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.
The minimum size criterion for a glacier is 0 .1 km², or about 25 acres. Below this size, the accumulated ice generally does not move and is stagnant. Many years of snow accumulation compacts the bottom layers of snow into ice. Under the huge weight of the snowpack (usually 100ft/30.5m thick or more), the ice becomes viscous and allows the mass to slide downhill. The appearance of crevasses and cracks attest to a glacier’s movement. Glaciers don't "bulldoze" landscape as much as they melt and re-freeze, plucking material from areas of snow deposition and moving it to other areas, like downhill conveyor belts.
The current glaciers in the park are estimated to be at least 7,000 years old and peaked in size in the mid-1800s, during the Little Ice Age. Millions of years before that, during a major glacial period known as the Pleistocene Epoch, enough ice covered the Northern Hemisphere to lower sea levels 300 feet. In places near the park, ice was a mile deep. The Pleistocene Epoch ended around 12,000 years ago.
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.
U-shaped ValleysLike any other form of water, glaciers follow the most direct course downward. This means they often fill areas previously filled by a river or stream. A river cuts a V-shape profile. The freeze/re-freeze glacial conveyor belt scours valleys into a U-shape, broad at the bases and sheer on the sides. The result (when the glacier is gone) is awesome verticality and/or long, deep lakes like Lake McDonald and Bowman.
Hanging ValleysWhen a small side-channel glacier feeds into a larger and deeper-cutting trunk glacier, the undercut forms a hanging valley, like the one above Bird Woman Falls and in hundreds of other places in the park.
Arêtes and HornsSaw-toothed arêtes, like the Garden Wall, mark places where two glaciers carved on each side of a ridge. Craggy horns are mountain tops that were scraped vertical by glaciers on three or more sides. Examples in Glacier include Flinsch Peak, Reynolds Mountain, and the Little Matterhorn.
Paternoster LakesA chain of small, successively lower lakes form where the glacier scoops a depression during its retreat. This string of bowls is known as paternoster lakes because of their resemblance to rosary beads.
MorainesAt a large terminal moraine, glaciers advanced and melted for a few hundred years at exactly the same rate, dumping their payload in one spot. Lateral moraines are made of debris pushed along a glacier's sides. Some "erratic" rocks in moraines are the size of houses. As you drive around the Blackfeet Reservation on the park's eastern boundary, you may notice such huge erratics sitting in fields, seemingly out of place, left there by long-ago glaciers.
Cirques and TarnsIce cream scoop-like amphitheaters, called cirques, are carved by glaciers sitting on a relatively protected slope where snow and ice can pile up and carve out a deep bowl. Many park cirques still hold glaciers, long-standing snowfields, or lakes. Tarns are the lakes which fill those cirques.
Last updated: November 15, 2019