Glacial Geology

Red rocks jumble and meet with blue snow and ice.
NPS photo by Tim Rains.

Evidence of glaciers is all around the park.

Glaciers are masses of ice that are so big they move under the influence of gravity. They grow when winter snow accumulation exceeds summer melting. They retreat when melting outpaces snowfall. Spotting an active glacier can be a challenge but the park's glacially carved landscapes are hard to miss. Once you know what to look for, viewing Glacier's landscape is like reading a textbook on the geologic effects of glaciation.

 

 
A u-shaped valley.
Watch for U-shaped valleys throughout the park.

NPS photo.

U-shaped Valleys

Like any other form of water, glaciers follow the most direct course downhill. This means they often flow in areas previously filled by a river or stream. When a river erodes a valley into the mountains, it cuts a V-shape profile. In contrast, a glacier scours valleys into a U-shape, broad at the base and sheer on the sides. The result (when the glacier is gone) is awesome verticality and/or long, deep lakes like Lake McDonald and St. Mary Lake.
 
A motorcyclist parked at an overlook along a short stone wall. A waterfall from a deep "hanging" valley positioned above another trickles down in the distance.
Bird Woman Falls flows from a higher-elevation “hanging valley” into a deeper valley carved by a larger glacier.

NPS photo.

Hanging Valleys

The smaller the glacier, the less power it has to erode the landscape. When a small, side-channel glacier feeds into a larger and deeper-cutting glacier (like a small stream flowing into a larger river), the smaller glacier often leaves behind a much higher-elevation valley that dramatically ends in mid-air. This is called hanging valley. The hanging valley above Bird Woman Falls is a textbook example, but they can be seen in many other places throughout the park.
 
A dark brown mountain with nearly vertical sides is illuminated by a golden sunrise.
Reynolds Mountain has a nearly vertical peak. It's an example of a horn left behind by glaciers on three or more of its sides.

NPS photo.

Arêtes and Horns

Saw-toothed arêtes, like the Garden Wall, mark places where two glaciers carved each side of a ridge. Craggy horns are mountain peaks that were scraped nearly vertical by glaciers on three or more sides. Examples in Glacier include Flinsch Peak, Reynolds Mountain, and the Little Matterhorn.
 
A gravel moraine.
Norris Glacier's moraine.

USGS photo by Lisa McKeon.

Moraines

The conveyor belt motion of a glacier carries debris downhill—from massive rocks to the smallest pebbles and grains of sand—which it then deposits at the far edge of the glacier where the ice is melting. When a glacier’s end (called its terminus) remains in the same place for a period of time, it dumps this material in a single spot where it accumulates in a sharp-edged ridge of loose rock, sand, and silt. This ridge is called a moraine. The longer a glacier stays in one spot—which requires a steady climate—the larger this moraine will be.
 
A series of lakes from a mountain top.
A chain of lakes left behind by glaciers stair-step down the Swiftcurrent Valley.

NPS photo.

Paternoster Lakes

As a massive valley glacier retreats at the end of an ice age, it doesn’t necessarily melt in a steady trajectory. During cooler climatic periods, its retreat may pause, with the glacier remaining in a steady state for decades, or even longer. Wherever the glacier’s tip remains in one place for an extended period of time, a small moraine will start to form. If this happens repeatedly, a series of these moraines will form as the ice retreats up the valley. They each fill with meltwater, creating a chain of small lakes in the glacial valley. This string of successive lakes is known as paternoster lakes, because of their resemblance to rosary beads.
 
A landscape view of a mountain with a scooped recess where snow and ice is visible.
Piegan Glacier sits in a cirque on Piegan Peak (seen from Preston Park).

NPS photo.

Cirques and Tarns

If you picture an ice cream scoop carving out a chunk of mountain, what’s left behind is a rounded amphitheater. Called a cirque, these round, steep-sided valleys are carved by the head of a glacier. Many park cirques still hold glaciers, long-standing snowfields, or lakes. These small alpine lakes are often called tarns.






 
 

Last updated: September 19, 2024

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West Glacier, MT 59936

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