Glaciers are perpetually moving, flowing frozen rivers, and like other rivers, they churn up dirt and rocks carry them "downstream." When a glacier retreats, the mud and rocks are often dumped at its edge, forming moraines. The moraines sometimes grow so large that they inhibit the advance of the glacier and cause the ice to thicken, like water filling a bathtub.
The surface of the rocks also tells a story. When a glacier melts, the newly exposed material becomes an inviting habitat for lichen and other organisms, which gradually darken the stone. Such growth can take 50 or 60 years to start, however, so bare rock inside the moraine signals that a glacier has retreated only within the last few decades. Shilts calls the light-colored moraine below the dark lichen-covered rock the glacier's "bathtub ring."
"1948 was the year that the first aerial photographs were taken of Bylot Island and most of northern Canada," Shilts said. "On those photographs you can see that the glaciers were considerably advanced over what they are now. And any boulders that were involved with glacier activities in the 1940s look as fresh as if they were broken off their outcrops yesterday.
They have no lichen or any sort of growth on them. As soon as you go beyond the 1948 boundary, the boulders are covered with black lichen. You can't even see the rock. And so it's a very clear demarcation on the ground."
Shilts photographed many of the same glaciers in the 1980s and 1990s, and other geologists have chronicled the changes up to the present. These photos show a steady and rapid decline in the extent of several glaciers: Stagnation Glacier, covered in a layer of rock and debris, has shrunk considerably since 1948.
Nearby Fountain Glacier seems more stable, but the outwash plain below it, a zone always coated in a thick layer of ice, even throughout the summer, w
|Contact: Diana Yates|
University of Illinois at Urbana-Champaign