What is a Glacier?

A glacier forms when more snow falls each winter than melts the next summer. The accumulation of snow presses down on the layers below, compacting them into ice. Ice near the surface of the glacier is often hard and brittle but, 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. Once this mass of snow and ice begins to move, it is called a glacier.

Glaciers Past and Present

The glaciers in Glacier National Park today are all geologically new, having formed in the last few thousand years. Presently, all the glaciers in the park are shrinking. More snow melts each summer than accumulates each winter. As the climate changed over the last two million years, glaciers formed and melted away several times. What will happen to today's glaciers if the climate becomes colder, wetter, or warmer?

Geologists theorize that about 20,000 years ago the climate became cooler and/or wetter, leading to the formation of huge glaciers that filled the valleys with thousands of feet of ice. Imagine the valleys of Glacier National Park filled with ice, and just the tops of the highest peaks sticking out. These giant rivers of ice sculpted the mountains and valleys into their present appearance. Today's glaciers are carving at the mountains as well. Although smaller, they work in the same way as the larger glaciers of the past, and teach us about Glacier National Park's geologic history.

Sculpting the Land

As the ice moves, it plucks rock and debris from the sides and bottom of the valleys. Rocks falling on the glacier from above mix with the glacial ice as well. A glacier is filled with rock and gravel. Over long periods of time the sandpaper-like quality of the moving ice scours and reshapes the land into broad U-shaped valleys, sharp peaks, and lake-filled basins, as at Glacier NP.

Glacial Landforms The park is filled with horns, cirques, arjtes, hanging valleys, and moraines, landforms given special names because they were produced by the action of glaciers.

• Horns

  A horn is a steep peak caused by several glaciers carving different sides of the same mountain. Mt. Reynolds at Logan Pass is a good example of a horn.

• Cirques

  A cirque is a large bowl formed at the head of a glacier. Often as the ice melts away a small lake will form in the depression gouged by the glacier. Avalanche, Iceberg, and Gunsight are all cirque lakes.

• Arjtes

  An arjte (French for fish-bone) forms when two glaciers work on opposite sides of the same wall, leaving a long thin ridge. One of Glacier National Parks more prominent features, the Garden Wall, is an arjte separating the Lake McDonald Valley from the Many Glacier Valley.

• Hanging Valleys

  Hanging Valleys are found throughout the park. As large glaciers scoured the main valleys, tributary glaciers worked the smaller side canyons. Unable to cut as deep as the valley glaciers, they left behind small valleys high up on the mountainsides. Hanging valleys often have waterfalls plunging out of their mouths into the valleys below. Birdwoman Falls, seen from the Going-to-the-Sun Road, plummets from a hanging valley on Mt. Oberlin.

• Moraines

  Moraines form at the sides and front of a glacier. In a glacier there is always a flow of ice from the head to the toe. This conveyor-belt-like flow brings with it the rock and debris trapped in the ice. As it reaches the sides or front and the ice melts, this trapped material is released, forming large piles. These piles of glacially-transported material are called moraines. Moraines from the present glaciers are visible as mounds of rock and gravel along the sides and front of the ice.