks (only 70 million years old!). Over millions of years, they would slide 50 miles (80 km) eastward along the fault to their present location. Geologists estimate that this movement occurred at a rate of millimeters per year, one earthquake at a time. The Lewis overthrust is proof of the tectonic events that created the mountains here in the Crown of the Continent. The immense pressure folded the rocks into massive s-curves that can be seen in many mountain faces if you look for them. Many smaller faults cracked and broke through the rock layers in various places throughout the park. As a result of the uplift along all these faults, erosive forces accelerated. Over several million years, the erosive power of water, wind, and glaciers removed the upper layers of rock, exposing the rock formations evident in the park today and carving them into unique shapes. Learn more about Glacier’s mountains on our Mountains page.
There are many places to see evidence of the Lewis overthrust, but the most famous and photographed view of it can be found at Marias Pass on US Highway 2 by looking at Summit Mountain across the road. Marias Pass is just outside the southeast corner on the park and can be found on our Maps page. The fault can be seen along Summit and Little Dog Mountains as a thin, tan line of rock roughly halfway between the peak and the ground below, separating the harder, older rocks that make up the craggy peaks and the softer, younger, more breakable rocks that make up the forested scree slopes beneath.
NPS photo. Glaciation: The Ice Age–2 million years agoThe most recent defining geological event that shaped this landscape began with a global cooling trend, or Ice Age, approximately 2 million years ago during the Pleistocene Epoch. This Ice Age saw large ice sheets repeatedly advance and retreat throughout the temperate regions of North America until about 10,000 years ago. The final retreat occurred at different times in different places. In the area that would become Glacier National Park, the Ice Age ended and ice retreated from the area about 12,000 years ago. During the ice advances, the lower-elevation valleys were filled with glaciers and only the very tops of the higher mountain peaks were visible. The "rivers of ice" would glide throughout the mountains and valleys, sculpting them into a variety of landforms associated with major alpine and valley glacial action. Even though the Ice Age glaciers are gone, the results of their presence are evident on the landscape. Massive U-shaped valleys, cirque lakes or tarns, horns, moraines, and arêtes are just a few of the glacially carved landforms that contribute to the beauty of Glacier National Park. Learn more on our Glacial Geology page. 
NPS photo. Recent Glaciation–Dating from about 7,000 years agoToday, we are living in a relatively warm interglacial period. All remnants of the Pleistocene ice have disappeared. The active glaciers we have today no longer fill the valley bottoms. However, there are currently about two dozen named alpine glaciers, which work the same way as larger glaciers of the past on a smaller scale. These alpine glaciers are of relatively recent origin, likely having formed in the last 6,000 to 8,000 years. They most likely grew rapidly during the Little Ice Age, which started about 400 to 500 years ago, and ended in about 1850. Tree ring studies indicate that retreat of these more recent glaciers began around 1850, near the end of the Little Ice Age. When Glacier National Park was established in 1910, there were around 80 glaciers within the park, compared to about two dozen now. Retreat rates appear to have been slow until about 1910, followed by a period of rapid retreat during the mid- to late 1920s. This corresponds to a period of warmer summer temperatures and decreased precipitation in this region. Several larger glaciers separated into two smaller glaciers at this time, such as the Jackson and Blackfoot Glaciers and the Grinnell and Salamander Glaciers. Learn more on our Glaciers page. A glacier forms when more snow falls each winter than melts the next summer. Over time, the snow metamorphoses to become firn, dense snow left behind from past seasons. As layers of snowpack and firn build up, the ice recrystallizes, becomes denser, and eventually forms into hard glacial ice. A glacier is a mass of ice so big that it flows under its own weight. A commonly used threshold for determining if a body of ice is big enough to flow under its own weight is an area of 0.1 km², which is about 25 acres. Below this size, the ice is less likely to move and is not considered a glacier. This general definition works most of the time, but there are exceptions. Some glaciers may be smaller than 0.1 km² and yet remain active. Others may stop moving under their own weight and remain larger than 0.1 km². Ice near the surface of the glacier is often hard and brittle. Due to the pressure of ice above, the ice near the bottom of the glacier becomes flexible. This flexible layer allows the ice to move. Depending on the amount of ice, the angle of the mountainside, and the pull of gravity, the ice may start to move downhill, both deforming internally and sliding downhill along its base. Once the ice begins to move, it is called a glacier. As the ice moves, it plucks rock from the sides and bottom of the valleys. Rocks falling on the glacier from above mix with the glacial ice as well. Over long periods of time, the moving ice and rock scours and reshapes the land into broad U-shaped valleys, sharp peaks, and lake-filled basins.
Some of the first Westerners who visited the park to document its glaciers, like George Bird Grinnell, arrived in the late 1800s, not long after the Little Ice Age ended. He and other naturalists and scientists began photographing alpine glaciers as they were at the time. Those photos allow for us to make some remarkable comparisons today. To see how these glaciers have melted and changed in the last 100 years, check out our Glacier Repeat Photos page.
Learn more about Glacier's geology
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Last updated: September 18, 2024