Glaciers 101
by Charles Finn
Broadcast 11.4.2018

Jackson Glacier terminus in Glacier National Park, Montana

Jackson Glacier terminus in Glacier National Park (public domain image).




Glaciers: they are sculptors, carvers and artists. When the Bitterroot Mountains, the Missions and the Rockies were raised from the floor of the ocean it was the glaciers that came behind, crawling over the surface, grinding and eroding the face of the land.

They buried our now green valleys under hundreds of tons of ice, filling these to the brim, inundating lakes and changing the paths of rivers. Crinkled with crevasses, burning the eyes with the reflected light of the sun; apparently, but not in repose – today’s glaciers hang in our mountains like clouds, draped seductively around pointed summits or fanned out in wide swaths of white against the cobalt fields of the sky.

A glacier is born when the summer sun shines but fails to fully melt the heavy snow pack. Winter comes again and deposits more snow. As time rolls on, the trend continues, and as the snow accumulates it’s pressed into denser and denser layers.

Gradually the lower layers become ice, and at some point the entire mass yields to the force of gravity. Flowing relentless as it is powerful, dangerous as it is serene, a glacier will shoulder its way into adjacent valleys, flowing together with other glaciers, marching onward in a path of blind, creative transformation.

The path of glaciers is easy to track in the alpine. At their furthest extent they leave piles of rock and rock debris called the terminal moraine. To either side are similar lateral moraines, wide stripes that outline its path. Medial moraines are where two glaciers have met and then melted back.

Crevasses, the slick-walled chasms that can open up instantaneously when a snow bridge collapses, are the product of ice moving at different rates and stretching over irregularities in the land. These slits lead to the internal plumbing of the glaciers, melt pool and streams that scurry well under the surface. Stories are told of victims who have fallen in and walked miles to emerge unscathed. More usually they will be regurgitated decades later, frozen corpses carried along then coughed up at the leading edge of the ice.

Glaciers live according to geological time, and scientists believe we may be living in an era of interglacial warmth. We’re making hay while the sun shines. But an overall cooling trend began 7,000 years ago and is expected to continue for another 23,000 years. Predictions, however, must now take into account the effects of global warming, a trend that could overwhelm the natural cycles of nature. Glaciers are shrinking worldwide. Huge chunks are calving from the ice shelves in the Arctic and Antarctic. Meanwhile, glaciers in the Rockies have shrunk by a third since the beginning of the century.

Scientists read glaciers like books. To thumb the pages they drill core samples up to 100 meters deep. Working back in time they can see the advent of acid rain in the 1980’s, pollution levels rising in the post war period, and a very distinct layer of radioactive fallout from Hiroshima and Nagasaki. Going further back they find evidence of different volcanic eruptions around the globe, including Iceland’s Laki eruption in 1783. By analyzing carbon dioxide and oxygen isotopes, also pollen levels, scientists can gauge global temperatures and patterns over the course of thousands of years. It’s even possible to deduce the atmosphere dinosaurs would have breathed.

Grinding mountains, impounding lakes, depositing immense amounts of sediments, glaciers are history books, fillers of steams, creators and destroyers. They walk the land, sticking a tongue out here, retracting a toe there, taking their time. As is evident, there’s no shortage of that. What is not so evident, is which way they’ll go.


Every week since 1991, Field Notes has inquired about Montana’s natural history. Field Notes are written by naturalists, students, and listeners about the puzzle-tree bark, eagle talons, woolly aphids, and giant puffballs of Western, Central and Southwestern Montana and aired weekly on Montana Public Radio.

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