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Home Archived Aug 4, 2016

USGS Geology in the Parks

North Cascades Geology

Granite polished by glaciers
Granite polished by glaciers.

The Work of Moving Ice

Visitors to the North Cascades can easily perceive the role of glaciers in creating the mountain scene. Glaciers have left their marks everywhere, either in erosional land forms or in glacial deposits. Views of glaciers (and their deposits) abound. Backcountry hikers and climbers learn the joys and hazards of traveling on glacier ice. In general, to see the larger glaciers and their moraines, the visitor must take to the trails and off-trail routes, but a few good distant views can be found with minimal effort. Mount Shuksan glaciers are on grand display from the road to the Mount Baker Ski Area (State Hwy 542) and from Artist Point. Hikes to Railroad Grade Moraine on Mount Baker, from Schriebers Meadow or Glacier Creek afford even closer views of active glaciers.
Borealis Glacier
Borealis Glacier flows into heavily crevassed icefall below Triconi Peak.

Formation of Glaciers

Glaciers form wherever winter snowfall exceeds summer melt for enough years to accumulate a thick mass of snow. As the snow deepens, the bottom layers metamorphose into dense ice. The weight of overlying ice and snow causes the lowermost ice to flow, and a glacier is born. As the glacier grows, it extends farther and farther downhill. In the summertime, all the snow may melt off the lower end of the glacier, leaving bare metamorphic ice, some of which may also melt. At higher elevations, snow accumulates, transforms to ice, and creeps slowly down to the zone of melting. The boundary between the upper part of the glacier, where snow accumulates from year to year, and the lower part, where melting exceeds accumulation, is called the firn line. If the overall rate of melting exceeds the rate of accumulation, the lower end of the glacier retreats; if accumulation dominates, the lower end of the glacier advances down the valley. But the ice and snow within the glacier always are moving downslope.

Profiles of V- and U-shaped valleys.
Profiles of V- and U-shaped valleys.

How a Glacier Sculpts and What It Leaves Behind

A glacier sculpts the land in ways different from running water. A river occupies a small U-shaped channel in the bottom of a V-shaped valley. A glacier carves the whole valley into a U-shape. The upper end of a river is a small stream in a small swale that merges with the drainage divide. The head of a glacier-carved valley is a steep-walled bowl or cirque. Glaciers are not as easily influenced by rock hardness as are rivers. They tend to bulldoze ahead, straightening out twisty river valleys.
Glacier and moraine nomenclature (modified from  Manning, 1967).
Glacier and moraine nomenclature (modified from Manning, 1967)

Glaciers move very slowly, but can carry large amounts of rock debris. Individual blocks can be huge compared to the largest rocks moved by running water. When glaciers melt, they drop their loads as masses of unsorted debris, called till, in piles of various shapes called moraines. Commonly, at the terminus, or snout, of a glacier, till forms a horseshoe-shaped moraine, with the open side facing upvalley. This end moraine forms as the glacier brings debris downvalley to the melting snout. The debris piles up as if delivered from a conveyor belt, albeit a slow one. Similarly, as a glacier moves downvalley, it also deposits till along its margins, forming sinuous ridges that parallel the glacier’s flow. These ridges are called lateral moraines. Running water from a melting glacier can sort the unsorted till sediment into more uniform layers or beds of sand, mud, and/or gravel. Till is commonly mixed with this sorted material from the melt streams.

On to Scenery Born of Ice

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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