Glaciers are famous stone carvers. Around eight percent of the world’s surface has experienced some aspect of glacial erosion. The three most common types of evidence of past glacial activity are smooth U-shaped valleys, deposits of till and outwash, and glacial grooves cut into a rock.

Along the edges of Blue Glacier one can find evidence of an older and bigger glacial past. Deep striations parallel to the direction of glacier flow cover the valley walls. At one time these surfaces were beneath the glacier and heavily abraded. Similar to coarse sandpaper, the rocks caught in the flowing ice sheet ground down the bedrock surface. The type of mark left depends on the rock and mineral particles at the ice-rock interface. Large angular fragments of a hard mineral such as quartz will carve a deep and continuous groove. Smaller particles, on the other hand, will polish the rock smooth rather than striate it. Most often, such as in the marks shown here, a random assortment of materials leave a complex variety of striations. Though the rate of erosion is slow, the process of abrasion is continual and over hundreds of years can profoundly alter the landscape.

Another way a glacier will erode material is a process called plucking. Instead of slowly grinding the valley rock down, a glacier will simply break off a large chunk. Melt water can penetrate small cracks or joints in a rock. When the water freezes it expands and breaks the rock free. Small gravel size fragments up to rocks the size of school buses are removed from the glacier floor or valley walls and carried off by the ice.

With such powerful erosional forces, a distinct difference is left between surfaces that have been run over by a glacier and those that were spared. In a valley glacier the horizontal boundary is known as the trim line—everything below having been trimmed by the glacier. On Blue Glacier it’s easy to get a sense of the maximum extent of the glacier by the sharp trim line 60 meters above the current surface of the ice.


intro | terminology | accumulation | firn | blue | ablation | water | equilibrium | massbalance | movement | crevasse | structure | algae | moraine | debris | erosion
pdf version | glacier glossary | bibliography | about blue ice

Benjamin Drummond 2002